JP2018169425A - Imaging lens system and imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image pickup lens system and an image pickup apparatus capable of suppressing a deviation between an image plane and a focus position due to a temperature change. An image pickup lens system 11 has a first lens L1 having a positive power, a second lens L2 having a negative power, a third lens L3 having a positive power, and an image side in order from the object side. It has a fourth lens L4 which is a concave surface, and the first lens L1 has a characteristic that the refractive index decreases as the temperature rises, and is connected to an image pickup lens system 11 due to thermal expansion of a lens barrel. The change in the distance to the image plane IMG and the change in the focal length due to the change in temperature cancel each other out. [Selection diagram] Fig. 1

Description

  The present invention relates to an imaging lens system and an imaging apparatus.

  In recent years, along with miniaturization of cameras, compact photographing lenses having a short overall lens length have been proposed. For example, as a photographic lens having a short lens total length, a compact photographic lens having a small number of lenses or a lens having a small focal length ratio to the total lens length is proposed.

  In general, the four-lens optical system shortens the overall length, and when attempting to widen the field angle of view by 60 degrees or more, distortion and astigmatism deteriorate, and good optical performance is obtained over the entire screen. It was difficult.

  Patent Document 1 discloses a lens having an imaging angle of view of about 65 degrees, an F number of 28, and a tele ratio of about 10 by appropriately setting the lens shape, aperture, and refractive power of each lens as described above. A compact photographic lens is described that has a short overall length or good optical performance over the entire screen.

JP-A-3-265809

  However, in these imaging lenses, when the temperature changes, the length of the lens barrel (lens barrel) that holds the lens also changes. Therefore, the focus position moves due to the temperature change and shifts to the imaging plane and the focus position. There was a problem that would occur.

The imaging lens system according to an embodiment includes, in order from the object side, a first lens having a positive power, a second lens having a negative power, a third lens having a positive power, and a concave surface on the image side. 4 lenses,
The first lens has a characteristic that the refractive index decreases as the temperature increases,
When the focal length of the first lens is defined as f1 and the focal length of the entire lens optical system is defined as f, the following expression (1) is satisfied:
1 <f1 / f <1.5 (1)
The focal length of the first lens is f1, the focal length of the second lens is f2, the focal length of the third lens is f3, the focal length of the fourth lens is f4, and the refractive index of the d-line of the first lens. n1, the refractive index of the d-line of the second lens is n2, the refractive index of the d-line of the third lens is n3, the refractive index of the d-line of the fourth lens is n4,
The environmental temperature of the lens optical system is defined as t,
The refractive index change amount of the d-line of the first lens is dn1 / dt, the refractive index change amount of the d-line of the second lens is dn2 / dt, the refractive index change amount of the d-line of the third lens is dn3 / dt, and the fourth The refractive index change amount of the d-line of the lens is defined as dn4 / dt,
The distance from the first lens to the image plane is defined as LL1, the distance from the second lens to the image plane as LL2, the distance from the third lens to the image plane as LL3, and the distance from the fourth lens to the image plane as LL4. ,
When the total length of the lens optical system is defined as TTL and the linear expansion coefficient of the lens barrel is defined as dL / dt, the following expression (2) is satisfied.
| (F / f1 × dn1 / dt × LL1 + f / f2 × dn2 / dt × LL2 + f / f3 × dn3 / dt × LL3 + f / f4 × dn4 / dt × LL4) + TTL / f * dL / dt | <20 × 10 ⁻⁶ (Mm / ° C) (2)

Preferably, in the imaging lens system of one embodiment, when the total length of the lens optical system is defined as TTL and the focal length of the lens is defined as f, the following expression (3) is satisfied.
1.5 <TTL / f <1.8 (3)
Preferably, in an imaging lens system according to an embodiment, the first lens has an Abbe number of 55 or more, and the fourth lens has a characteristic that a refractive index decreases as a temperature increases. The number was made to be 55 or more.

  According to the imaging lens system of an embodiment, the first lens has a characteristic that the refractive index decreases as the temperature increases, and the change in focal length and the focal length due to thermal expansion of the lens barrel. By canceling out the changes, it is possible to suppress a shift between the imaging plane and the focus position.

  According to the imaging lens system of one embodiment, the image plane can be corrected more easily by making the refractive index of the third lens larger than that of the first lens.

An imaging apparatus according to an embodiment includes any one of the imaging lens systems described above,
A lens barrel holding the imaging lens system;
A flat cover glass disposed on the object side of the imaging lens system;
And an image sensor arranged at a focal position of the imaging lens system.

  According to the imaging apparatus system of one embodiment, the first lens has a characteristic that the refractive index decreases as the temperature increases, and the imaging lens system and the imaging plane caused by thermal expansion of the lens barrel Since the change in the distance between the two and the change in the focal length due to the change in temperature cancel each other, it is possible to suppress the deviation between the imaging plane and the focus position.

  According to the imaging lens system and the imaging apparatus of the present invention, it is possible to suppress a shift between the imaging plane and the focus position due to a temperature change.

1 is a cross-sectional view of an imaging lens system according to Example 1. FIG. FIG. 3 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram at room temperature of the imaging lens system of Example 1. 3 is a lateral aberration diagram of the imaging lens system according to Example 1. FIG. 3 is a lateral aberration diagram of the imaging lens system according to Example 1. FIG. 3 is a lateral aberration diagram of the imaging lens system according to Example 1. FIG. 3 is a lateral aberration diagram of the imaging lens system according to Example 1. FIG. 3 is a lateral aberration diagram of the imaging lens system according to Example 1. FIG. FIG. 3 is a spherical aberration diagram of the imaging lens system of Example 1 at normal temperature, low temperature, and high temperature. 6 is a diagram illustrating MTF defocus characteristics of the imaging lens system at a half angle of view of 0 ° in Example 1. FIG. 6 is a cross-sectional view of an imaging lens system according to Example 2. FIG. FIG. 4 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram at room temperature of the imaging lens system of Example 2. 6 is a lateral aberration diagram of the imaging lens system according to Example 2. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 2. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 2. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 2. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 2. FIG. 6 is a spherical aberration diagram at normal temperature, low temperature, and high temperature of the imaging lens system of Example 2. FIG. 6 is a diagram illustrating MTF defocus characteristics of an imaging lens system at a half field angle of 0 ° in Embodiment 2. FIG. 6 is a cross-sectional view of an imaging lens system according to Example 3. FIG. FIG. 6 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram at room temperature of the imaging lens system of Example 3. 6 is a lateral aberration diagram of the imaging lens system according to Example 3. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 3. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 3. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 3. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 3. FIG. 6 is a spherical aberration diagram of the imaging lens system of Example 3 at normal temperature, low temperature, and high temperature. FIG. FIG. 10 is a diagram illustrating MTF defocus characteristics of the imaging lens system at a half field angle of 0 ° in Example 3. 6 is a cross-sectional view of an imaging lens system according to Example 4. FIG. FIG. 6 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram at room temperature of the imaging lens system of Example 4. 6 is a lateral aberration diagram of the imaging lens system according to Example 4. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 4. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 4. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 4. FIG. 6 is a lateral aberration diagram of the imaging lens system according to Example 4. FIG. FIG. 6 is a spherical aberration diagram of the imaging lens system of Example 4 at normal temperature, low temperature, and high temperature. FIG. 10 is a diagram illustrating MTF defocus characteristics of the imaging lens system at a half angle of view of 0 ° in Example 4. 10 is a cross-sectional view of an imaging apparatus according to Embodiment 5. FIG.

