JPH04174807A - Retro-focus type lens - Google Patents

Abstract

PURPOSE:To obtain a high optical performance projected image with aberrations such as distortion aberration and flare corrected properly by providing at least one aspherical lens, the surface of at least one side of which is aspheric in the second group and third group. CONSTITUTION:When a plurality of lenses are divided into three groups, the first group I, second group II and third group III, in the order from the first conjugate point side of longer distance, and the focal distance of a total system is taken as fT, back focus SK is set so as to be SK>0.7fT, and each of the second group II and third group III is provided with at least one aspherical lens, the surface of at least one side of which is aspheric. By thus setting lens constitution, a high optical performance projected image with aberrations such as distortion aberration and flare corrected properly can be obtained.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a retrofocus lens, which combines, for example, a plurality of images having different color information using a compositing mirror, and then enlarges and projects the images onto a screen. The present invention relates to a retrofocus lens suitable for use as a projection lens for color liquid crystal projection televisions.

(Prior Art) Various color liquid crystal projection systems have been proposed in which images displayed on multiple color liquid crystals (liquid crystal light valves) are optically superimposed and projected onto a screen using a projection lens. There is.

FIG. 7 is a schematic diagram of the main parts of a color liquid crystal projection television that projects images formed on a general color liquid crystal onto a screen surface (not shown).

In the figure, reference numeral 1 indicates a white light source that emits a collimated light beam. 2a, 2b, and 2c are liquid crystal display elements for red, edge, and blue, respectively, on which projected images are displayed. 3a, 3
Reference numerals b denote reflecting mirrors, and 4 a red reflecting dichroic mirror, which illuminates the liquid crystal display element 2a for red. 5 is a green reflective dichroic mirror that illuminates the liquid crystal display element 2b on the periphery.

The blue liquid crystal display element 2c is illuminated with blue light that has passed through the red reflective dichroic mirror 4 and the green reflective dichroic mirror 5. 6 is a blue-transmitting dichroic mirror.

7 is a projection lens.

In the figure, the white light from the white light source 1 is separated into red, edge, and blue color lights by the tichroic mirrors (4, 5), and these red, edge, and blue color lights are used for red, green, and blue, respectively. liquid crystal display elements (2a, 2b.

2c), and the images of the liquid crystal display elements (2a, 2b, 2c) based on these colored lights are superimposed and projected onto a screen surface (not shown) by a projection lens 7 to obtain a color image.

The projection lens in such a configuration requires a retrofocus type with a long back focus because it is necessary to arrange various optical members such as a reflective mirror and a tichroic mirror between the final lens surface and the liquid crystal display element (between the back focuses). Many lenses are used.

(Problem to be solved by the invention) Generally, a retrofocus lens has a lens group with negative refractive power arranged on the object side (longer distance conjugate point side), and a lens group with negative refractive power on the image side (longer distance conjugate point side).
It consists of a lens configuration in which a lens group with positive refractive power is arranged on the conjugate point side (with a shorter distance). Therefore, it has the advantage that a relatively long back focus can be easily obtained.

However, since the lens structure is asymmetrical, a lot of asymmetric aberrations such as distortion and astigmatism tend to occur.

When a retrofocus lens is used in a color LCD projection TV, a dichroic mirror for color separation is placed on the image plane side as shown in Figure 7, which eliminates color unevenness and improves color reproduction across the screen. In order to achieve this, it is necessary that the angle of incidence of the light beam onto the dichroic mirror be approximately equal over the entire screen. That is, it is necessary to put the lens system in a state close to a so-called exit telecentric system in which the chief ray exiting from the image plane side is substantially parallel to the optical axis.

However, when an exit telecentric system is used, the off-axis light beam enters the lens group with positive refractive power on the image plane side at a high position on the optical axis. For this reason, many barrel-shaped (negative distortion aberrations and flares) occur, resulting in the problem that it is very difficult to obtain a good projected image.

Therefore, the present applicant previously proposed in Japanese Patent Application No. 1-286058 by appropriately setting the lens configuration, which is suitable for a projection lens for color LCD projection televisions, in which the aforementioned various aberrations such as distortion and flare are well corrected. We proposed a retrofocus lens with a lens configuration similar to that of an exit telecentric system.

