JP2001004928A - Real image type finder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a finder by which an excellent image is always observed without being influenced by ambient brightness. SOLUTION: This finder 10 is composed of a diaphragm 11 whose aperture diameter is fixed, an objective lens 12 and an ocular block 13 in order from an object side. The lens 12 is composed of one convex lens, and both surfaces 12a and 12b are formed to be aspherical. The block 13 is constituted by forming a field lens surface 15 on the object-side surface of an erect prism 14 and an ocular lens surface 16 on the image-side surface thereof. When fo represents the focal distance of the lens 12, d1 represents a distance on an optical axis between the diaphragm 11 and the lens 12, ED represents the aperture diameter of the diaphragm 11, and Δasp represents the aspherical amount in an optical axis direction of the object-side surface 12a of the lens 12, they satisfy each of conditions; 0.3f0<d1<2.0f0, 3.0<f0/ED<6.0 and -0.06<Δasp<-0.01. Even when it is somewhat dark on the periphery, a very bright image is formed at a fixed position, and the change of the pupil diameter of an observer is restrained to prevent the deterioration of the image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a real image type finder used for a camera or the like.

[0002]

2. Description of the Related Art A real image finder is an objective lens system,
It is composed of an erecting optical system and an eyepiece lens system, and requires at least three optical components. In addition, a field lens is generally arranged on the object side of the erecting optical system in order to efficiently guide the light beam that has passed through the objective lens system to the eyepiece. For this reason, the conventional real-image finder has a high manufacturing cost due to a large number of component parts, and moreover,
There is a drawback that an image becomes dark due to surface reflection due to an increase in the number of interfaces with air.

Japanese Unexamined Patent Publication No. 11-109446 discloses that an erect prism has a field lens surface on the object side surface and an eyepiece lens surface on the image side, and the field lens and the erecting optical system have an eyepiece. A finder integrating a lens system is described. According to this finder, not only the number of parts is reduced, but also the interface with air is reduced due to the reduction in the number of parts, so that the image becomes bright.

[0004]

However, in the above configuration, since the size of the light beam is determined by the pupil diameter of the observer, when the pupil diameter of the observer changes due to the surrounding brightness, unnecessary light rays are generated. The light enters the pupil of the observer, causing flare and degradation of resolution.

The present invention has been made in view of the above circumstances, and has as its object to provide a real image type finder that can always observe a good image without being affected by ambient brightness.

[0006]

In order to achieve the above object, a finder according to the present invention comprises, in order from the object side, a stop having a fixed aperture diameter, an objective lens having an aspherical convex lens on both surfaces, and a field lens. It comprises a lens, an erecting prism having four reflecting surfaces and inverting the inverted object image formed by the objective lens up, down, left and right, and an eyepiece having a positive refractive power. The erecting prism and the eyepiece are integrally formed to form a field lens surface on the object side surface of the erecting prism and an eyepiece surface on the image side. Further, the focal length of the objective lens is f ₀ , the distance between the stop and the objective lens on the optical axis is d1, the aperture diameter of the stop is ED, and the aspherical amount of the object-side surface of the objective lens in the optical axis direction is Δasp. Where 0.3f ₀ <d1 <2.0 3.0 <f ₀ /ED<6.0 -0.04 <Δasp <−0.01

[0007]

According to the present invention, since the field lens is integrated with the erect prism, a strong refracting power cannot be given to the field lens. The exit pupil position of the objective lens needs to be at an appropriate position in order for a light beam entering the field lens, passing through the erecting prism, and exiting the eyepiece to enter a predetermined pupil.

The condition 0.3f ₀ <d1 <2.0f ₀ defines the position of the exit pupil of the objective lens, and is a condition necessary for satisfying telecentricity. If the lower limit of this condition is exceeded, the exit pupil position of the objective lens will be too close, and the divergence of light incident on the field lens will be too strong. If the upper limit of the condition is exceeded, the size of the objective lens becomes too large, and the size of the viewfinder is increased.

The condition 3.0 <f ₀ /ED<6.0 is to ensure sufficient brightness of the objective lens so that a bright image can be observed even when the surroundings are slightly dark. If the lower limit of this condition is exceeded, it becomes difficult to correct spherical aberration to a favorable state when the objective lens is formed of a single lens. If the upper limit of the condition is exceeded, the brightness of the viewfinder cannot be sufficiently secured, and if the surroundings are dark, it is not suitable for observation with the naked eye.

