JPH0943511A - Retro focus lens - Google Patents

Abstract

(57) Abstract: To obtain a retrofocus type lens suitable as a projection lens for a color liquid crystal projector having a long back focus. SOLUTION: The first lens group having a negative refractive power and the stop are arranged in order from the first conjugate point having a longer distance, and the second lens group having a positive refractive power is divided into two.
The first lens group has two lens groups, the first lens group 1a and the first lens group 1b, and the first lens group 1b is small on the side of the second conjugate point having a short distance. A first b1 lens having a convex radius on both lens surfaces, a negative 1b2 lens having a concave surface on the second conjugate point side, a 1b3 lens having a concave surface on both lens surfaces, and a small radius of curvature on the first conjugate point side. The first lens includes a first b4 lens whose both lens surfaces have a convex surface.
The air distance Dab between the a group and the 1b group, the distance from the final lens surface of the second group to the second conjugate point, bf, the focal length f of the entire system, etc. are set appropriately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus type lens, which is suitable for a color liquid crystal projection television in which a plurality of images having different color information are combined by a combining mirror and then enlarged and projected on a screen surface. The present invention relates to a retrofocus lens having a long back focus.

[0002]

2. Description of the Related Art Conventionally, various color liquid crystal projections have been proposed in which images displayed on a plurality of color liquid crystals (liquid crystal light valves) are optically superposed and projected on a screen surface by a projection lens. There is.

FIG. 11 is a schematic view of a main part of a color liquid crystal projection television which projects an image formed on a general color liquid crystal onto a screen surface (not shown).

In the figure, reference numeral 1 denotes a white light source which emits a collimated light beam. 2a, 2b, 2c are for red, green,
It is a liquid crystal display element for blue and displays a projected image. 3a and 3b are reflection mirrors, and 4 is a red reflection dichroic mirror for illuminating the red liquid crystal display element 2a. A green reflection dichroic mirror 5 illuminates the liquid crystal display element 2b for green. 5a is a green reflection dichroic mirror.

The blue liquid crystal display element 2c is illuminated with blue light which has passed through the red reflection dichroic mirror 4 and the green reflection dichroic mirror 5. 6 is a blue transmission dichroic mirror. 7 is a projection lens.

In the figure, white light from the white light source 1 is separated into red, green and blue color lights by the dichroic mirrors (4, 5), and the red, green and blue color lights respectively emit red,
The liquid crystal display elements (2a, 2b, 2c) for green and blue are illuminated, and the liquid crystal display elements (2a, 2c) based on the respective color lights are illuminated.
The images b, 2c) are projected on the screen surface (not shown) by the projection lens 7 to obtain a color image.

In the projection lens having such a structure, it is necessary to dispose various optical members such as a reflection mirror and a dichroic mirror between the final lens surface and the liquid crystal display element (between the back focus). Focus type lenses are often used.

Such a retrofocus type lens for a liquid crystal projector is disclosed in, for example, Japanese Unexamined Patent Publication No. 5-113534.
Japanese Patent Laid-Open Publication No. 5-188287.

[0009]

Generally, in a retrofocus type lens, a lens unit having a negative refractive power is arranged on the object side (the conjugate point side having a long distance, the first conjugate point side), and the image plane side (the distance is short). It has a lens configuration in which lens groups having a positive refractive power are arranged on the conjugate point side and the second conjugate point side). Therefore, there is a feature that a relatively long back focus can be obtained.

However, there is a problem that asymmetrical aberrations such as distortion and chromatic aberration of magnification often occur due to the asymmetrical lens structure.

For example, if the back focus is made longer with respect to the screen size (effective screen), the asymmetry of the lens structure is increased, and the optical performance is significantly deteriorated.

Further, in order to secure sufficient illuminance on the screen surface, a stack for projecting images formed on the liquid crystal display surface by superimposing the respective images on the screen by a plurality of lenses whose optical axes are shifted from each other. In the method, it is necessary to minimize the amount of change in distortion aberration in the entire screen area in order to superimpose images by a plurality of projection lenses. Also, in a retrofocus lens having a particularly large angle of view, lateral chromatic aberration increases near the periphery of the screen. Generally, unless this amount is reduced to about half the size of each pixel on the liquid crystal display surface, a phenomenon such as unsightly color bleeding in a projected image becomes noticeable.

