TWI664469B - Fixed-focus lens - Google Patents

Abstract

The fixed-focus lens includes an aperture, a first lens group, and a second lens group. The first lens group is disposed between a first side and the diaphragm. The first lens group includes a first lens and a second lens. The first lens is disposed between the first side and the second lens, the first lens and the second lens are two lenses with refractive power closest to the first side, and the first lens and the second lens are plastic lenses. The second lens group is disposed between a second side and the aperture. The second lens group includes a first combined lens and a second combined lens. The diaphragm separates the first lens group and the second lens group.

Description

Fixed focus lens

The present invention relates to a lens, and particularly to a fixed focus lens.

Compared with a TV, a projector can project a large-size image without using too much space, so the projector occupies a certain proportion in the market. The micro projector has the advantages of light, thin and short, and has gradually become the mainstream in the market.

The micro projector is usually based on the lens and light valve (such as digital micro-mirror device (DMD), liquid-crystal-on-silicon panel, LCOS panel) and other optical components ) There must be enough space to place the optical lens, so in the optical design, the back focal length (BFL) of the lens of the micro projector is usually greater than the effective focal length (EFL) of the lens, so the lens architecture Usually retrofocus. The above design methods are often accompanied by large distortion aberrations, and the application of projectors often requires a large aperture and low dispersion design, so micro projectors usually use multiple aspheric lenses and cemented lenses to deal with distortion images Poor problem.

In order to effectively eliminate or reduce distortion aberrations, conventional techniques usually use aspherical lenses for the first lens and the last lens of the lens. However, if the first aspherical lens is a molded glass aspherical lens, there are problems of manufacturability and high cost. If the last aspheric lens is a plastic lens, due to the high brightness of the projector, the plastic lens will be easily deformed under high-temperature use conditions, causing the lens to shift focus (ie, thermal drift), The projection quality is reduced; but if the last aspheric lens is a molded glass aspheric lens, there is a problem of high cost.

Therefore, how to solve the problems arising from the optical design and use requirements of the above-mentioned micro projectors has become an urgent issue in the industry.

The invention provides a fixed-focus lens, which can effectively reduce distortion aberration and shift focus, and can have lower cost.

A fixed focus lens according to an embodiment of the present invention includes an aperture and two lens groups. One of the two lens groups is located between one side of the fixed focus lens and the aperture, and this one lens group includes at least two lenses. One of the two lenses is located between the side of the fixed focus lens and the other lens. The two lenses are two diopter lenses closest to the side of the fixed focus lens, and the two lenses are plastic lens. The fixed focus lens satisfies the following conditions: -5 <R1/RL <-1, and -10 <R2/RL <-5 where R1 is the effective focal length of the lens closer to the fixed lens side of the two lenses, R2 Is the effective focal length of the other of the two lenses, and RL is the effective focal length of the fixed focus lens. The other lens group of the two lens groups is provided between the other side of the fixed focus lens and the aperture, and the other lens group includes two combined lenses. One of the two combined lenses is composed of the other two negative refractive power lenses and the positive refractive power lenses, and the Abbe number difference of the other two lenses falls within the range of 40 to 50. The other of the two combined lenses is composed of two more positive dioptric power lenses and a negative dioptric power lens, and the Abbe number difference of the other two lenses falls within the range of 25 to 35. The aperture divides the two lens groups, and the total number of dioptric lenses of the two lens groups falls within the range of 6 and 10.

A fixed focus lens according to an embodiment of the present invention includes two lens groups and an aperture. One of the two lens groups has negative refractive power, and this one lens group is disposed between an enlarged side and a reduced side. This lens group includes two plastic aspherical lenses in order from the magnification side to the reduction side. The curvature radius of the surface of the plastic aspherical lens closer to the magnification side facing the magnification side is negative , And the radius of curvature of the surface facing the reduced side is a positive value. The other of the two lens groups has positive refractive power and is provided between the aforementioned one lens group and the reduced side. The other lens group includes two glass doublets and a glass spherical lens in order from the magnification side to the reduction side. The aperture is set between the two lens groups. Among them, the refractive power of at least one of the two glass double cemented lenses is positive. In a fixed focus lens, the total number of lenses with diopters is less than 11, and when the fixed focus lens is in focus, the distance between the two lens groups does not change.