Hereinafter, the imaging lens system and the imaging apparatus according to the present embodiment will be described.
(Example 1: Imaging lens system)
FIG. 1 is a cross-sectional view of the imaging lens system according to the first embodiment. In FIG. 1, an imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a positive power, a stop STOP, a second lens L2 having a negative power, and a third lens having a positive power. A lens L3, and a fourth lens L4 having a concave surface on the image side. The imaging plane of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12, a cover glass 13, and a lens barrel 14.

  The first lens L1 is an aspheric lens having positive power. The object side lens surface S1 of the first lens L1 is an aspherical surface and has a convex curved surface portion protruding toward the object side. The image side lens surface S2 has a concave curved surface portion that is recessed toward the object side.

  The stop STOP adjusts the amount of light passing therethrough. For example, the diaphragm STOP preferably has a plate shape having holes.

  The second lens L2 is a lens having negative power. The object side lens surface image S4 of the second lens L2 has a concave curved surface portion that is recessed toward the image side, and the image side lens surface S5 has a concave surface portion that is recessed toward the object side.

  The third lens L3 is an aspheric lens having positive power. The object side lens surface S6 of the third lens L3 has a concave curved surface portion that is recessed toward the image side, and the image side lens surface S7 has a convex curved surface portion that protrudes toward the image side.

  The fourth lens L4 is an aspheric lens having positive power. The object side lens surface S8 of the fourth lens L4 has a convex curved surface portion protruding toward the object side, and the image side lens surface S9 has a concave curved surface portion recessed toward the object side.

The IR cut filter 12 is a filter that cuts infrared light.
The cover glass 13 is a glass plate for protecting the image sensor. The cover glass 13 is handled as an integral part of the imaging lens system 11 when the imaging lens system 11 is designed. However, the cover glass 13 is not an essential component of the imaging lens system 11.

  The lens barrel 14 is a flange that fixes at least the positions of the first lens L1, the stop STOP, the second lens L2, the third lens L3, and the fourth lens L4. Specifically, the lens barrel 14 has a hollow cylindrical shape, and has a shape that holds the outer periphery of the lens and the diaphragm inside. The lens barrel 14 may have a shape that directly or indirectly fixes the positional relationship between the IR cut filter 12, the cover glass 13, and the imaging plane.

  Any material can be applied to the lens barrel 14 as long as the lens and the diaphragm can be fixed. For example, the material of the lens barrel 14 is preferably metal or resin. The material of the lens barrel 14 is particularly preferably aluminum or plastic.

  When the temperature rises, the lens barrel 14 is thermally expanded and elongated. Therefore, the distance between the lens held by the lens barrel 14 and the imaging surface becomes longer as the temperature rises. As a result, the distance between the lens held by the lens barrel 14 and the imaging plane deviates from the focal length of the imaging lens system.

  On the other hand, in the imaging lens system 11, since the first lens L1 is located farthest from the imaging surface, a change in refractive index due to a temperature change greatly affects the focal length. Therefore, the first lens L1 has a characteristic that the refractive index decreases as the temperature increases, thereby canceling the change in the distance between the imaging lens system and the imaging plane due to the thermal expansion of the lens barrel. Thus, it becomes easy to change the focal length.

  As the first lens L1 having the characteristic that the refractive index decreases as the temperature increases, it is preferable to use fluorophosphate glass (or phosphate glass). In Example 1, an example in which a fluorophosphate glass is used for the first lens L1 will be described.

  In the imaging lens system 11, the change in the distance between the imaging lens system and the imaging plane due to the thermal expansion of the lens barrel 14 is such that the first lens L1, the stop STOP, the second lens L2, and the third lens L3. Further, by canceling out the change in the focal length of the image pickup lens system by the fourth lens L4, the shift of the imaging plane and the focus position is suppressed. Detailed numerical conditions will be described later.

Hereinafter, characteristic data of the imaging lens system 11 will be described.
First, Table 1 shows lens data of each lens surface of the imaging lens system 11. The lens data includes the radius of curvature of each surface, the surface spacing with the next surface, the refractive index at 25 ° C. for the d-line, the Abbe number, and the amount of change in the refractive index for each temperature zone. The amount of change in refractive index for each temperature zone indicates the amount of change in refractive index with temperature in the temperature range shown in the table. For example, the column of −40 ° C. to −20 ° C. indicates the amount of change in the refractive index when the temperature is in the range of −40 ° C. to −20 ° C. When the refractive index at 100 ° C. of the d-line of the first lens L1 is obtained using Table 1, the refractive index at 25 ° C. of the d-line of the first lens L1 is 1.55332. The refractive index of the d-line at 100 ° C. is 1.553332+ (40−25) × −0.0000062 + (60−40) × −0.0000064 + (80−60) × −0.0000064 + (100−80) × −. 0.0000064 = 1.552843. In Table 1, although -40 degrees C or less and 100 degrees C or more are not defined, about -40 degrees C or less change amount, -20 degreeC--40 degreeC, about 80 degrees C or more, temperature change coefficient of 60 degrees C-80 degrees C Will be used. The same applies to the embodiments described later. A surface with “*” indicates that the surface is aspherical.