Further, since the liquid crystal display element has an angular characteristic, it is necessary to make the incident angle of the off-axis light beam incident on the display surface small. For example, a projection lens for a liquid crystal projector needs to be composed of an image-side telecentric lens so that an off-axis light beam is incident substantially perpendicularly to a display surface. Therefore, the asymmetry of the lens configuration of the projection lens is further increased, and it is very difficult to obtain good optical performance.

According to the present invention, a predetermined back focus can be easily obtained by appropriately setting the lens configuration, and the image side has a high optical performance with a large aperture in which the asymmetric aberration which is substantially telecentric is well corrected. An object of the present invention is to provide a retrofocus type lens suitable as a projection lens for a color liquid crystal projection TV.

[0015]

The retrofocus type lens of the present invention comprises a first lens unit having a negative refractive power, an aperture stop, and a second lens unit having a positive refractive power in order from a first conjugate point having a longer distance. Two
The first lens group has two lens groups, the first lens group 1a and the first lens group 1b, and the first lens group 1b is small on the side of the second conjugate point having a short distance. A first b1 lens having a convex radius on both lens surfaces, a negative 1b2 lens having a concave surface on the second conjugate point side, a 1b3 lens having a concave surface on both lens surfaces, and a small radius of curvature on the first conjugate point side. Has a first b4 lens whose both lens surfaces have a convex surface,
4 <bf / f, where Dab is the air gap between the first group and the first group, bf is the distance from the final lens surface of the second group to the second conjugate point, and f is the focal length of the entire system. ... (1) 0.5 <Dab / f <1.3 (2) It is characterized by satisfying the condition.

[0016]

1 to 5 are lens cross-sectional views of Numerical Embodiments 1 to 5 of the present invention. 6 to 10 are aberration charts when the object distances (distances to the first conjugate point S side) when the numerical examples 1 to 5 of the present invention are expressed in mm are 5 m.

In the figure, LFL is a retrofocus type lens of the present invention. S is a screen, which is arranged on the first conjugate point side (hereinafter also referred to as “object side”) having a long distance. LC is a projection surface of a liquid crystal display element or the like, and is arranged on the side of the second conjugate point having a short distance (hereinafter also referred to as “image plane side”).

On the image side, for example, in the case of a color liquid crystal projection as shown in FIG. 11, each element such as a liquid crystal display element which is a projected image, a light source and a filter is arranged. The retrofocus type lens LFL is composed of two lens units, a first lens unit L1 having a negative refractive power and a second lens unit having a positive refractive power. SP is a diaphragm, and the first lens unit L1 and the second lens unit
It is arranged between the group L2. The first lens unit L1 is composed of two lens units, a first lens unit L1a and a first lens unit L1b, which are separated by the largest air gap.

In the present embodiment, the first group b is a first b1 lens having both convex lens surfaces having a small radius of curvature on the side of the second conjugate point having a short distance, and a negative first lens having a concave surface on the side of the second conjugate point. It is composed of four lenses, a 1b2 lens, a first b3 lens whose both lens surfaces are concave surfaces, and a first b4 lens whose both lens surfaces having a small radius of curvature on the first conjugate point side are convex surfaces.

By setting each element so as to satisfy the conditional expressions (1) and (2), a predetermined back focus is ensured, the occurrence of asymmetric aberration is reduced, and high optical performance is obtained over the entire screen. ing.

When used as a projection lens for a color liquid crystal projection TV, a long back focus is required because a dichroic mirror or the like needs to be arranged on the image plane side. In addition, it is necessary to configure the image side telecentric according to the angle characteristic of the liquid crystal display element.

Conditional expression (1) is for obtaining a predetermined back focus (distance from the final lens surface of the second lens unit to the second conjugate point). If the conditional expression (1) is deviated, it becomes difficult to obtain a predetermined back focus enough to dispose the dichroic prism and the like.

Conditional expression (2) is defined by the first group a and the first group a in the first group.
It relates to the air gap with the group b. If the lower limit of conditional expression (2) is exceeded, not only the lens diameter of the 1b-th group will become large, but also the balance of various aberrations will be greatly disturbed, and many under (barrel-shaped) distortion aberrations will occur. If the upper limit of conditional expression (2) is exceeded, the entire length of the lens will become long, and in order to obtain a sufficient amount of peripheral light, it is necessary to increase the diameter of the front lens, and the overall lens system becomes large.

In this embodiment, the retrofocus type lens suitable as a projection lens for a color liquid crystal projector or the like is achieved by specifying the lens configuration of each lens group as described above.