Based on the above, in a fixed-focus lens according to an embodiment of the present invention, a lens group of two combined lenses, since the two diopter lenses closest to the fixed-focus lens are plastic lenses, they are both manufacturable. And low cost. In addition, the other lens group of the two combined lenses includes four lenses with diopters, and the numerical design of Abbe number can be used to solve chromatic aberration. Furthermore, the fixed-focus lens in an embodiment of the present invention includes two plastic aspheric lenses in order from the zoom-in side to the zoom-out side, so it has both manufacturability, low cost, and can avoid distortion aberration and chromatic aberration. advantage. In addition, since the lens group near the reduction side includes two glass doublets and a glass spherical lens, the problem of lens shifting can be reduced.

In order to make the above-mentioned features and advantages of the present invention more obvious and understandable, the embodiments are specifically described below in conjunction with the accompanying drawings for detailed description as follows.

FIG. 1 is a schematic cross-sectional view of a fixed focus lens according to an embodiment of the invention. Please refer to FIG. 1. In this embodiment, the fixed focus lens 100 includes an aperture stop (Aperture stop) 150, a lens group 110 and a lens group 120.

In this example, the lens group 110 (which may be referred to as a first lens group) includes a lens L1 (which may be referred to as a first lens) and a lens L2 (which may be referred to as a second lens). The lens group 120 (which may be referred to as a second lens group) includes a compound lens 121 (which may be referred to as a first compound lens) and a compound lens 122 (which may be referred to as a second compound lens).

In this example, the lens L1 and the lens L2 are plastic lenses. The compound lens 121 is composed of a lens L4 (which may be called a third lens) of negative power and a lens L5 (which may be called a fourth lens) of positive power. The combined lens 122 is composed of a positive power lens L6 (may be referred to as a fifth lens) and a negative power lens L7 (may be referred to as a sixth lens). Moreover, in this example, the total number of lenses of the lens group 110 and the lens group 120 with diopters falls within the range of 6 and 10.

In this embodiment, the combined lens 121 and the combined lens 122 may be cemented lenses, but the invention is not limited thereto. The lenses L4 and L5 in the combined lens 121 and the lenses L6 and L7 in the combined lens 122 may also be used It is fixed by other means (for example, a fixed frame) and connected along the optical axis.

In the present embodiment, the lens L1 has negative refractive power, and the lens L2 has negative refractive power.

In this example, the lens group 110 is disposed between the side 130 and the aperture 150. The side 130 may be referred to as the first side or the magnification side, which is the light output side of the fixed focus lens 100. The lens L1 is provided between the side 130 and the lens L2. The lens L1 and the lens L2 are the two dioptric lenses closest to the side 130.

In this embodiment, since the lenses L1 and L2 can be plastic aspheric lenses, distortion aberration and chromatic aberration can be avoided, but the refractive index gradient Dn/Dt (that is, the refractive index change per unit temperature rise of the plastic lens) Quantity), therefore, the lens group 120 of the fixed focus lens 100 is provided with two sets of combined lenses, each with a positive diopter lens, and a specific Abbe number material is used to offset the refraction of the lens group 110 The lens is out of focus due to thermal drift caused by the gradient.

In detail, the lens group 120 is disposed between the side 140 and the aperture 150. The side 140 may be referred to as the second side or the reduced side, which is the light input side of the fixed focus lens 100. The Abbe number of lens L5 is greater than the Abbe number of lens L4, and the difference between the Abbe number of lens L4 and lens L5 falls within the range of 40 to 50. The Abbe number of lens L6 is greater than the Abbe number of lens L7, and the difference between the Abbe number of lens L6 and lens L7 falls within the range of 25 to 35.

It is worth mentioning that in this example, the refractive index gradient Dn/Dt of the positive refractive power lens L5 in the combined lens 121 and the positive refractive power lens L6 in the combined lens 122 needs to be negative.

In this example, the aperture 150 separates the lens group 110 and the lens group 120. The fixed focus lens 100 satisfies the following conditions: -5 <R1/RL <-1, and -10 <R2/RL <-5 where R1 is the effective focal length (EFL) of lens L1 and R2 is the lens L2 Effective focal length, and RL is the effective focal length of the fixed focus lens 100.

In this embodiment, the effective focal lengths of the lens group 110 and the lens group 120 are -87.598 mm and 11.058 mm, respectively, wherein the effective focal length R1 of the lens L1 is -14.992 mm, and the effective focal length R2 of the lens L2 is -49.995 mm, and The effective focal length RL of the fixed focus lens 100 is 6.256 mm. Therefore, the ratio R1/RL of the effective focal length of the lens L1 to the effective focal length of the fixed focus lens 100 is -2.40, and the ratio R2/RL of the effective focal length of the lens L2 to the effective focal length of the fixed focus lens 100 is -7.99.