The present invention further improves the retrofocus type lens previously proposed by the applicant, and allows a predetermined hack focus to be easily obtained while reducing the number of lenses, and also satisfactorily corrects various aberrations such as distortion and flare. The purpose of the present invention is to provide a retrofocus type lens suitable for a projection lens for a color liquid crystal projection telescope.

(Means for Solving the Problems) The retrofocus type lens of the present invention has a plurality of lenses, and the lenses are arranged in order from the first conjugate point side with the longest distance.
Divided into three lens groups, the third group and the third group, the focal length of the entire system is fT, and the distance from the lens surface on the side of the second conjugate point, which is shorter in distance, to the second conjugate point is SK> 0, 7 f T −・−・−=
(a+), and the second group and the third group are each provided with at least one aspherical lens having at least one lens surface or an aspherical surface.

In particular, in the present invention, the focal length of at least one aspherical lens provided in the second group and the third group is fAL2. fA
When L3 is set, 1fAL21>3fT -=...-(a2)l
It is characterized by satisfying the following condition: fAL31>3fT (a3).

(Example) 1st. FIG. 2 is a cross-sectional view of the lens of Numerical Example 1.2 of the present invention, taking as an example a case where it is applicable to projection, and also shows a part of the lens.

In the figure, LFL is a retrofocus type lens of the present invention. S is a screen, and the first conjugate point side (
Hereinafter, this will be referred to as the "object side." ). F is a projection plane, which is located on the first and second conjugate point side (hereinafter referred to as "image plane side"), which is shorter in distance. On this image plane side, in the case of a color liquid crystal projection as shown in FIG. 7, for example, various elements such as a liquid crystal display element which is a projected image, a light source, a filter, etc. are arranged. ■ indicates the first group, ■ indicates the second group, and ■ indicates the third group.

By appropriately setting the refractive power, lens shape, etc. of a plurality of lenses, the retrofocus type lens of the present invention has a back focus (the lens surface on the second conjugate point side) when the focal length of the entire lens system is fT. The distance (SK) from a certain final lens surface to the second conjugate point is set such that SK>0.7fT.

At least one aspherical lens with an aspherical surface on at least one side is placed in each of the second and third groups, which effectively corrects various aberrations such as distortion and flare, resulting in high optical performance. The projected image is obtained.

In particular, the aspherical lens (21st aspherical lens) provided in the second group mainly corrects spherical aberration. Further, an aspherical lens (31st aspherical lens) provided in the third group mainly corrects distortion.

At this time, the refractive powers of the 21st aspherical lens and the 31st aspherical lens are made to satisfy conditional expressions (a2>, (a3)).

Conditional expressions (a2) and (a3) are used to prevent the refractive index of the material from changing due to environmental changes such as temperature changes, resulting in focus shift and optical performance deterioration when an aspherical lens is manufactured by molding, for example, a plastic material. It is for the purpose of

In other words, by applying an aspherical surface to a lens with relatively small refractive power (power) that satisfies conditional expressions (a2) and (a3), a predetermined optical performance can be maintained while reducing optical performance due to environmental changes. is effectively prevented. Furthermore, by satisfying conditional expressions (a2) and (a3), the selection range of materials can be expanded.

In Numerical Example 1 shown in FIG. 1, the first group m is a positive lens, a meniscus-shaped negative lens with a convex surface facing the object side, and the second group m
is composed of a single aspherical lens, and the third group m is composed of a composite lens of a negative lens and a positive lens, a positive lens, and an aspherical lens.

Further, in Numerical Example 2 shown in Fig. 2, the first group m is a negative meniscus lens with a convex surface facing the object side, a positive meniscus lens with a convex surface facing the image plane side, and the second group m is a single lens. The third group m is composed of a bonded lens of a negative lens and a positive lens, a positive lens, and an aspherical lens.

Next, the features of the lens configuration of the retrofocus type lens of the present invention will be explained.

In general, in an imaging optical system, when the imaging magnification between the two conjugate points is made sufficiently large or sufficiently small, and the conjugate point of the imaging optical system is short (here, the imaging optical system is The conjugate point that forms a smaller image among the two conjugate points is hereinafter referred to as the "second conjugate point.") and the distance SK to the lens surface on the second conjugate point side (herein, this distance SK
When the focal length f of the imaging optical system is made sufficiently long compared to the focal length f of the imaging optical system, the lens configuration of the imaging optical system should be The conjugate point that forms a large image among the two conjugate points is hereafter referred to as the first conjugate point J), and a lens or lens group with negative refractive power is placed on the side of the second conjugate point. It is necessary to use a so-called retrofocus type, in which a lens or lens group with positive refractive power is arranged.