In the present invention, a predetermined back focus can be easily obtained as a projection lens for a color liquid crystal projector or the like, and off-axis aberrations such as distortion aberration can be corrected well, and the optical performance of the entire screen can be corrected in a well-balanced manner. Preferably satisfies at least one of the following conditions.

(1-1) Both the lens surfaces of the first b2 lens have a concave surface or a meniscus shape.

(1-2) When the focal lengths of the first group and the second group are f1 and f2, respectively, -2.5 <f2 / f1 <-2.0 ... ( 3) To satisfy the following condition.

Conditional expression (3) relates to the ratio of the refractive powers of the first and second groups. When the upper limit of conditional expression (3) is exceeded and the refractive power of the first lens unit becomes small, sufficient back focus cannot be obtained. If the lower limit of conditional expression (3) is exceeded and the refractive power of the first lens group becomes large, the asymmetry of the lens system becomes large, and it becomes difficult to correct distortion, coma, chromatic aberration of magnification, and the like. The distance between the principal points of the first and second lens groups may be increased in order to favorably correct aberrations, but this method is not preferable because the total lens length becomes long.

(1-3) When the focal lengths of the first group a and the first group b are f1a and f1b, 0.7 <f1b / f1a <3.5 (4) To satisfy the condition.

Conditional expression (4) relates to the ratio of the refracting powers of the first group a and the first group b in the first group. Conditional expression (4)
If the lower limit of is exceeded, the balance of various aberrations deteriorates, and sufficient projection performance cannot be obtained. If the upper limit of conditional expression (4) is exceeded, the refractive power of the 1a-th group will become large, so that a large amount of under (barrel-shaped) distortion aberration and downward coma will occur.

(1-4) The radii of curvature on the first conjugate point side and the second conjugate point side of the first b1 lens are R1b1 _f and R, respectively.
1b1 _R ,

[0032]

[Equation 2] Satisfying the following conditions.

Conditional expression (5) relates to the shape of the first b1 lens arranged on the first conjugate point side in the first b group. In this embodiment, the radius of curvature R1b of the first b1 lens is
By reducing 1 _R , a higher-order (pincushion type) distortion aberration is generated, and an under distortion aberration generated in the 1a group is corrected. Radius of curvature R1b1 _R
Becomes larger and the shape factor of the conditional expression (5) exceeds the lower limit value of the conditional expression (5), it is not possible to satisfactorily correct the under-distortion aberration that occurs in the first-a lens group, but also the astigmatism is overcorrected. As a result, the image quality around the screen deteriorates. On the contrary, the radius of curvature R1b1 _R becomes too small and exceeds the upper limit of the conditional expression (5), and over distortion aberration remains, which is not preferable because the variation amount of distortion aberration becomes too large in the entire screen area.

(1-5) When the average values of the refractive indices of the materials of the positive lens and the negative lens in the first group are N1P and N1N respectively, 0.04 <N1N-N1P <0.13 ... (6) To satisfy the following condition.

In the present invention, in order to satisfactorily correct the distortion aberration, in order to suppress the large barrel distortion that occurs in the first group, the first
A high refractive index glass material is used as the material of the negative lens of the group.

Conditional expression (6) relates to the refractive indexes of the materials of the positive lens and the negative lens in the first lens group. When the lower limit of conditional expression (6) is exceeded and the refractive index of the material of the negative lens becomes low, a large amount of under distortion remains. Further, as can be seen from the refractive power arrangement of the conditional expression (3), the negative refractive power of the first group is considerably larger than the positive refractive power of the second group in order to secure the back focus. Therefore, the Petzval sum takes a large value in the negative direction and the image plane falls over, so that the image quality is deteriorated in the periphery of the screen. On the other hand, when a glass material having a high refractive index exceeding the upper limit of conditional expression (6) is used, it is advantageous for aberration correction and image plane correction, but this is not preferable because the spectral transmittance of the glass material deteriorates.

(1-6) The second group has a plurality of positive lenses and a plurality of negative lenses, and the average value of the refractive index and the Abbe number of the material of the plurality of positive lenses is N2P, ν2P, The average value of the refractive index of the material of the negative lens and the Abbe number are N2N, ν2
When N is set, 0.2 <N2N-N2P ······················································································································································· ((8) Is.