In this example, the actual design of the aforementioned components can be seen in Table 1 below.

Table I element surface Radius of curvature (mm) Interval (mm) Refractive index (nd) Abbe number (vd) Lens L1 S1 -11.805 4.108 1.525 56.282 S2 26.821 3.341 Lens L2 S3 -5.907 3.501 1.525 56.282 S4 -9.170 0.100 Lens L3 S5 12.342 2.601 1.847 23.785 S6 29.544 5.153 Aperture 150 S7 Unlimited 3.395 Lens L4 S8 -62.649 0.800 1.834 37.229 Lens L5 S9 12.634 3.176 1.497 81.608 S10 -12.634 0.100 Lens L6 S11 66.503 3.896 1.618 63.406 Lens L7 S12 -7.690 0.800 1.850 32.308 S13 -20.398 0.119 Lens L8 S14 24.217 2.782 1.773 49.613 S15 -24.217 1.229 Penetrating smooth image device 160 S16 Unlimited 2.000 1.509 56.475 S17 Unlimited 0.800 珜鏡170 S18 Unlimited 11.200 1.723 37.956 S19 Unlimited 0.700 Cover glass 180 S20 Unlimited 1.100 1.510 62.911 S21 Unlimited 0.303 Digital Micromirror Element 190 S22 Unlimited

Please also refer to Figure 1 and Table 1. Specifically, in the fixed-focus lens 100 in this embodiment, the lens L1 from the side 130 to the side 140 is the surface S1 and the surface S2 in sequence, and the lens L2 from the side 130 to the side 140 is the surface S3 and the surface S4 in sequence. By analogy, the surface corresponding to each element will not be repeated. Among them, the display surfaces of the aperture 150 and the Digital Micromirror Device (DMD) 190 are respectively represented by the surface S7 and the surface S22, and the radius of curvature thereof is infinite (that is, the plane perpendicular to the optical axis); and the lens L4 The lens L5, the lens L6, and the lens L7 have a common surface S9 and a surface S12, respectively, meaning that the lens L4 and the lens L5, the lens L6, and the lens L7 are two lenses connected along the optical axis, or respectively A pair of cemented lenses.

In addition, the interval in Table 1 is the interval between the next surface from the side 130 to the side 140 of the surface, for example, the thickness of the lens L1 is 4.108 mm, the distance between the lens L1 and the lens L2 is 3.341 mm, and the thickness of the lens L2 It is 3.501 mm, the distance between lens L2 and lens L3 is 0.100 mm, and so on without repeating it.

Among them, the radius of curvature of the surface S1 of the lens L1 is negative, and the radius of curvature of the surface S2 of the lens L1 is positive, therefore, the lens L1 is a biconcave lens, where the radius of curvature is exactly the center of the surface shifted toward the side 130, As with surface S2, in addition, a negative radius of curvature indicates that the center of the surface is offset toward side 140, as with surface S1. The radius of curvature of the surface S3 of the lens L2 is negative, the radius of curvature of the surface S3 of the lens L2 is negative, and the absolute value of the radius of curvature of the surface S3 of the lens L2 is smaller than the absolute value of the radius of curvature of the surface S4 of the lens L2. Lens L2 is a negative meniscus lens with a concave surface facing side 130; and so on, lens L3 is a positive meniscus lens with a concave surface facing side 140; lens L4 is a biconcave lens; lens L5 is a biconvex lens; lens L6 is a biconvex lens; The lens L7 is a negative meniscus lens with a concave facing side 130; the lens L8 is a biconvex lens.

It is worth mentioning that, in this embodiment, the total length of the projection system (TTL, the distance from the surface S1 of the lens L1 to the surface S22 of the digital micromirror element 190) is less than 60 mm, and the total length of the fixed focus lens 100 (the length of the lens L1) The distance from the surface S1 to the surface S15 of the lens L8) is less than 40 mm. For example, in the embodiment of Table 1, the total length of the projection system is 51.2 mm (the total value of the interval from surface S1 to surface S21 in Table 1), and the total length of the fixed focus lens 100 is 33.872 mm (the surface of S1 to S14 in Table 1) The total interval value).