If the telecentricity of the objective lens is satisfied, light rays pass through the periphery of the objective lens as the angle of view increases. On the other hand, in the present invention, the objective lens is constituted by a single lens, and its brightness is relatively bright as a single lens. For this reason, the objective lens must be configured so as to correct the spherical aberration of the bright on-axis light beam at the center portion and to correct the aberration of the off-axis light beam at the peripheral portion.

Therefore, the lens surface of the objective lens is formed to be aspherical on both surfaces, and by satisfying the condition of -0.06 <Δasp <-0.01, spherical aberration is reduced in all of the on-axis light beam and the off-axis light beam. It can be corrected to a good state. If the upper limit of the condition is exceeded, it becomes impossible to correct spherical aberration, and if the lower limit is exceeded, off-axis aberration correction becomes excessive.

In a camera or an electronic still camera employing such a real image type finder, since the optical axis of the taking lens and the optical axis of the finder do not match,
Causes parallax. In order to correct parallax, a field mask that determines the field of view is often arranged at the image forming position of the objective lens. In this case, since the light beam incident on the erecting optical system becomes asymmetric with respect to the optical axis of the erecting optical system,
The erecting optical system has to be configured to be large in accordance with a larger effective diameter. However, the erecting optical system can be made smaller by decentering the optical axis of the erecting optical system with respect to the optical axis of the objective lens system, and arranging it to match the effective beam diameter inside the erecting optical system. It is.

[0013]

FIG. 1 shows the configuration of a real image type finder according to the present invention. The finder 10 includes an aperture 11 having a fixed aperture, an objective lens 12 having a positive refractive power, and an eyepiece block 13 in order from the object side. The objective lens 12 is composed of one convex lens,
Both surfaces 12a and 12b are formed in an aspherical shape. The eyepiece block 13 is formed by integrally forming a field lens and an eyepiece with an erect prism 14, and a field lens surface 15 is formed on an object side surface, and an eyepiece lens surface 16 is formed on an image side surface. ing.

The erecting prism 14 has four reflecting surfaces,
The inverted object image formed by the objective lens 12 is inverted up, down, left, and right. The eyepiece lens surface 16 has a positive refractive power and is formed in an aspherical shape.

The finder 10 has a focal length f _{0 of} the objective lens 12, a distance d 1 between the aperture 11 and the objective lens 12 on the optical axis, an aperture diameter of the aperture 11 ED, and an objective lens 1.
The amount of aspherical surface of the object-side surface 12a in the optical axis direction is Δ
Asp, the following conditions are satisfied: 0.3f ₀ <d1 <2.0f ₀ 3.0 <f ₀ /ED<6.0−0.06<Δasp<−0.01 The aspherical amount represents a distance on the optical axis between the aspherical surface and the paraxial spherical surface.

[0016]

[First Embodiment] The specifications of the viewfinder of the first embodiment are as follows. f ₀ = 7.6 mm ED = 2.0 mm Δasp = −0.026 mm

Table 1 shows lens data of the finder of the first embodiment.

[0018]

[Table 1]

The symbol "*" shown in the column of curvature radius indicates an aspherical surface. The aspherical surface is defined by the following equation: Z = (H ² / R) / [1 + √ ｛1− (1 + K) · (H
² / R ² )｝] + AH ⁴ + BH ⁶ + CH ⁸ + DH ¹⁰ + E
H ¹² is formed.

In the above equation, Z is the amount of displacement in the optical axis direction, H is the height from the optical axis, R is the paraxial radius of curvature, A is the fourth order aspherical coefficient, B is the sixth order aspherical coefficient, C is the 8th order aspheric coefficient, D
Is a 10th order aspherical coefficient, and E is a 12th order aspherical coefficient. It should be noted that the use of a definition expression other than the above as the definition expression of the aspheric surface does not hinder the effects of the present invention.

Table 2 shows the aspherical surface coefficients of the finder of the first embodiment.

[0022]

[Table 2]

The lower limit value defining the distance d1 on the optical axis between the aperture 11 and the objective lens 12 '0.3f ₀ "and the upper limit value" 2.0f ₀ "is, 0.3f ₀ = 0.3 × 7 6.6 = 2.28 2.0f ₀ = 2.0 × 7.6 = 15.2. The value of the interval d1 is “3.74 m” from Table 1.
m ”, which satisfies the condition 0.3f ₀ <d1 <2.0f ₀ .