Here, the aspherical shape adopted for the lens surface is that the sag amount in the optical axis direction is Y (h), c is the reciprocal of the radius of curvature, the height from the optical axis in the direction perpendicular to the optical axis is h, and the cone When the coefficients are K, fourth order, sixth order, eighth order, tenth order, twelfth order, fourteenth order, and sixteenth order aspheric coefficients are A4, A6, A8, A10, A12, A14, and A16, respectively, Is done. The meaning of each symbol and the expression representing the aspherical shape are the same in the examples described later.

Table 2 shows the aspheric coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 1. In Table 2, for example, “−6.522528E-03” means “−6.522528 × 10 ⁻³ ”.

  FIG. 2 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram of the imaging lens system of Example 1 at room temperature. As shown in FIG. 2, in the imaging lens system 11 of Example 1, the half field angle is 13 ° and the F value is 1.8. In the field curvature diagram of FIG. 2, the horizontal axis indicates the distance in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In the field curvature diagram of FIG. 2, Sag represents the field curvature in the sagittal plane, and Tan represents the field curvature in the tangential plane. As shown in the field curvature diagram of FIG. 2, according to the imaging lens system 11 of the present embodiment, the field curvature is well corrected. Therefore, the imaging lens system 11 has a high resolution.

  In the distortion diagram of FIG. 2, the horizontal axis indicates the amount of image distortion (%), and the vertical axis indicates the image height (angle of view). The field curvature diagram and the distortion diagram of FIG. 2 show the simulation results with a light beam having a wavelength of 558 nm. In the spherical aberration diagram of FIG. 2, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in the spherical aberration diagram of FIG. 2, in the imaging lens system 11 of the present embodiment, occurrence of defocusing at different wavelengths is suppressed.

  3 to 7 are lateral aberration diagrams of the imaging lens system according to Example 1. FIG. 3 to 7, the horizontal axis represents the relative pupil X coordinate or the relative pupil Y coordinate, and the vertical axis represents the lateral aberration amount. FIG. 3 is a lateral aberration diagram of the imaging lens system at a half angle of view of 0 °. FIG. 4 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half field angle of 5 °. FIG. 5 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half field angle of 5 °. FIG. 6 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half field angle of 8.8 °. FIG. 7 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half field angle of 8.8 °. As shown in FIGS. 3 to 7, there is little variation due to wavelength, and color bleeding is suppressed.

Next, Table 3 shows the result of calculating the characteristic value of the imaging lens system 11 of Example 1. In the imaging lens system 11, the focal length of the entire lens system to f, and the focal length _{f 1} of the first lens L1, the focal distance _{f 2} of the second lens L2, the focal length of the third lens L3 _{f 3,} 4 Table 3 shows these characteristic values when the focal length of the lens L4 is f ₄ and the refractive indexes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

Table 4 shows the temperature characteristics. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

In Table 4, dn / dT (10 ^ -6) is the refractive index when the temperature of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is changed from 20 ° C. to 40 ° C. ^Is shown on the order of 10 ⁻⁶ .

1 / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the reciprocal of the focal length of the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1, and further multiplying by 10 ⁶ . 1 / f2 * dn / dt × 10 ^ 6, 1 / f3 * dn / dt × 10 ^ 6 and 1 / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

f / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the value obtained by dividing the focal length f of the entire lens system by the focal length f _{1 of} the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1. , And multiplied by 10 ⁶ .

  f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6 and f / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

  L1 to image plane (= LL1) indicate the distance on the optical axis from the image-side lens surface S2 of the first lens L1 to the imaging plane. Similarly, L2 to the image plane (= LL2), L3 to the image plane (= LL3), and L4 to the image plane (= LL4) from the image side lens surface S2 of the second lens L2, the third lens L3, and the fourth lens L4. The distance on the optical axis to the image plane is shown.

  Here, in Table 4, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, the reciprocal of the focal length is multiplied by the amount of change in the refractive index with respect to the lens temperature, and further multiplied by 106. The values f / f1 * dn / dt × 10 ^ 6, f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6, and f / f4 * dn / dt × 10 ^ 6 Define a, b, c, and d, and define the distances LL1, LL2, LL3, and LL4 from the lens to the image plane as e, g, h, and i, respectively.

  The change in the focal length due to the temperature change in the first lens L1 is obtained by a value obtained by multiplying the above 1 / f1 * dn / dt × 10 ^ 6 by L1 to the image plane (= LL1). A change in focal length due to a temperature change in the second lens L2, the third lens L3, and the fourth lens L4 can be obtained by the same calculation.

  That is, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, changes in focal length due to temperature changes are values A multiplied by a and e, and values multiplied by b and g, respectively. A value C obtained by multiplying B, c and h, and a value D obtained by multiplying d and i.

  On the other hand, the change in the focal length due to the thermal expansion of the lens barrel 14 is obtained by multiplying the value obtained by dividing the total length TTL of the lens optical system by the focal length f of the entire lens system and the linear expansion coefficient of the barrel. . In Table 4, the change in focal length due to the thermal expansion of the lens barrel 14 is defined as F.

  The total change in focal length of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is indicated by sum (A to D) in Table 4. sum (A to D) is a value obtained by adding A, B, C, and D. The sum of the focal length changes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 described above cancels the focal length change caused by the thermal expansion of the lens barrel 14. Set. For example, if the absolute value of the value indicated by E + F in Table 4 is less than or equal to a predetermined numerical value, it is considered that the two have canceled each other.

  FIG. 8 is a spherical aberration diagram of the imaging lens system of Example 1 at normal temperature, low temperature, and high temperature. In the spherical aberration diagram of FIG. 8, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in FIG. 8, the imaging lens system 11 of the present embodiment suppresses occurrence of defocusing at different wavelengths in any state of normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature. Is done.

  FIG. 9 shows the MTF defocus characteristic of the imaging lens system at a half angle of view of 0 ° in the present embodiment. In FIG. 9, the horizontal axis represents the image plane position change amount, and the vertical axis represents MTF (Modulation Transfer Function). As shown in FIG. 9, the change in the peak position is suppressed at normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature.

  Thus, according to the imaging lens system of Example 1, the first lens has a characteristic that the refractive index decreases as the temperature increases, and the imaging lens system resulting from the thermal expansion of the lens barrel By canceling out the change in the distance to the imaging plane and the change in the focal length, it is possible to suppress the deviation between the imaging plane and the focus position.