In addition, in this embodiment, the fixed focus lens 100 is a whole group of focusing, so the relative distance between the lenses with diopters in the fixed focus lens 100 does not change when focusing, and the diopters in the fixed focus lens 100 have diopters The lens has the same distance change value as the distance from the surface S22 of the digital micromirror element 190 when focusing.

Furthermore, in this embodiment, a digital micromirror element 190 with a smaller size can be selected, and a transmissive smooth picture (TSP) 160 is used to replace the existing products with higher specifications, so it can be cut costs. The transmissive smooth image device 160 is a flat plate, and the pixel points of the projected image can be flicked to a small extent through vibration to prevent the granularity of the pixel from being observed by the user.

Table 2 below lists the conic coefficient K and the aspheric coefficients of each order for the surfaces S1 and S2 of the lens L1 and the surfaces S3 and S4 of the lens L2. The aspheric polynomial can be expressed by the following formula: (1) where x is the offset (sag) in the direction of the optical axis I, c'is the reciprocal of the radius of the close spherical surface (Osculating Sphere), that is, the reciprocal of the radius of curvature near the optical axis, and K is the quadric surface The coefficient, y is the height of the aspheric surface, which is the height from the center of the lens to the edge of the lens. AF represents the aspheric coefficients of each order of the aspheric polynomial.

Table II Aspheric coefficient Surface S1 Surface S2 Surface S3 Surface S4 K -1.545 31.932 -0.383 -0.386 A 2.44E-03 2.99E-03 1.34E-03 4.61E-04 B -8.35E-05 4.07E-05 -9.88E-05 -4.56E-05 C 2.41E-06 -1.71E-05 8.90E-06 5.12E-06 D -4.64E-08 1.76E-06 -4.84E-07 -3.34E-07 E 5.11E-10 -8.30E-08 1.15E-08 1.12E-08 F -2.39E-12 1.52E-09 -3.06E-11 -1.47E-10

Based on the above, due to the fixed focus lens of an embodiment of the present invention, the two lenses in the lens group near the magnification side are plastic lenses, so they have the advantages of manufacturability and low cost; moreover, the two plastic lenses can be The plastic aspheric lens has the advantages of manufacturability, low cost, and effective reduction of distortion and chromatic aberration. In addition, the numerical design of the Abbe number of the two combined lenses near the reduction side lens group can be used to solve the chromatic aberration problem.

In addition, the fixed focus lens 100 according to an embodiment of the present invention includes a lens group 110, a lens group 120, and an aperture 150.

In this example, the lens group 110 includes a plastic aspherical lens 111 (which may be referred to as a first plastic aspherical lens) and a plastic aspherical lens 112 in order from the side 130 to the side 140 in a plurality of diopter lenses (Can be called the second plastic aspheric lens). The lens group 120 includes, from the side 130 to the side 140, a glass double cemented lens 123 (which may be called a first glass double cemented lens), a glass double cemented lens 124 (which may be called a second glass double cemented lens), and a glass spherical lens in order 125.

In this example, the lens group 110 (which may be referred to as the first lens group) has negative refractive power, wherein the radius of curvature of the surface S1 of the plastic aspheric lens 111 facing the side 130 is negative, and the radius of curvature of the surface S2 facing the side 140 Is a positive value. The lens group 120 has a positive refractive power, and at least one of the glass double cemented lens 123 and the glass double cemented lens 124 has a positive refractive power.

In addition, in the fixed-focus lens 100, the lens groups 110 and 120 in the above-mentioned related embodiments, the total number of lenses of the dioptric lenses falls within the range of 6 and 8.

In the fixed-focus lens 100 in the above-mentioned related embodiments, the total number of lenses of the dioptric lenses falls within the range of 6 and 8.

The number of aspheric lenses of the fixed focus lens 100 in the above-mentioned related embodiments is 2 or less.

In the fixed-focus lens 100 in the above related embodiment, the lens closest to the side 140 in the lens group 120 is a glass spherical lens (for example, the glass spherical lens 125 in FIG. 1 ).

The number of plastic lenses of the fixed focus lens 100 in the above-mentioned related embodiments is less than or equal to 2.

In this example, the lens group 110 is provided between the side 130 (which may be referred to as an enlarged side) and the side 140 (which may be referred to as a reduced side). The lens group 120 is provided between the lens group 110 and the side 140. The aperture 150 is provided between the lens group 110 and the lens group 120. In the fixed focus lens 100, the total number of lenses with diopters is less than 11, and when the fixed focus lens 100 is in focus, the distance between the lens group 110 and the lens group 120 does not change.