As mentioned above, a retrofocus lens is a lens with negative refractive power, or a lens group with a positive refractive power, or a lens group with positive refractive power, or because the lens group is arranged asymmetrically, it is susceptible to asymmetry in the optical system. Based on this, many aberrations, especially distortion, tend to occur. This distortion has a strong tendency to become barrel-shaped at the second conjugate point, and it has been difficult to satisfactorily correct this distortion with a compact lens configuration using a small number of spherical lenses.

In the present invention, the distance from the first lens surface on the object side to the final lens surface on the optical axis is divided into approximately three equal parts, and the first lens surface is divided into thirds from the object side.
It is divided into three subsystems: group, second group, and third group. And the third group is an aspherical lens (31st aspherical lens)
By using an aspherical lens (21st aspherical lens 9), the above-mentioned barrel-shaped distortion can be effectively corrected, and by using an aspherical lens (21st aspherical lens 9) as the 2nth aberration in the center of the lens system, spherical aberration and comatic aberration can be mainly corrected. etc. are effectively corrected.

As shown in FIG. 3, the aspheric surface used in this example has a paraxial radius of curvature R1 at the apex of the aspheric surface. , the H axis passes through the apex of the aspherical surface and is perpendicular to the optical axis, and B,
C2D is expressed as an aspherical coefficient by the following equation (1).

In equation (1), the first term is a term related to the paraxial radius of curvature R, and gives the amount of the aspheric surface, which is less than or equal to the second term. Regarding the media before and after the aspherical surface, the refractive index of the medium on the side where the light beam enters the aspherical surface is N, and the refractive index of the medium on the exit side is N'. At this time, the third-order aspherical aberration coefficient! is the aspheric coefficient B in equation (1) above, and the refractive index N and N of the medium before and after the aspheric surface.
′ is expressed as follows.

'I'=8 (N'-N) B...
・(2) [The present invention requires only the results described here, so the derivation of equation (2) will be omitted, but (3.l. )
It was derived from equations (4.3b) and (4.5). ] Furthermore, the aspherical aberration coefficient type brings about the amount of change shown below with respect to the third-order aberration coefficient, that is, the amount of change in the third-order aberration caused by making the surface aspherical.

However, ΔI is the amount of change in the spherical aberration coefficient, Δ■ is the amount of change in the coma aberration coefficient, 6m is the amount of change in the astigmatism coefficient, Δ■ is the amount of change in the spherical field curvature aberration coefficient, and Δ■ is the distortion aberration. Represents the amount of change in the coefficient.

Also, h and h are the paraxial ray tracing amounts shown in Fig. 4, and h
represents the height (distance from the optical axis) where I (paraxial ray 1□) intersects each lens surface, and h represents the diagonal direction. It shows the height at which a ray (pupil paraxial ray IL2) that enters the lens and passes through the center of the aperture SP intersects each lens surface.

Therefore, h changes depending on the position of the aperture in the optical system, and cannot be defined in an optical system without an aperture. The explanation will be based on the assumption that there is a temporary aperture in the middle.

Generally, in a lens system, if the diaphragm is placed in a position biased toward either the first lens surface or the final lens surface of the lens system, the lens diameter of the lens placed near the diaphragm must be made smaller. It becomes possible. However, it is necessary to considerably increase the lens diameter of the lens disposed on the side opposite to the side on which the aperture is disposed. Therefore, in order to make the entire lens system compact, it is desirable to place the aperture near the center of the lens system, as assumed here.

Now, let us consider correcting barrel-shaped distortion, which often occurs in retrofocus lenses, using an aspheric surface based on equation (3). As can be seen from equation (3), by using an aspheric surface, the spherical aberration coefficient, comatic aberration coefficient, astigmatism coefficient, and spherical field curvature coefficient can be calculated at the height h at which the paraxial axial ray intersects each lens surface. 4th power, 3rd power, 2nd power, 2nd power respectively
In contrast, the distortion aberration coefficient changes in proportion to the first power of h. Also, spherical aberration coefficient, coma aberration coefficient,
While the astigmatism coefficient and the spherical field curvature coefficient change in proportion to the 0th power, 1st power, 2nd power, and 2nd power of the height h at which the pupil paraxial ray intersects each lens surface, The distortion coefficient is h
It changes in proportion to the third power of Therefore, by using an aspherical surface, the spherical aberration coefficient, coma aberration coefficient, astigmatism coefficient,
In order to greatly change the distortion aberration coefficient while suppressing the change in the spherical field curvature aberration coefficient, it is necessary to reduce the height h at which the paraxial ray intersects each lens surface, and to increase the height h at which the paraxial ray intersects each lens surface. The lens surface that intersects with the height Y or larger may be made an aspherical surface.