(Example 2: imaging lens system)
FIG. 10 is a cross-sectional view of the imaging lens system according to the second embodiment. In FIG. 10, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a positive power, a stop STOP, a second lens L2 having a negative power, and a third lens having a positive power. The lens L3 includes a fourth lens L4 having a concave surface on the image side. The imaging plane of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12, a cover glass 13, and a lens barrel 14 (not shown).

  The first lens L1 is an aspheric lens having positive power. The object side lens surface S1 of the first lens L1 is an aspherical surface and has a convex curved surface portion protruding toward the object side. The image side lens surface S2 has a convex curved surface portion protruding toward the image side. The first lens L1 preferably has a characteristic that the refractive index decreases as the temperature increases. As the first lens L1 having the characteristic that the refractive index decreases as the temperature increases, it is preferable to use fluorophosphate glass (or phosphate glass). In Example 1, an example in which a fluorophosphate glass is used for the first lens L1 will be described.

  The stop STOP adjusts the amount of light passing therethrough. For example, the diaphragm STOP preferably has a plate shape having holes.

  The second lens L2 is a lens having negative power. The object side lens surface image S4 of the second lens L2 has a concave curved surface portion that is recessed toward the image side, and the image side lens surface S5 has a concave surface portion that is recessed toward the object side.

  The third lens L3 is an aspheric lens having positive power. The object side lens surface S6 of the third lens L3 has a concave curved surface portion that is recessed toward the image side, and the image side lens surface S7 has a convex curved surface portion that protrudes toward the image side.

  The fourth lens L4 is an aspheric lens having positive power. The object side lens surface S8 of the fourth lens L4 has a convex curved surface portion protruding toward the object side, and the image side lens surface S9 has a concave curved surface portion recessed toward the object side.

The IR cut filter 12 is a filter that cuts infrared light.
The cover glass 13 is a glass plate for protecting the image sensor. The cover glass 13 is handled as an integral part of the imaging lens system 11 when the imaging lens system 11 is designed. However, the cover glass 13 is not an essential component of the imaging lens system 11.

  The lens barrel 14 (not shown) is a flange that fixes at least the positions of the first lens L1, the stop STOP, the second lens L2, the third lens L3, and the fourth lens L4, as in the first embodiment.

  The imaging lens system 11 has an imaging lens system in which the change in focal length due to thermal expansion of the lens barrel 14 is the first lens L1, the stop STOP, the second lens L2, the third lens L3, and the fourth lens L4. By canceling out the change in the focal length, the deviation between the image plane and the focus position is suppressed. Detailed numerical conditions will be described later.

Hereinafter, characteristic data of the imaging lens system 11 will be described.
First, Table 5 shows lens data of each lens surface of the imaging lens system 11. The lens data includes the curvature radius of each surface, the surface spacing, the refractive index, the Abbe number, and the amount of change in refractive index for each temperature zone. The amount of change in refractive index for each temperature zone indicates the amount of change in refractive index with temperature in the temperature range shown in Table 5. For example, the column of −40 ° C. to −20 ° C. indicates the amount of change in the refractive index when the temperature is in the range of −40 ° C. to −20 ° C. A surface with “*” indicates that the surface is aspherical.

Table 6 shows aspheric coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 2. In Table 6, for example, “−6.522528E-03” means “−6.522528 × 10 ⁻³ ”.

  FIG. 11 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram of the imaging lens system of Example 2 at normal temperature. As shown in FIG. 11, in the imaging lens system 11 of Example 2, the half field angle is 13 ° and the F value is 1.8. In the field curvature diagram of FIG. 11, the horizontal axis indicates the distance in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In the field curvature diagram of FIG. 11, Sag represents field curvature on the sagittal plane, and Tan represents field curvature on the tangential plane. As shown in the field curvature diagram of FIG. 11, according to the imaging lens system 11 of the present embodiment, the field curvature is corrected well. Therefore, the imaging lens system 11 has a high resolution.

  In the distortion diagram of FIG. 11, the horizontal axis indicates the amount of image distortion (%), and the vertical axis indicates the image height (angle of view). The field curvature diagram and the distortion diagram in FIG. 11 show the simulation results with a light beam having a wavelength of 558 nm. In the spherical aberration diagram of FIG. 11, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in the spherical aberration diagram of FIG. 11, in the imaging lens system 11 of this example, occurrence of defocusing at different wavelengths is suppressed.

  12 to 16 are lateral aberration diagrams of the imaging lens system according to Example 2. FIGS. 12 to 16, the horizontal axis represents the relative pupil X coordinate or the relative pupil Y coordinate, and the vertical axis represents the lateral aberration amount. FIG. 12 is a lateral aberration diagram of the imaging lens system at a half angle of view of 0 °. FIG. 13 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half field angle of 5 °. FIG. 14 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half field angle of 5 °. FIG. 15 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half field angle of 8.8 °. FIG. 16 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half field angle of 8.8 °. As shown in FIGS. 12 to 16, color bleeding is suppressed.

Next, Table 7 shows the result of calculating the characteristic value of the imaging lens system 11 of Example 2. The focal length of the entire lens system f in the imaging lens system 11, the focal length _{f 1} of the first lens L1, the focal distance _{f 2} of the second lens L2, the focal length _{f 3} of the third lens L3, the fourth lens Table 7 shows these characteristic values when the focal length of L4 is f ₄ and the refractive indexes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

Table 8 shows the temperature characteristics. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

In Table 8, dn / dT (10 ^ -6) is the refractive index when the temperature of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is changed from 20 ° C. to 40 ° C. ^Is shown on the order of 10 ⁻⁶ .

1 / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the reciprocal of the focal length of the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1, and further multiplying by 10 ⁶ . 1 / f2 * dn / dt × 10 ^ 6, 1 / f3 * dn / dt × 10 ^ 6 and 1 / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

f / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the value obtained by dividing the focal length f of the entire lens system by the focal length f _{1 of} the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1. , And multiplied by 10 ⁶ .

  f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6 and f / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

  L1 to image plane (= LL1) indicate the distance from the first lens L1 to the image plane. Similarly, L2 to the image plane (= LL2), L3 to the image plane (= LL3), and L4 to the image plane (= LL4) are distances from the second lens L2, the third lens L3, and the fourth lens L4 to the image formation plane. Indicates.