The conditional expressions (7) and (8) relate to the refractive indexes and Abbe numbers of the glass materials of the positive lens and the negative lens which form the second lens group. In order to correct the negative Petzval sum by the first group, it is preferable that the glass material of the lens in the second group satisfies conditional expression (7). When the lower limit of conditional expression (7) is exceeded and the refractive index of the material of the negative lens becomes low, not only the image surface falls over, but also the conditional expression (8) is deviated, causing color blur on the screen. It is not good because it occurs. If the upper limit of conditional expression (8) is exceeded, a high-dispersion glass material must be used as the glass material for the negative lens, which not only leads to an increase in cost, but also leaves a large secondary spectrum of color, which is not preferable for chromatic aberration correction.

(1-7) The focal length of the second lens unit is f2,
The distance from the diaphragm to the front main plane of the second group is O21.
Then, 0.9 <O21 / f2 <1.10 (9) is satisfied.

Conditional expression (9) relates to the peripheral light amount. If the lower limit of conditional expression (9) is exceeded, the off-axis chief ray angle with respect to the optical axis becomes large, and it is not possible to secure a sufficient illumination light flux to enter the liquid crystal display element, and the amount of peripheral light decreases on the screen. If the upper limit of conditional expression (9) is exceeded, the diaphragm position will come forward and the balance of off-axis aberrations, especially astigmatism, will be lost, which is not desirable.

(1-8) In order from the first conjugate point side, the first
The group has a meniscus negative lens whose convex surface faces the first conjugate point side, a positive lens whose both lens surfaces are convex surfaces, and a meniscus negative lens whose convex surface faces the first conjugate point side. . As a result, good optical performance is obtained over the entire screen.

(1-9) From the first conjugate point side, the second
The group is a cemented lens in which a positive lens with both convex lens surfaces and a negative lens with both concave lens surfaces are cemented, a cemented lens in which a negative lens with both concave lens surfaces and a positive lens with both convex lens surfaces are cemented together. A lens, a cemented lens in which a positive lens having a convex surface with a small radius of curvature facing the second conjugate point side and a meniscus negative lens having a convex surface facing the second conjugate point side are cemented, and at least one of both lens surfaces is That is, it has a positive lens having a convex surface. As a result, good optical performance is obtained over the entire screen. In particular, spherical aberration and coma that occur when the aperture is increased are well corrected.

Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the screen side, Di is the i-th lens thickness and air gap from the screen side, and Ni and νi are the values of the i-th lens in order from the screen side, respectively. The refractive index of glass and the Abbe number. Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples. (Numerical Example 1) f = 17.508 fno = 1: 3.0 2ω = 71.1 ° bf = 72.83mm R 1 = 75.29 D 1 = 3.10 N 1 = 1.72342 ν 1 = 38.0 R 2 = 23.88 D 2 = 10.05 R 3 = 77.74 D 3 = 8.05 N 2 = 1.64769 ν 2 = 33.8 R 4 = -141.59 D 4 = 0.20 R 5 = 59.58 D 5 = 2.20 N 3 = 1.72342 ν 3 = 38.0 R 6 = 27.95 D 6 = 16.10 R 7 = 56.77 D 7 = 9.20 N 4 = 1.51633 ν 4 = 64.2 R 8 = -38.08 D 8 = 0.20 R 9 = 515.41 D 9 = 1.40 N 5 = 1.61800 ν 5 = 63.4 R10 = 17.42 D10 = 6.38 R11 = -26.53 D11 = 1.40 N 6 = 1.61800 ν 6 = 63.4 R12 = 26.58 D12 = 6.35 N 7 = 1.53172 ν 7 = 48.9 R13 = -33.32 D13 = 22.20 R14 = (aperture) D14 = 5.76 R15 = 44.46 D15 = 4.95 N 8 = 1.69895 ν 8 = 30.1 R16 = -27.96 D16 = 1.10 N 9 = 1.65844 ν 9 = 50.9 R17 = 89.42 D17 = 19.32 R18 = -171.77 D18 = 1.30 N10 = 1.83400 ν10 = 37.2 R19 = 23.54 D19 = 9.15 N11 = 1.51633 ν11 = 64.2 R20 = -39.02 D20 = 0.20 R21 = -505.58 D21 = 8.15 N12 = 1.48749 ν12 = 70.2 R22 = -21.19 D22 = 1.60 N13 = 1.83400 ν13 = 37.2 R23 = -48.65 D23 = 0.20 R24 = 180.68 D24 = 11.00 N14 = 1.49700 ν14 = 81.6 R25 = -30.25 (Numerical example 2) f = 17.510 fno = 1: 3.0 2ω = 71.0 ° bf = 72.84mm R 1 = 58.76 D 1 = 3.10 N 1 = 1.72342 ν 1 = 38.0 R 2 = 28.16 D 2 = 10.53 R 3 = 70.15 D 3 = 7.30 N 2 = 1.67270 ν 2 = 32.1 R 4 = -212.44 D 4 = 0.20 R 5 = 71.34 D 5 = 2.20 N 3 = 1.72342 ν 3 = 38.0 R 6 = 25.45 D 6 = 19.41 R 7 = 69.37 D 7 = 9.50 N 4 = 1.51633 ν 4 = 64.2 R 8 = -34.97 D 8 = 0.20 R 9 = -607.33 D 9 = 1.40 N 5 = 1.61800 ν 5 = 63.4 R10 = 19.44 D10 = 25.23 R11 = -39.32 D11 = 1.40 N 6 = 1.61800 ν 6 = 63.4 R12 = 21.70 D12 = 4.26 N 7 = 1.67270 ν 7 = 32.1 R13 = -95.20 D13 = 0.20 R14 = (Aperture) D14 = 5.22 R15 = 51.99 D15 = 7.00 N 8 = 1.67270 ν 8 = 32.1 R16 = -34.42 D16 = 1.10 N 9 = 1.80610 ν 9 = 41.0 R17 = 1437.33 D17 = 16.25 R18 = -87.99 D18 = 1.45 N10 = 1.83400 ν10 = 37.2 R19 = 44.00 D19 = 8.31 N11 = 1.48749 ν11 = 70.2 R20 = -36.15 D20 = 0.20 R21 = 321.53 D21 = 10.12 N12 = 1.49700 ν12 = 81.6 R22 = -22.83 D22 = 1.90 N13 = 1.83400 ν13 = 37.2 R23 = -89.73 D23 = 0.20 R24 = -972.43 D24 = 8.27 N14 = 1.48749 ν14 = 70.2 R25 = -33.04 D25 = 0.03 R26 = 95.91 D26 = 5.05 N15 = 1.61800 ν15 = 63.4 R27 = -208.41 ( Numerical Example 3) f = 17.511 fno = 1: 3.0 2ω = 71.0 ° bf = 72.85mm R 1 = 61.31 D 1 = 3.10 N 1 = 1.83400 ν 1 = 37.2 R 2 = 30.72 D 2 = 9.58 R 3 = 76.79 D 3 = 8.56 N 2 = 1.74664 ν 2 = 32.4 R 4 = -266.04 D 4 = 0.20 R 5 = 60.65 D 5 = 2.20 N 3 = 1.83400 ν 3 = 37.2 R 6 = 26.76 D 6 = 22.48 R 7 = 185.10 D 7 = 3.00 N 4 = 1.51600 ν 4 = 64.5 R 8 = -34.17 D 8 = 