As mentioned above, in a retrofocus lens, a lens or lens group with negative refractive power is placed on the side of the first conjugate point of the imaging optical system, and a lens with positive refractive power is placed on the side of the second conjugate point. , or a configuration in which lens groups are arranged, the height h at which the paraxial ray intersects each lens surface is generally a small value on the lens surface on the first conjugate point side, and becomes smaller on the lens surface on the second conjugate point side. It becomes a large value. Further, near the center of the lens system, the value is often intermediate between these values.

On the other hand, the height h at which the pupil paraxial ray intersects each lens surface generally increases as the distance from the diaphragm position increases. If it is assumed that the aperture is placed at a position where the diaphragm is divided, the value will be large at both ends of the lens system, and the value will be small at the center of the lens system.

Incidentally, here, the value of 100 at the height at which the pupil paraxial ray intersects each lens surface is said to be a large value and a small value, or the value of 100 changes its sign before and after the aperture, so it is a large value, a small value. Hani indicates the magnitude of its absolute value.

From the above, in the present invention, in order to correct the barrel-shaped distortion that often occurs in retrofocus lenses,
The height h at which the paraxial ray intersects each lens surface is relatively small, and the height h at which the pupil paraxial ray intersects each lens surface is relatively large, that is, the lens surface Of the three partial systems created by dividing the distance from the surface to the final lens surface into approximately three equal parts, an aspherical surface is used for the partial system of the third group on the second conjugate point side.

That is, in the present invention, attention is paid to the height h at which the paraxial axial ray intersects each lens surface, and the value of this height h is arranged on the second conjugate point side. 2nd from lens group
Focusing on the fact that it rapidly becomes smaller while reaching the conjugate point,
The distance between this paraxial ray and the optical axis becomes relatively small,
In the third group, in which the distance between the pupil paraxial ray and the optical axis becomes large, the third group has a relatively weak refractive power that satisfies the above-mentioned conditional expression (a3).
1 Distortion aberration is well corrected by arranging an aspherical lens.

The aspherical lens here is a lens with a weak refractive power that satisfies conditional expression (a3) because if the aspherical lens placed here has a strong negative refractive power, it will no longer be a retrofocus type. , the back focus SK is set to a focal pressure of 111
This is because it is difficult to make the length sufficiently long compared to f. In addition, in the case of strong positive refractive power, the above-mentioned paraxial axial ray cannot sufficiently reduce the height h at which it intersects with this aspheric surface, causing aberrations other than distortion to change significantly. This is because it will end up being a problem.

In the present invention, as described above, the spherical aberration coefficient, coma aberration coefficient,
Is the 31st aspherical lens placed in the third group that does not significantly change the astigmatism coefficient and the spherical field curvature coefficient to correct distortion?As shown in equation (3), the spherical surface The amount of change in the aberration coefficient, coma aberration coefficient, astigmatism coefficient, and truncated field curvature aberration coefficient is not zero.

Naturally, these aberrations still remain even in retrofocus lenses that do not use an aspherical surface.

Therefore, in the present invention, by providing a 21st aspherical lens in the second group, these remaining aberrations are effectively corrected.

This 21st aspherical lens is different from the above-mentioned 31st aspherical lens for correcting distortion aberration, and mainly corrects other aberrations such as spherical aberration, coma aberration, astigmatism, and spherical field curvature aberration. It is for the purpose of

For this reason, it is desirable to arrange the lens at a position where the height h at which the paraxial ray intersects each lens surface is large and the height h at which the pupil paraxial ray intersects each lens surface is small.

Therefore, in the present invention, the second aspherical surface is placed in the second group near the center of the lens system, where the height of the paraxial ray is medium or the height of the paraxial ray is extremely small. are doing.

Moreover, if the 21st aspherical lens placed here is also a lens with a strong refractive power, the retrofocus type refractive power arrangement tends to weaken and the huck focus tends to become short, so the above-mentioned conditional expression (a2) is satisfied. The lens has a relatively weak refractive power.