  Here, in Table 8, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, the reciprocal of the focal length is multiplied by the amount of change in the refractive index with respect to the lens temperature, and further multiplied by 106. The values f / f1 * dn / dt × 10 ^ 6, f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6 and f / f4 * dn / dt × 10 ^ 6 Define a, b, c, and d, and define the distances LL1, LL2, LL3, and LL4 from the lens to the image plane as e, g, h, and i, respectively.

  The change in the focal length due to the temperature change in the first lens L1 is obtained by a value obtained by multiplying the above 1 / f1 * dn / dt × 10 ^ 6 by L1 to the image plane (= LL1). A change in focal length due to a temperature change in the second lens L2, the third lens L3, and the fourth lens L4 can be obtained by the same calculation.

  That is, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, changes in focal length due to temperature changes are values A multiplied by a and e, and values multiplied by b and g, respectively. A value C obtained by multiplying B, c and h, and a value D obtained by multiplying d and i.

  On the other hand, the change in the focal length due to the thermal expansion of the lens barrel 14 is obtained by multiplying the value obtained by dividing the total length TTL of the lens optical system by the focal length f of the entire lens system and the linear expansion coefficient of the barrel. . In Table 8, the change in focal length due to the thermal expansion of the lens barrel 14 is defined as F.

  The total change in focal length of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is indicated by sum (A to D) in Table 8. sum (A to D) is a value obtained by adding A, B, C, and D. The sum of the focal length changes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 described above cancels the focal length change caused by the thermal expansion of the lens barrel 14. Set. For example, if the absolute value of the value indicated by E + F in Table 8 is equal to or less than a predetermined numerical value, it is considered that they are canceling each other.

  FIG. 17 is a spherical aberration diagram of the imaging lens system of Example 2 at normal temperature, low temperature, and high temperature. In the spherical aberration diagram of FIG. 17, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in FIG. 17, in the imaging lens system 11 of the present embodiment, occurrence of defocusing at different wavelengths is suppressed in any state of normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature. Is done.

  FIG. 18 is a diagram illustrating the MTF defocus characteristic of the imaging lens system at the half field angle of 0 ° in the second embodiment. In FIG. 18, the horizontal axis represents the image plane position change amount, and the vertical axis represents the MTF. As shown in FIG. 18, the change in the peak position is suppressed at normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature.

  Thus, according to the imaging lens system of Example 2, the first lens has a characteristic that the refractive index decreases as the temperature increases, and the change in focal length due to the thermal expansion of the lens barrel. By canceling out changes in the focal length, it is possible to suppress a shift between the image plane and the focus position.

(Example 3: imaging lens system)
FIG. 19 is a sectional view of the imaging lens system according to Example 3. In FIG. 19, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a positive power, a stop STOP, a second lens L2 having a negative power, and a third lens having a positive power. The lens L3 includes a fourth lens L4 having a concave surface on the image side. The imaging plane of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12, a cover glass 13, and a lens barrel 14 (not shown).

  The first lens L1 preferably has a characteristic that the refractive index decreases as the temperature increases. As the first lens L1 having the characteristic that the refractive index decreases as the temperature increases, it is preferable to use fluorophosphate glass (or phosphate glass). In Example 1, an example in which a fluorophosphate glass is used for the first lens L1 will be described.

  The first lens L1 is an aspheric lens having positive power. The object side lens surface S1 of the first lens L1 is an aspherical surface and has a convex curved surface portion protruding toward the object side. The image side lens surface S2 has a convex curved surface portion protruding toward the image side.

  The stop STOP adjusts the amount of light passing therethrough. For example, the diaphragm STOP preferably has a plate shape having holes.

  The second lens L2 is a lens having negative power. The object-side lens surface image S4 of the second lens L2 has a convex curved surface portion protruding toward the object side, and the image-side lens surface S5 has a concave curved surface portion recessed toward the object side.

  The third lens L3 is an aspheric lens having positive power. The object side lens surface S6 of the third lens L3 has a concave curved surface portion that is recessed toward the image side, and the image side lens surface S7 has a convex curved surface portion that protrudes toward the image side.

  The fourth lens L4 is an aspheric lens having positive power. The object side lens surface S8 of the fourth lens L4 has a convex curved surface portion protruding toward the object side, and the image side lens surface S9 has a concave curved surface portion recessed toward the object side.

The IR cut filter 12 is a filter that cuts infrared light.
The cover glass 13 is a glass plate for protecting the image sensor. The cover glass 13 is handled as an integral part of the imaging lens system 11 when the imaging lens system 11 is designed. However, the cover glass 13 is not an essential component of the imaging lens system 11.

  The lens barrel 14 (not shown) is a flange that fixes at least the positions of the first lens L1, the stop STOP, the second lens L2, the third lens L3, and the fourth lens L4, as in the first embodiment.

  The imaging lens system 11 has an imaging lens system in which the change in focal length due to thermal expansion of the lens barrel 14 is the first lens L1, the stop STOP, the second lens L2, the third lens L3, and the fourth lens L4. By canceling out the change in the focal length, the deviation between the image plane and the focus position is suppressed. Detailed numerical conditions will be described later.

Hereinafter, characteristic data of the imaging lens system 11 will be described.
First, Table 9 shows lens data of each lens surface of the imaging lens system 11. The lens data includes the curvature radius of each surface, the surface spacing, the refractive index, the Abbe number, and the amount of change in refractive index for each temperature zone. The amount of change in refractive index for each temperature zone indicates the amount of change in refractive index with temperature in the temperature range shown in Table 9. For example, the column of −40 ° C. to −20 ° C. indicates the amount of change in the refractive index when the temperature is in the range of −40 ° C. to −20 ° C. A surface with “*” indicates that the surface is aspherical.

Table 10 shows the aspheric coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 3. In Table 10, for example, “−6.522528E-03” means “−6.522528 × 10 ⁻³ ”.

  FIG. 20 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram of the imaging lens system of Example 3 at normal temperature. As shown in FIG. 20, in the imaging lens system 11 of Example 3, the half angle of view is 13 ° and the F value is 1.8. In the field curvature diagram of FIG. 20, the horizontal axis indicates the distance in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In the field curvature diagram of FIG. 20, Sag represents field curvature on the sagittal plane, and Tan represents field curvature on the tangential plane. As shown in the field curvature diagram of FIG. 20, according to the imaging lens system 11 of the present embodiment, the field curvature is corrected well. Therefore, the imaging lens system 11 has a high resolution.