0.20 R 9 = 1111.53 D 9 = 1.40 N 5 = 1.61800 ν 5 = 63.4 R10 = 20.33 D10 = 21.90 R11 = -28.40 D11 = 1.40 N 6 = 1.60906 ν 6 = 63.5 R12 = 23.39 D12 = 5.54 N 7 = 1.67169 ν 7 = 32.0 R13 =- 46.56 D13 = 0.20 R14 = (Aperture) D14 = 0.20 R15 = 73.16 D15 = 3.79 N 8 = 1.64815 ν 8 = 34.1 R16 = -27.75 D16 = 1.10 N 9 = 1.83400 ν 9 = 37.2 R17 = -154.96 D17 = 23.37 R18 = -75.43 D18 = 1.30 N10 = 1.83400 ν10 = 37.2 R19 = 45.03 D19 = 8.38 N11 = 1.48700 ν11 = 70.2 R20 = -35.93 D20 = 0.20 R21 = 1862.71 D21 = 9.76 N12 = 1.48700 ν12 = 81.6 R22 = -23.80 D22 = 1.50 N13 = 1.83400 ν13 = 37.2 R23 = -63.09 D23 = 0.20 R24 = -643.09 D24 = 8.81 N14 = 1.48749 ν14 = 70.2 R25 = -34.57 D25 = 0.03 R26 = 97.14 D26 = 5.21 N15 = 1.48749 ν15 = 70.2 R27 = -182.12 (numerical value Example 4) f = 17.520 fno = 1: 3.0 2ω = 71.0 ° bf = 72.88mm R 1 = 76.39 D 1 = 3.30 N 1 = 1.72342 ν 1 = 38.0 R 2 = 31.34 D 2 = 9.47 R 3 = 69.90 D 3 = 8.28 N 2 = 1.64769 ν 2 = 33.8 R 4 = -188.38 D 4 = 0.20 R 5 = 58.97 D 5 = 2.30 N 3 = 1.72342 ν 3 = 38.0 R 6 = 27.33 D 6 = 19.00 R 7 = 67.23 D 7 = 8.00 N 4 = 1.51633 ν 4 = 64.2 R 8 = -39.73 D 8 = 0.20 R 9 =- 721.43 D 9 = 1.40 N 5 = 1.61800 ν 5 = 63.4 R10 = 18.20 D10 = 6.09 R11 = -26.74 D11 = 1.40 N 6 = 1.61800 ν 6 = 63.4 R12 = 26.74 D12 = 5.82 N 7 = 1.53172 ν 7 = 48.9 R13 = -32.61 D13 = 21.91 R14 = (Aperture) D14 = 7.62 R15 = 50.17 D15 = 4.85 N 8 = 1.69895 ν 8 = 30.1 R16 = -26.25 D16 = 1.10 N 9 = 1.65844 ν 9 = 50.9 R17 = 160.40 D17 = 18.03 R18 = -158.53 D18 = 1.50 N10 = 1.83400 ν10 = 37.2 R19 = 23.03 D19 = 9.14 N11 = 1.51633 ν11 = 64.2 R20 = -40.58 D20 = 0.20 R21 = -404.86 D21 = 8.45 N12 = 1.48749 ν12 = 70.2 R22 = -20.24 D22 = 1.90 N13 = 1.83400 ν13 = 37.2 R23 = -46.18 D23 = 0.20 R24 = 229.73 D24 = 9.58 N14 = 1.49700 ν14 = 81.6 R25 = -29.28 (Numerical Example 5) f = 17.504 fno = 1: 3.0 2ω = 71.1 ° bf = 70.02 mm R 1 = 40.49 D 1 = 2.40 N 1 = 1.84666 ν 1 = 23.8 R 2 = 19.86 D 2 = 11.58 R 3 = 10 8.61 D 3 = 6.00 N 2 = 1.68893 ν 2 = 31.1 R 4 = -81.05 D 4 = 0.20 R 5 = 79.78 D 5 = 1.70 N 3 = 1.84666 ν 3 = 23.8 R 6 = 27.79 D 6 = 10.29 R 7 = 57.53 D 7 = 7.40 N 4 = 1.51633 ν 4 = 64.2 R 8 = -27.10 D 8 = 0.20 R 9 = -120.14 D 9 = 1.20 N 5 = 1.61800 ν 5 = 63.4 R10 = 18.07 D10 = 4.87 R11 = -27.65 D11 = 1.40 N 6 = 1.61800 ν 6 = 63.4 R12 = 30.40 D12 = 0.20 R13 = 29.25 D13 = 5.41 N 7 = 1.64769 ν 7 = 33.8 R14 = -33.60 D14 = 29.28 R15 = (Aperture) D15 = 0.20 R16 = 39.79 D16 = 4.90 N 8 = 1.69895 ν 8 = 30.1 R17 = -31.85 D17 = 1.10 N 9 = 1.74400 ν 9 = 44.8 R18 = 121.83 D18 = 29.03 R19 = -179.33 D19 = 1.50 N10 = 1.83400 ν10 = 37.2 R20 = 24.37 D20 = 8.96 N11 = 1.51633 ν11 = 64.2 R21 = -38.39 D21 = 0.20 R22 = 191.54 D22 = 9.00 N12 = 1.48749 ν12 = 70.2 R23 = -23.30 D23 = 1.90 N13 = 1.83400 ν13 = 37.2 R24 = -76.14 D24 = 0.20 R25 = 169.57 D25 = 10.50 N14 = 1.49700 ν14 = 81.6 R26 = -30.58