Also, this has the effect of widening the selection range of materials. This is the same as in the case of the 31st aspherical lens for correcting distortion aberration described above.

Next, the shape of the aspherical lens, which is a further feature of the present invention, will be explained. First, for the sake of simplicity, the paraxial radius of curvature of the two lens surfaces of the 31st aspherical lens disposed in the third group is assumed to be infinite. Further, it is assumed that the aspherical surface is formed only on one lens surface, and that only the value of B in the above-mentioned equation (1) is given as the aspherical surface coefficient. At this time, the aspherical shape is expressed as R in equation (1).
= ω, substitute C=D=O and get x=BH' ・・・・・・・・・・・・(
4).

At this time, the focal length fAL3 of this 31st aspherical lens is infinite, and its refractive power is sufficiently weak, so it does not cause any harm to the retrofocus type configuration, and therefore it is possible to maintain the back focus for a sufficiently long time. This also makes it possible to satisfactorily correct distortion for the reasons mentioned above.

Assume that the 31st aspherical lens is placed on the second conjugate point side of the lens system, and further consider that an aspherical surface is used as the lens surface on the second conjugate point side.

In general, the distortion aberration at the second conjugate point of a retrofocus lens has a significant barrel shape, so the aspherical shape to correct this is such that the amount of change ΔV of the distortion aberration coefficient in equation (3) is a negative value. You can give it to Generally, at the position on the second conjugate point side of the lens system where the 31st aspherical lens is placed, the height h at which the paraxial ray intersects with the lens surface is a positive value, and the pupil paraxial ray is Height h where it intersects with the lens surface
is also a positive value. Therefore, in order to make the amount of change ΔV of the distortion aberration coefficient a negative value, the aspheric aberration coefficient! It is necessary to set the value of to a negative value.

On the other hand, the aspheric aberration coefficient! is expressed as in equation (2) using the refractive indices N, N- of the medium before and after the aspherical surface and the aspherical coefficient B. Here, the medium in front of the aspherical surface is an optical material such as glass or plastic, and the medium in the rear is air, so generally N>N'. Therefore, in order to correct the barrel distortion aberration, it is necessary to set the value of the aspherical coefficient B to a positive value. At this time, the shape of the aspherical surface is (
From equation 4), it becomes a quartic curved surface with the concave surface facing the second conjugate point side, and the thickness of the 31st aspherical lens measured in the optical axis direction increases as the distance from the optical axis increases.

Similarly, consider a case where the aspherical surface of the 31st aspherical lens is used as the lens surface on the first conjugate point side instead of the lens surface on the second conjugate point side. In this case, the refractive index N of the medium before and after the aspherical surface
. It is necessary to take a negative value.In this case, the shape of the aspherical surface is a quartic curved surface with the convex surface facing the second conjugate point according to equation (4), and the thickness measured in the optical axis direction of the 31st aspherical lens The intensity increases as the distance from the optical axis increases.

For the above reasons, the shape of the 31st aspherical lens is changed to correct barrel-shaped distortion by making one or both lens surfaces of the 31st aspherical lens aspherical. It has a shape in which the thickness measured in the distal axial direction increases as it goes away from the optical axis.

In addition, when the 31st aspherical lens is placed in the convergent optical path on the second conjugate point side of the lens system, it is particularly effective in correcting distortion aberration and does not worsen spherical aberration, coma, etc. In order to achieve this, it is more effective to form a meniscus shape with the concave surface substantially facing the second conjugate point side.

On the other hand, the 21st lens is located in the second group near the center of the lens system.
Aspherical lenses are primarily intended to correct spherical aberration, coma, and the like.

In lenses and lens groups with positive refractive power placed on the second conjugate point side of a retrofocus lens, under-corrected spherical aberration and extroverted coma aberration are likely to occur, and the lens system as a whole is , these aberrations tend to remain.

The 21st aspherical lens placed in the second group effectively corrects these aberrations, and also satisfactorily and effectively corrects various aberrations generated by the 31st aspherical lens placed for correcting distortion. It is something to do. To achieve this, it is sufficient to set the amount of change in the spherical aberration coefficient ∆ and the amount of change in the coma aberration coefficient ∆ to negative values. Accordingly, the aspherical shape is such that the thickness increases as measured in the optical axis direction.