  In the distortion diagram of FIG. 20, the horizontal axis indicates the amount of distortion (%) of the image, and the vertical axis indicates the image height (angle of view). The field curvature diagram and the distortion diagram of FIG. 20 show the simulation results with a light beam having a wavelength of 558 nm. In the spherical aberration diagram of FIG. 20, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in the spherical aberration diagram of FIG. 20, in the imaging lens system 11 of the present example, occurrence of defocusing at different wavelengths is suppressed.

  21 to 25 are lateral aberration diagrams of the imaging lens system according to Example 3. FIGS. 21 to 25, the horizontal axis represents the relative pupil X coordinate or the relative pupil Y coordinate, and the vertical axis represents the lateral aberration amount. FIG. 21 is a lateral aberration diagram of the imaging lens system when the half field angle is 0 °. FIG. 22 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half angle of view of 5 °. FIG. 23 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half field angle of 5 °. FIG. 24 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half field angle of 8.8 °. FIG. 25 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half field angle of 8.8 °. As shown in FIGS. 21 to 25, color bleeding is suppressed.

Next, Table 11 shows the result of calculating the characteristic value of the imaging lens system 11 of Example 3. The focal length of the entire lens system f in the imaging lens system 11, the focal length _{f 1} of the first lens L1, the focal distance _{f 2} of the second lens L2, the focal length _{f 3} of the third lens L3, the fourth lens Table 11 shows these characteristic values when the focal length of L4 is f ₄ and the refractive indexes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

Table 12 shows the temperature characteristics. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

In Table 12, dn / dT (10 ^ -6) is the refractive index when the temperature of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is changed from 20 ° C. to 40 ° C. ^Is shown on the order of 10 ⁻⁶ .

1 / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the reciprocal of the focal length of the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1, and further multiplying by 10 ⁶ . 1 / f2 * dn / dt × 10 ^ 6, 1 / f3 * dn / dt × 10 ^ 6 and 1 / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

f / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the value obtained by dividing the focal length f of the entire lens system by the focal length f _{1 of} the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1. , And multiplied by 10 ⁶ .

  f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6 and f / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

  L1 to image plane (= LL1) indicate the distance from the first lens L1 to the image plane. Similarly, L2 to the image plane (= LL2), L3 to the image plane (= LL3), and L4 to the image plane (= LL4) are distances from the second lens L2, the third lens L3, and the fourth lens L4 to the image formation plane. Indicates.

  Here, in Table 12, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, the reciprocal of the focal length is multiplied by the amount of change in the refractive index with respect to the lens temperature, and further multiplied by 106. The values f / f1 * dn / dt × 10 ^ 6, f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6 and f / f4 * dn / dt × 10 ^ 6 Define a, b, c, and d, and define the distances LL1, LL2, LL3, and LL4 from the lens to the image plane as e, g, h, and i, respectively.

  The change in the focal length due to the temperature change in the first lens L1 is obtained by a value obtained by multiplying the above 1 / f1 * dn / dt × 10 ^ 6 by L1 to the image plane (= LL1). A change in focal length due to a temperature change in the second lens L2, the third lens L3, and the fourth lens L4 can be obtained by the same calculation.

  That is, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, changes in focal length due to temperature changes are values A multiplied by a and e, and values multiplied by b and g, respectively. A value C obtained by multiplying B, c and h, and a value D obtained by multiplying d and i.

  On the other hand, the change in the focal length due to the thermal expansion of the lens barrel 14 is obtained by multiplying the value obtained by dividing the total length TTL of the lens optical system by the focal length f of the entire lens system and the linear expansion coefficient of the barrel. . In Table 12, the change in focal length due to the thermal expansion of the lens barrel 14 is defined as F.

  The total change in focal length of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is indicated by sum (A to D) in Table 12. sum (A to D) is a value obtained by adding A, B, C, and D. The sum of the focal length changes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 described above cancels the focal length change caused by the thermal expansion of the lens barrel 14. Set. For example, if the absolute value of the value indicated by E + F in Table 12 is equal to or less than a predetermined numerical value, it is considered that they are canceling each other.

  FIG. 26 is a spherical aberration diagram of the imaging lens system of Example 3 at normal temperature, low temperature, and high temperature. In the spherical aberration diagram of FIG. 26, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in FIG. 26, in the imaging lens system 11 of this embodiment, the occurrence of defocusing at different wavelengths is suppressed in any state of normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature. Is done.

  FIG. 27 is a diagram illustrating the MTF defocus characteristic of the imaging lens system at the half field angle of 0 ° in the third embodiment. In FIG. 27, the horizontal axis represents the image plane position change amount, and the vertical axis represents the MTF. As shown in FIG. 27, the change in peak position is suppressed at normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature.

  Thus, according to the imaging lens system of Example 3, the first lens has a characteristic that the refractive index decreases as the temperature increases, and the change in focal length due to the thermal expansion of the lens barrel. By canceling out changes in the focal length, it is possible to suppress a shift between the image plane and the focus position.

(Example 4: imaging lens system)
FIG. 28 is a cross-sectional view of the imaging lens system according to Example 4. In FIG. 28, the imaging lens system 11 includes, in order from the object side to the image side, a first lens L1 having a positive power, a stop STOP, a second lens L2 having a negative power, and a third lens having a positive power. The lens L3 includes a fourth lens L4 having a concave surface on the image side. The imaging plane of the imaging lens system 11 is indicated by IMG. The imaging lens system 11 includes an IR cut filter 12, a cover glass 13, and a lens barrel 14 (not shown).

  The first lens L1 preferably has a characteristic that the refractive index decreases as the temperature increases. As the first lens L1 having the characteristic that the refractive index decreases as the temperature increases, it is preferable to use fluorophosphate glass (or phosphate glass). In Example 1, an example in which a fluorophosphate glass is used for the first lens L1 will be described.

  The first lens L1 is an aspheric lens having positive power. The object side lens surface S1 of the first lens L1 is an aspherical surface and has a convex curved surface portion protruding toward the object side. The image side lens surface S2 has a convex curved surface portion protruding toward the image side.