Based on the above, due to the fixed focus lens of an embodiment of the present invention, the two lenses in the lens group near the magnification side are plastic aspheric lenses, so they are both manufacturable, low cost, and can effectively reduce distortion and chromatic aberration. advantage. In addition, among the lens groups near the reduction side, the lens closest to the reduction side is a glass spherical lens, which is less likely to be deformed under a high-temperature use environment, so the problem of lens shifting can be alleviated.

The fixed-focus lens of another embodiment of the present invention will be described below. The actual design of each component can be seen in Table 3 below.

Table 3 element surface Radius of curvature (mm) Interval (mm) Refractive index (nd) Abbe number (vd) Lens L1 S1 -48.873 5.846 1.525 56.282 S2 7.187 5.609 Lens L2 S3 -7.981 3.874 1.497 81.546 S4 -13.616 0.205 Lens L3 S5 29.904 2.396 1.850 32.270 S6 -29.904 8.759 Aperture 150 S7 Unlimited 4.789 Lens L4 S8 75.214 0.800 1.847 23.778 Lens L5 S9 12.059 3.330 1.497 81.546 S10 -21.801 0.200 Lens L6 S11 51.980 3.843 1.618 63.334 Lens L7 S12 -9.446 0.800 1.847 23.778 S13 -31.792 0.200 Lens L8 S14 23.906 2.680 1.847 23.778 S15 -33.577 0.564 Penetrating smooth image device 160 S16 Unlimited 2.000 1.509 56.470 S17 Unlimited 0.800 珜鏡170 S18 Unlimited 11.200 1.723 37.956 S19 Unlimited 0.700 Cover glass 180 S20 Unlimited 1.100 1.510 62.900 S21 Unlimited 0.303 Digital Micromirror Element 190 S22 Unlimited

2 is a schematic cross-sectional view of a fixed focus lens according to another embodiment of the invention. Also refer to Figure 2 and Table 3. In this example, lens L1 is a biconcave lens; lens L2 is a negative meniscus lens with concave side 130; lens L3 is a biconvex lens; lens L4 is a negative meniscus lens with concave side 140; lens L5 is a biconvex lens ; Lens L6 is a biconvex lens; lens L7 is a negative meniscus lens with a concave facing side 130; lens L8 is a biconvex lens.

It is worth mentioning that in the embodiment of Table 3, the total length of the projection system is 59.998 mm (the total value of the interval from surface S1 to surface S21 in Table 3), and the total length of the fixed focus lens 100 is 43.331 mm (Table 3 The total value of the interval from surface S1 to surface S14).

Table 4 below lists the quadric coefficient K and the aspheric coefficients of each order for the surfaces S1 and S2 of the lens L1 and the surfaces S3 and S4 of the lens L2, where the aspheric surface equation can refer to the above formula (1).

Table 4 Aspheric coefficient Surface S1 Surface S2 K 0.000 0.503 A 4.64E-04 4.89E-04 B -6.51E-06 3.23E-06 C 7.48E-08 -1.10E-06 D -5.02E-10 4.81E-08 E 1.46E-12 -8.57E-10

In addition, in the fixed focus lens 200 of this embodiment, in the lens group 110 near the side 130, only the lens L1 is a plastic aspheric lens, and the lens L2, lens L3, lens L4, lens L5, lens L6, lens L7 and The lenses L8 are all glass spherical lenses.

In summary, in the fixed-focus lens of the embodiment of the present invention, the two lenses in the lens group near the magnification side are plastic lenses, so they have the advantages of manufacturability and low cost; moreover, the two plastic lenses can It is a plastic aspheric lens, so it has the advantages of manufacturability, low cost and effective reduction of distortion and chromatic aberration. In addition, the numerical design of the Abbe number of the two combined lenses near the reduction side lens group can be used to solve the chromatic aberration problem. In a fixed-focus lens according to an embodiment of the present invention, the two lenses in the lens group near the magnification side are plastic aspheric lenses, so they have the advantages of manufacturability, low cost, and effective reduction of distortion and chromatic aberration. In addition, among the lens groups near the reduction side, the lens closest to the reduction side is a glass spherical lens, which is less likely to be deformed under a high-temperature use environment, so the problem of lens shifting can be alleviated.

Although the present invention has been disclosed as above with examples, it is not intended to limit the present invention. Any person with ordinary knowledge in the technical field can make some changes and modifications without departing from the spirit and scope of the present invention. The scope of protection of the present invention shall be subject to the scope defined in the appended patent application.