The aspherical shape of this 21st aspherical lens changes depending on the refractive power arrangement of the other lenses and the shape of the other aspherical lenses, and it does not necessarily have to be this shape, but it is a well-balanced shape. Such a shape is desirable for correcting aberrations.

Further, in the present invention, when the center of the off-axis light flux is taken as the principal ray, the angle θ between the principal ray of the maximum off-axis light flux emitted from the lens surface on the image plane side and the optical axis is set such that θ<20 degrees. Each element is set. That is, when the height from the optical axis of the principal ray of the maximum off-axis luminous flux emerging from the lens surface on the image plane side is h, the maximum image height on the image plane is D, and the back focus is SK (D
-h)/S, K<0.364. As a result, a predetermined telecentric system is obtained.

Next, numerical examples of the present invention will be shown. In numerical examples R
i is the radius of curvature of the i-th lens surface in order from the object side, D
i is the i-th lens thickness and air distance from the object side, Ni
and νi are the refractive index and Abbe number of the glass of the i-th lens, respectively, in order from the object side.

Note that the aspheric coefficient is expressed based on the above-mentioned equation (1).

Numerical Example 1 1-90. OFNo=l:4 2ω-52,8'
R]-147,40D I-9,ON +=1.784
72ν l-25, 7R2-1117, 99D 2-0
．． 5 R3-222, 16D 3-5. ON 2-1.7]2
99shi 2-53.8R4-45, 38D4-70.0 R5-106, 6705-3,0, N 3-1.491
71 shi 3-57.488-106.87 D 6-5
5.0R7--98,34D 7-5.0 N 4
-1.78472ν4-25.7R8-43+, 72
D 8-10.0 N 5-1.77250ν5
-49. IiR9-133,00D9- o,5 RIO-299,99010-17,086-1,7+
299 Jiro 53.8R11--90,72Dll-0
,5 R12-734,12012-5,0N 7-1.47
171 shear 57.4RI3- '734.12 SK -186 (Aspheric coefficient) 5th surface 6th surface R106,67106,67 89,3+4x 10-' 1.010x 1O
-5C3,030x 10-93.808x 1O-9
D 1.904x 10-” 2.835x
10-12 12th surface 13th surface R734, 12734, 12 B ], 946X 10-62.324X In
-'C-4,265x 10-" 9.915x
10-”D-1,258X 10-13-1.5
+9x 10-+3fAL2 Tan 23400 fA
L3 diameter 665,000 ray effective diameter φ40 ray effective diameter φ90 (Dh)/SK10', 135 (thickness measured in the optical axis direction) HD ALII D ALII+5 3.
00050 5.0002410 3.00
g73 5.0039215 3.05+0
4 5.0207+20 3.19931
5.0689525 5, +7
84530 5.3927635
5.7698040
6.3787345 7.2
9] 03 Numerical Example 2 F-90, OFNo-1:4 2ω-62,8'
R1-203,27D I-5,0N I-1,7]2
99ν l-53,8R2-59,4502-65,0 R3-139,45D 3-5.0 N 2-1.
80518ν2~25.4R4~-84,9104-4
3,0 R5--146,29D 5-3.0 N 3”1
．． 49171 v 3-57.486-146.29
D 6-55.087~-69.93 0 7・
5. ON4~1.72825 ν 4・28,5R
8-197, 91D 8-1fi, ON 5-1.65
844shi5・50.9R9--84,2809-0,5 RIO-211,75DIG-14,0N 6-1.6
9680 Jiro 55.5R11-148,66Dll-
0,5 R12=-474,26DI2-5.0 N 7-
1.49171 v 7-57.4R13-474,2
6 SK -186 (Aspheric coefficient) 5th surface 6th surface R-146, 29-146, 29 B -1,952X 10-' -1,71
8x IQ-6C6,801X 10-” 5.7
31x 10-”D 2.532x 10-12
2.545x 10-" 12th surface 13th surface R-474, 26-474, 26 B 3.424X 10-' 3.659
X 10-'C-1,292x 10-th 1.5
42x 10-”D 1.899x 10-”
3.078x 10-”fAL2-44000
fAL3-278000 ray effective diameter φ40
Effective beam diameter φ9゜(Dh)/SK -0,146 (Thickness measured in the optical axis direction) HDALU DALm o 3 5 5 3.000+5 5.0001510
3.00224 5.0025215
3.01068 5.0139020 3
．． 03097 5.0491425
5.1374330 5.3
325935 5.7294240
6.4878445
7.86789 In Numerical Example 1, the fifth surface (
The focal lengths of the 21st aspherical lens composed of R5) and the 6th surface (R6) and the 31st aspherical lens composed of the 12th surface (R12) and the 13th surface (R13) are approximately fAL2= 23400. fAL3=665000, which is a sufficiently large value with respect to the focal length f=90 of the entire lens system. The effective diameter of the rays of the 21st aspherical lens and the 31st aspherical lens is approximately $40 each.
and 90, and in this range, the thickness of the aspherical lens measured in the optical axis direction increases as the distance from the optical axis increases, as described above.