[0044]

[Table 1]

[0045]

As described above, according to the present invention, by properly setting the lens configuration, a predetermined back focus can be easily obtained, and a large aperture having a substantially telecentric asymmetry aberration on the image side is well corrected. Therefore, it is possible to achieve a retrofocus type lens having high optical performance, which is particularly suitable as a projection lens for a color liquid crystal projection TV.

[Brief description of drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 3 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 4 is a lens cross-sectional view of Numerical Example 4 of the present invention.

FIG. 5 is a lens cross-sectional view of Numerical Example 5 of the present invention.

FIG. 6 is an aberration diagram of Numerical example 1 of the present invention.

FIG. 7 is an aberration diagram of Numerical example 2 of the present invention.

FIG. 8 is an aberration diagram of Numerical example 3 of the present invention.

FIG. 9 is an aberration diagram of Numerical example 4 of the present invention.

FIG. 10 is an aberration diagram of Numerical example 5 of the present invention.

FIG. 11 is a schematic view of a conventional liquid crystal projector.

[Explanation of symbols]

 L1 1st group L2 2nd group L1a 1a group L1b 1b group SP Aperture LC Projected surface S screen d d line F F line C C line ΔS Sagittal image surface ΔM Meridional image surface

Claims (10)

[Claims] 1. A first lens group having a negative refractive power and an aperture stop in order from a first conjugate point having a longer distance, and a second lens group having a second lens group having a positive refractive power.
The first lens group has two lens groups, the first lens group 1a and the first lens group 1b, and the first lens group 1b is small on the side of the second conjugate point having a short distance. A first b1 lens having a convex radius on both lens surfaces, a negative 1b2 lens having a concave surface on the second conjugate point side, a 1b3 lens having a concave surface on both lens surfaces, and a small radius of curvature on the first conjugate point side. Has a first b4 lens whose both lens surfaces have a convex surface,
4 <bf / f, where Dab is the air gap between the first group and the first group, bf is the distance from the final lens surface of the second group to the second conjugate point, and f is the focal length of the entire system. A retrofocus type lens characterized by satisfying a condition of 0.5 <Dab / f <1.3. 2. The retrofocus lens according to claim 1, wherein both lens surfaces of the first b2 lens have a concave surface or a meniscus shape. 3. The focal lengths of the first lens unit and the second lens unit are f
The retrofocus lens according to claim 1 or 2, wherein a condition of -2.5 <f2 / f1 <-2.0 is satisfied, where f1 and f2 are satisfied. 4. When the focal lengths of the first group a and the first group b are f1a and f1b, respectively, the condition 0.7 <f1b / f1a <3.5 is satisfied. 2 or 3 retrofocus lenses. 5. When the radii of curvature on the first conjugate point side and the second conjugate point side of the first b1 lens are R1b1 _f and R1b1 _R , respectively, The following conditions are satisfied: Claims 1, 2, 3
Or 4 retrofocus type lens. 6. When the average values of the refractive indices of the materials of the positive lens and the negative lens in the first group are N1P and N1N, respectively, the condition 0.04 <N1N−N1P <0.13 is satisfied. The retrofocus lens according to any one of claims 1 to 5, characterized in that: 7. The second group has a plurality of positive lenses and a plurality of negative lenses, and the average values of the refractive index and the Abbe number of the materials of the plurality of positive lenses are N2P and ν2P. 7. When the average value of the refractive index of the material and the Abbe number are N2N and ν2N, 0.2 <N2N-N2P18 <ν2P-ν2N <30 is satisfied, and the condition is satisfied. A retrofocus lens according to the item. 8. When the focal length of the second lens unit is f2 and the distance from the diaphragm to the front main plane of the second lens unit is O21, the condition of 0.9 <O21 / f2 <1.10 is satisfied. The retrofocus lens according to any one of claims 1 to 7, wherein 9. The first group is a first group in order from the first conjugate point side.
A meniscus negative lens having a convex surface directed to the conjugate point side, a positive lens having both convex lens surfaces, and a meniscus negative lens having a convex surface directed to the first conjugate point side. 9. The retrofocus lens according to any one of 1 to 8. 10. A cemented lens in which, in order from the first conjugate point side, the second lens group is a cemented lens in which a positive lens whose both lens surfaces are convex surfaces and a negative lens whose both lens surfaces are concave surfaces are cemented, and a negative lens whose both lens surfaces are concave surfaces. A cemented lens in which a positive lens whose both lens surfaces are convex surfaces are cemented, a positive lens having a convex surface having a small radius of curvature on the second conjugate point side, and a meniscus-shaped negative lens having a convex surface on the second conjugate point side. 10. The retrofocus type lens according to claim 1, further comprising a cemented lens having a cemented lens and a positive lens having at least one lens surface having a convex surface.