  The stop STOP adjusts the amount of light passing therethrough. For example, the diaphragm STOP preferably has a plate shape having holes.

  The second lens L2 is a lens having negative power. The object-side lens surface image S4 of the second lens L2 has a convex curved surface portion protruding toward the object side, and the image-side lens surface S5 has a concave curved surface portion recessed toward the object side.

  The third lens L3 is an aspheric lens having positive power. The object side lens surface S6 of the third lens L3 has a concave curved surface portion that is recessed toward the image side, and the image side lens surface S7 has a convex curved surface portion that protrudes toward the image side.

  The fourth lens L4 is an aspheric lens having positive power. The object side lens surface S8 of the fourth lens L4 has a convex curved surface portion protruding toward the object side, and the image side lens surface S9 has a concave curved surface portion recessed toward the object side.

The IR cut filter 12 is a filter that cuts infrared light.
The cover glass 13 is a glass plate for protecting the image sensor. The cover glass 13 is handled as an integral part of the imaging lens system 11 when the imaging lens system 11 is designed. However, the cover glass 13 is not an essential component of the imaging lens system 11.

  The lens barrel 14 (not shown) is a flange that fixes at least the positions of the first lens L1, the stop STOP, the second lens L2, the third lens L3, and the fourth lens L4, as in the first embodiment.

  The imaging lens system 11 has an imaging lens system in which the change in focal length due to thermal expansion of the lens barrel 14 is the first lens L1, the stop STOP, the second lens L2, the third lens L3, and the fourth lens L4. By canceling out the change in the focal length, the deviation between the image plane and the focus position is suppressed. Detailed numerical conditions will be described later.

  Hereinafter, characteristic data of the imaging lens system 11 will be described.

First, Table 13 shows lens data of each lens surface of the imaging lens system 11. The lens data includes the curvature radius of each surface, the surface spacing, the refractive index, the Abbe number, and the amount of change in refractive index for each temperature zone. The amount of change in refractive index for each temperature zone indicates the amount of change in refractive index with temperature in the temperature range shown in Table 13. For example, the column of −40 ° C. to −20 ° C. indicates the amount of change in the refractive index when the temperature is in the range of −40 ° C. to −20 ° C. A surface with “*” indicates that the surface is aspherical.

Table 14 shows the aspheric coefficients for defining the aspherical shape of the aspherical lens surface in the imaging lens system 11 of Example 4. In Table 14, for example, “−6.522528E-03” means “−6.522528 × 10 ⁻³ ”.

  FIG. 29 is a field curvature diagram, a distortion diagram, and a spherical aberration diagram at room temperature of the imaging lens system of Example 4. As shown in FIG. 29, in the imaging lens system 11 of Example 4, the half angle of view is 13 ° and the F value is 1.8. In the field curvature diagram of FIG. 29, the horizontal axis indicates the distance in the optical axis Z direction, and the vertical axis indicates the image height (field angle). In the field curvature diagram of FIG. 29, Sag represents the field curvature in the sagittal plane, and Tan represents the field curvature in the tangential plane. As shown in the field curvature diagram of FIG. 29, according to the imaging lens system 11 of the present embodiment, the field curvature is well corrected. Therefore, the imaging lens system 11 has a high resolution.

  In the distortion diagram of FIG. 29, the horizontal axis indicates the amount of distortion (%) of the image, and the vertical axis indicates the image height (field angle). In the field curvature diagram and the distortion diagram of FIG. 29, the simulation result with a light beam having a wavelength of 558 nm is shown. In the spherical aberration diagram of FIG. 29, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in the spherical aberration diagram of FIG. 29, in the imaging lens system 11 of this example, occurrence of defocusing at different wavelengths is suppressed.

  30 to 25 are lateral aberration diagrams of the imaging lens system according to Example 4. FIGS. 30 to 25, the horizontal axis represents the relative pupil X coordinate or the relative pupil Y coordinate, and the vertical axis represents the lateral aberration amount. FIG. 30 is a lateral aberration diagram of the imaging lens system when the half field angle is 0 °. FIG. 31 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half angle of view of 5 °. FIG. 32 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half angle of view of 5 °. FIG. 33 is a lateral aberration diagram on the tangential surface of the imaging lens system at a half field angle of 8.8 °. FIG. 34 is a lateral aberration diagram on the sagittal surface of the imaging lens system at a half field angle of 8.8 °. As shown in FIGS. 30 to 25, color bleeding is suppressed.

Next, Table 15 shows the result of calculating the characteristic value of the imaging lens system 11 of Example 4. In the imaging lens system 11, the focal length of the entire lens system to f, and the focal length _{f 1} of the first lens L1, the focal distance _{f 2} of the second lens L2, the focal length of the third lens L3 _{f 3,} 4 Table 15 shows these characteristic values when the focal length of the lens L4 is f ₄ and the refractive indexes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

Table 16 shows the temperature characteristics. Various focal lengths were calculated using light rays with a wavelength of 558 nm.

In Table 16, dn / dT (10 ^ -6) is the refractive index when the temperature of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is changed from 20 ° C. to 40 ° C. ^Is shown on the order of 10 ⁻⁶ .

1 / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the reciprocal of the focal length of the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1, and further multiplying by 10 ⁶ . 1 / f2 * dn / dt × 10 ^ 6, 1 / f3 * dn / dt × 10 ^ 6 and 1 / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

f / f1 * dn / dt × 10 ^ 6 is obtained by multiplying the value obtained by dividing the focal length f of the entire lens system by the focal length f _{1 of} the first lens L1 by the amount of change in the refractive index with respect to the temperature of the first lens L1. , And multiplied by 10 ⁶ .

  f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6 and f / f4 * dn / dt × 10 ^ 6 are also the second lens L2, the third lens L3, and the fourth lens. It is the result of having performed the same calculation about L4.

  L1 to image plane (= LL1) indicate the distance from the first lens L1 to the image plane. Similarly, L2 to the image plane (= LL2), L3 to the image plane (= LL3), and L4 to the image plane (= LL4) are distances from the second lens L2, the third lens L3, and the fourth lens L4 to the image formation plane. Indicates.