FIG. 5 is an aberration diagram of Numerical Example 1, in which the distance from the first conjugate point to the Luns surface is M3000, and the back focus is 5K.
=186.

In Numerical Example 2, the fifth surface (R5) and the sixth surface (R6)
The 21st aspherical lens and the 12th surface (R12
) and the 13th surface (R13), the focal length of the 31st aspherical lens is approximately fAL2 = 44000, respectively.
．． fAL3=278000, which is a sufficiently large value with respect to the focal length f=90 of the entire lens system. The effective diameters of the rays of the 21st aspherical lens and the '1J31 aspherical lens are about 40 and 90 mm to the right, respectively, and the thickness measured in the optical axis direction of the aspherical lens in this range is far from the optical axis as described above. It is increasing according to the following.

FIG. 6 is an aberration diagram of Numerical Example 2, in which the distance from the first conjugate point to the Luns surface is 3113000. This shows a state where the back focus is 5K=186.

Numerical Example 1.2 is a retrofocus type in which the back focus SK is sufficiently long compared to the focal length f', and although it has a compact configuration, the 5th. As shown in FIG. 6, various aberrations including distortion are well corrected.

(Effects of the Invention) According to the present invention, by setting the lens configuration as described above, various aberrations such as distortion and flare can be effectively corrected with a relatively small number of lenses, and a back focus with high optical performance can be achieved. A retrofocus type lens suitable for a long color LCD projection television projection lens can be achieved.

Further, according to the present invention, by setting the refractive power of the aspherical lens provided with the aspherical surface as described above, it is possible to effectively prevent focus shift due to temperature changes when the lens is constructed from a plastic molded lens such as acrylic. It is possible to achieve a retrofocus lens having features such as the ability to reduce the weight of the entire lens system.

[Brief explanation of drawings]

FIGS. 1 and 2 are cross-sectional views of lenses of numerical embodiment 1.2 of the present invention, and FIG. 3. Figure 4 is an explanatory diagram regarding the aspherical shape;
5 and 6 are respectively shown at a projection magnification of −3/100 times of Numerical Example 1.2 of the present invention. FIG. FIG. 7 is a block diagram of a conventional color liquid crystal projector. In the figure, LFL is a retrofocus type lens, S is a screen, F is a projected image, F is a first group, ■ is a second group, ■ is a third group, 1 is a light source, 2a, 2b. 2c is a liquid crystal display element, 3a and 3b are mirrors, 4 is a red-reflecting dichroic mirror, 5 is a string-reflecting dichroic mirror, 6 is a blue-transmitting dichroic mirror, 7 is a projection lens, ΔS is a sagittal image plane, and ΔM is a meridional image plane. be. Patent applicant: Canon Corporation Figure 1 Figure 2 Figure 3 Figure 5 Figure 6

Claims (4)

[Claims] (1) It has multiple lenses, and is divided into three lens groups, the first, second, and third groups, in order from the first conjugate point side with the longest distance, and the focal length of the entire system is fT, and the distance is the second shorter of
The distance from the lens surface on the conjugate point side to the second conjugate point is SK
A retrofocus type that satisfies SK>0.7fT when lens. (2) When the focal lengths of at least one aspherical lens provided in the second group and the third group are fAL2 and fAL3, respectively, the following conditions are satisfied: |fAL2|>3fT |fAL3|>3fT The retrofocus type lens according to claim 1. (3) The aspherical lens provided in the second group has a shape in which the thickness measured in the optical axis direction increases within the effective area as the distance from the optical axis increases. retro focus lens. (4) The aspherical lens provided in the third group has a shape in which the thickness measured in the optical axis direction increases within the effective area as the distance from the optical axis increases. retro focus lens.