100, 200‧‧‧ fixed focus lens

110, 120‧‧‧ lens group

111、112‧‧‧Plastic aspheric lens

121, 122‧‧‧Combination lens

123、124‧‧‧glass double cemented lens

125‧‧‧glass spherical lens

130, 140‧‧‧

150‧‧‧ Aperture

160‧‧‧penetrating smooth image device

170‧‧‧珜鏡

180‧‧‧ Cover glass

190‧‧‧Digital micromirror device

L1, L2, L3, L4, L5, L6, L7, L8

S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S13, S14, S15, S16, S17, S18, S19, S20, S21, S22

FIG. 1 is a schematic cross-sectional view of a fixed focus lens according to an embodiment of the invention. 2 is a schematic cross-sectional view of a fixed focus lens according to another embodiment of the invention.

Claims (10)

A fixed-focus lens includes: an aperture; a first lens group disposed between a first side and the aperture; the first lens group includes: a first lens; and a second lens, the first lens is provided Between the first side and the second lens, the first lens and the second lens are two dioptric lenses closest to the first side, and the first lens and the second lens are plastic lenses And a second lens group disposed between a second side and the aperture, the second lens group includes: a first combined lens, a third lens with negative refractive power, and a fourth lens combination with positive refractive power The difference between the Abbe number of the third lens and the fourth lens is in the range of 40 to 50; and a second combination lens is composed of a fifth lens with positive refractive power and a sixth lens with negative refractive power. As a result, the Abbe number difference between the fifth lens and the sixth lens falls within a range of 25 to 35; the diaphragm separates the first lens group and the second lens group, the first lens group and the first lens group The total number of diopter lenses in the two lens group falls within the range of 6 and 10; the fixed focus lens The following conditions are sufficient: -5 <R1 / RL <-1, and -10 <R2 / RL <-5, where R1 is the effective focal length of the first lens, R2 is the effective focal length of the second lens, and RL is the Effective focal length of a fixed focus lens. The fixed-focus lens according to item 1 of the scope of the patent application, wherein the first side and the second side are an enlarged side and a reduced side, respectively, and when the fixed-focus lens is in focus, the first lens group and the second The distance between the lens groups does not change. The fixed focus lens according to item 2 of the scope of patent application, wherein the first combination lens and the second combination lens are cemented lenses. The fixed-focus lens according to item 3 of the scope of patent application, wherein the first lens has a negative refractive power, and the second lens has a negative refractive power. A fixed-focus lens includes: a first lens group having a negative refractive power, disposed between an enlarged side and a reduced side, and the first lens group has a plurality of lenses with refractive power from the enlarged side to the reduced side In this order, a first plastic aspheric lens and a second plastic aspheric lens are included in order. The curvature radius of the surface of the first plastic aspheric lens facing the magnifying side is negative, and the curvature of the surface facing the reducing side is The radius is a positive value; a second lens group having a positive refractive power is disposed between the first lens group and the reduced side, and the second lens group includes a first glass double cemented lens and a second glass double cemented lens And a glass spherical lens, wherein the Abbe number difference of two lenses in the first glass double cemented lens falls within a range of 40 to 50; and an aperture is provided in the first lens group and the second lens group Among them, the diopter of at least one of the first glass double cemented lens and the second glass double cemented lens is positive; in the fixed focus lens, the total number of lenses having diopter is less than 11, and the fixed focus lens is focusing , A distance between the first lens group and the second lens group is constant. The fixed-focus lens according to item 2 or item 5 of the scope of patent application, wherein the total number of lenses of the first lens group and the second lens group with refractive lenses falls within the range of 6 and 8. The fixed-focus lens according to item 6 of the scope of patent application, wherein the total number of lenses of the fixed-focus lens with refractive lenses falls within the range of 6 and 8. The fixed-focus lens according to item 7 of the scope of the patent application, wherein the number of aspheric lenses of the fixed-focus lens is less than or equal to two, and the number of plastic lenses of the fixed-focus lens is less than or equal to two. The fixed-focus lens according to item 8 of the scope of patent application, wherein the lens closest to the reduction side in the second lens group is a glass spherical lens. The fixed-focus lens according to item 9 of the scope of the patent application, wherein the fixed-focus lens includes eight lenses, and the diopters of the eight lenses are arranged from the magnification side to the reduction side, and are negative, negative, positive, negative, positive, Positive, negative, positive.