The aspherical amount Δasp of the object-side surface 12a of the objective lens 12 is “−0.026 mm”, which satisfies the condition −0.06 <Δasp <−0.01.

Further, the characteristic value of the present invention, "f ₀ / E
D value of "is a _{f 0 / ED = 7.6 / 2.0} = 3.8, meets the conditions 3.0 <f ₀ /ED<6.0.

FIG. 2 shows aberration diagrams of the finder of the first embodiment.
Shown in In the figure, (A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion, respectively. Symbols S and T in the astigmatism diagram of (B) represent spherical defects, respectively. It represents aberrations with respect to the image plane and the meridional image plane.

[Second Embodiment] The viewfinder of the second embodiment has the same lens configuration as the first embodiment. The specifications of the finder of the second embodiment are as follows. f ₀ = 7.67 mm ED = 2.0 mm Δasp = −0.046 mm

Tables 3 and 4 show lens data and aspheric coefficients of the finder of the second embodiment. The aspherical surface is defined in the same manner as in the first embodiment.

[0029]

[Table 3]

[0030]

[Table 4]

The lower limit value "0.3f ₀ " that defines the interval d1
And the upper limit value “2.0f ₀ ” is 0.3f ₀ = 0.3 × 7.67 = 2.301 2.0f ₀ = 2.0 × 7.67 = 15.34. The value of the interval d1 is “4.54 m” from Table 3.
m ”, which satisfies the condition 0.3f ₀ <d1 <2.0f ₀ .

The aspherical amount Δasp of the object-side surface 12a of the objective lens 12 is “−0.046 mm”, which satisfies the condition −0.06 <Δasp <−0.01.

The value of the characteristic value “f ₀ / ED” is f ₀ /ED=7.67/2.0=3.835, and the condition 3.0 <f ₀ /ED<6.0 is satisfied. Meets

FIG. 3 shows aberration diagrams of the finder of the second embodiment.
Shown in (A), (B), (C) in the aberration diagram
Spherical aberration, astigmatism, and distortion are respectively represented in the same manner as in the first embodiment, and symbols S,
T represents aberrations with respect to the spherical and meridional image planes, respectively.

[0035]

As described above, according to the real image type finder of the present invention, by adjusting the distance between the stop and the objective lens according to the focal length of the objective lens, the light beam emitted from the eyepiece can be adjusted to a predetermined value. It is possible to make the light enter the pupil at the position. In addition, by forming the lens surface of the objective lens on both surfaces into an aspherical shape and adjusting the amount of aspherical surface of the object-side lens surface, the objective lens was constituted by a single lens, and the brightness was sufficiently increased. Even in this case, the spherical aberration can be corrected to a favorable state on all the axes, both off-axis and on-axis. As a result, a sufficiently bright image is formed at a fixed position without being affected by the surrounding brightness, so that a change in the pupil diameter of the observer can be suppressed, and a good image is always observed. be able to.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram of a finder of the present invention.

FIG. 2 is an aberration diagram of a finder according to the first embodiment;
(A) represents spherical aberration, (B) represents astigmatism, and (C) represents distortion.

FIG. 3 is an aberration diagram of a finder according to a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Viewfinder 11 Aperture 12 Objective lens 13 Eyepiece block 14 Erect prism 15 Field lens surface 16 Eyepiece lens surface

Claims (1)

[Claims] 1. An object having an aperture having a fixed aperture diameter, an objective lens formed of an aspherical convex lens on both sides, a field lens, and four reflecting surfaces in order from the object side. An upright prism for inverting the inverted object image vertically and horizontally, and an eyepiece having a positive refractive power, and the field lens, the upright prism, and the eyepiece are integrally formed, A field lens surface is formed on the object-side surface of the prism, an eyepiece lens surface is formed on the image-side surface, and the focal length of the objective lens is f ₀ , and the distance between the stop and the objective lens on the optical axis is d1. And the aperture diameter of the diaphragm is E
D, when the amount of aspherical surface in the optical axis direction of the object-side surface of the objective lens is Δasp, 0.3f ₀ <d1 <2.0f ₀ 3.0 <f ₀ /ED<6.0-0. A real image finder, wherein each condition of 06 <Δasp <−0.01 is satisfied.