  Here, in Table 16, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, the reciprocal of the focal length is multiplied by the amount of change in the refractive index with respect to the lens temperature, and further multiplied by 106. The values f / f1 * dn / dt × 10 ^ 6, f / f2 * dn / dt × 10 ^ 6, f / f3 * dn / dt × 10 ^ 6 and f / f4 * dn / dt × 10 ^ 6 Define a, b, c, and d, and define the distances LL1, LL2, LL3, and LL4 from the lens to the image plane as e, g, h, and i, respectively.

  The change in the focal length due to the temperature change in the first lens L1 is obtained by a value obtained by multiplying the above 1 / f1 * dn / dt × 10 ^ 6 by L1 to the image plane (= LL1). A change in focal length due to a temperature change in the second lens L2, the third lens L3, and the fourth lens L4 can be obtained by the same calculation.

  That is, in the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4, changes in focal length due to temperature changes are values A multiplied by a and e, and values multiplied by b and g, respectively. A value C obtained by multiplying B, c and h, and a value D obtained by multiplying d and i.

  On the other hand, the change in the focal length due to the thermal expansion of the lens barrel 14 is obtained by multiplying the value obtained by dividing the total length TTL of the lens optical system by the focal length f of the entire lens system and the linear expansion coefficient of the barrel. . In Table 16, the change in focal length due to the thermal expansion of the lens barrel 14 is defined as F.

  The total change in focal length of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 is indicated by sum (A to D) in Table 16. sum (A to D) is a value obtained by adding A, B, C, and D. The sum of the focal length changes of the first lens L1, the second lens L2, the third lens L3, and the fourth lens L4 described above cancels the focal length change caused by the thermal expansion of the lens barrel 14. Set. For example, when the absolute value of the value indicated by E + F in Table 16 is equal to or less than a predetermined numerical value, it is considered that they are canceling each other.

  FIG. 35 is a spherical aberration diagram of the imaging lens system of Example 4 at normal temperature, low temperature, and high temperature. In the spherical aberration diagram of FIG. 35, the horizontal axis indicates the position where the light beam intersects the optical axis, and the vertical axis indicates the height at the pupil diameter. As shown in FIG. 35, in the imaging lens system 11 of the present embodiment, occurrence of defocusing at different wavelengths is suppressed in any state of normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature. Is done.

  FIG. 36 is a diagram illustrating the MTF defocus characteristic of the imaging lens system at the half field angle of 0 ° in the fourth embodiment. In FIG. 36, the horizontal axis represents the image plane position change amount, and the vertical axis represents the MTF. As shown in FIG. 36, the change in the peak position is suppressed at normal temperature, a low temperature lower than normal temperature, and a high temperature higher than normal temperature.

  As described above, according to the imaging lens system of Example 4, the first lens has the characteristic that the refractive index decreases as the temperature increases, and the change in focal length due to the thermal expansion of the lens barrel. By canceling out changes in the focal length, it is possible to suppress a shift between the image plane and the focus position.

(Example 5: Application example to imaging device)
FIG. 37 is a cross-sectional view of the imaging apparatus according to the fifth embodiment. The imaging device 20 includes an imaging lens system 11 and an imaging element 21. The imaging lens system 11 and the imaging element 21 are accommodated in a housing (not shown). The imaging lens system 11 is the imaging lens system 11 described in the first embodiment.

  The imaging element 21 is an element that converts received light into an electrical signal, and for example, a CCD image sensor or a CMOS image sensor is used. The imaging element 21 is disposed at the imaging position of the imaging lens system 11. The horizontal angle of view is an angle of view corresponding to the horizontal direction of the image sensor 21.

  Note that the present invention is not limited to the above-described embodiments, and can be modified as appropriate without departing from the spirit of the present invention. For example, Example 5 may be applied to Embodiments 2 to 4.

DESCRIPTION OF SYMBOLS 11 Imaging lens system 12 IR cut filter 13 Cover glass 14 Lens barrel 20 Imaging device 21 Imaging element L1, L2, L3, L4 Lens

Claims (5)

In order from the object side, a first lens having a positive power, a second lens having a negative power, a third lens having a positive power, and a fourth lens having a concave surface on the image side,
The first lens has a characteristic that the refractive index decreases as the temperature increases,
When the focal length of the first lens is defined as f1 and the focal length of the entire lens optical system is defined as f, the following formula (1) is satisfied:
1 <f1 / f <1.5 (1)
The focal length of the first lens is f1, the focal length of the second lens is f2, the focal length of the third lens is f3, the focal length of the fourth lens is f4, and the focal length of the entire lens optical system is f,
The refractive index of the d-line of the first lens is n1, the refractive index of the d-line of the second lens is n2, the refractive index of the d-line of the third lens is n3, and the refractive index of the d-line of the fourth lens. n4,
The environmental temperature of the lens optical system is defined as t,
The first lens d-line refractive index change amount is dn1 / dt, the second lens d-line refractive index change amount is dn2 / dt, and the third lens d-line refractive index change amount is dn3 / dt. The amount of change in the refractive index of the d-line of the fourth lens is defined as dn4 / dt,
The distance from the first lens to the image plane is LL1, the distance from the second lens to the image plane is LL2, the distance from the third lens to the image plane is LL3, and the distance from the fourth lens to the image plane is LL4,
An imaging lens system that satisfies the following expression (2) when the total length of the lens optical system is defined as TTL and the linear expansion coefficient of the lens barrel is defined as dL / dt.
| (F / f1 × dn1 / dt × LL1 + f / f2 × dn2 / dt × LL2 + f / f3 × dn3 / dt × LL3 + f / f4 × dn4 / dt × LL4) + TTL / f * dL / dt | <20 × 10 ⁻⁶ (Mm / ° C) (2) The imaging lens system according to claim 1, wherein the following expression (3) is satisfied when the total length of the lens optical system is defined as TTL and the focal length of the lens is defined as f.

1.5 <TTL / f <1.8 (3) The first lens has an Abbe number of 55 or more;
The imaging lens system according to claim 1, wherein the fourth lens has a characteristic that a refractive index decreases as a temperature increases, and an Abbe number is 55 or more.   The imaging lens system according to claim 1 or 2, wherein a refractive index of the third lens is larger than a refractive index of the first lens. An imaging lens system according to any one of claims 1 to 4,
A lens barrel holding the imaging lens system;
A flat cover glass disposed on the object side of the imaging lens system;
An imaging device comprising: an imaging element disposed at a focal position of the imaging lens system.

Images