JPH02284108A - Retrofocus type lens - Google Patents

Abstract

PURPOSE:To attain the higher performance with the smaller number of lens elements by consisting the above lens of a 1st lens group having >=1 pieces of negative meniscus lenses and a 2nd lens group having >=2 pieces of positive lenses including >=1 faces of joint surfaces and specifying the shape coeffts. of the lenses and the power of the joint surfaces of the 2nd lens group to a range of specific values. CONSTITUTION:This lens is constituted of the 1st lens group 1 having >=1 pieces of the negative meniscus lenses and the 2nd lens group 2 having >=2 pieces of the positive lenses including >=1 faces of the joint surfaces. The lens is so formed as to satisfy the conditions -3<S1<-1, -7<S2.F<-0.2, -0.5<S2.R<0.5, PHI2.X<0 where the shape coefft. of the negative meniscus lenses of the 1st lens group 1 is designated as S1, the shape coefft. of the positive lens of the 2nd lens group 2 in proximate to the 1st lens group side as S2.F, the negative meniscus lens of the positive lens of the 2nd lens group in the position apart from the 1st lens group as S2.R and the power of the joint surfaces of the 2nd lens group as PHI2.X. The fairly large back focus is obtd. in this way and various aberrations are well corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a retrofocus lens, and particularly to a lens whose back focus is considerably larger than its focal length.

[Conventional technology]

In a color TV imaging camera, a three-color separation prism is provided on the image plane side of the lens, and a lens with a large back focus is required.

Also, in color scanners, when a positive or negative film image is imaged by an imaging lens onto a line sensor or area sensor, the colors are separated by a three-color separation prism. After all, a lens with a large back focus is required.

Furthermore, dichroic prisms and other devices are used in color LCD projection televisions, in which three LCD panels are illuminated with three-color light of B, G, and R, and the images on the LCD panels are projected onto a screen using a projection lens for viewing. A color synthesis system such as a dichroic mirror is provided between the liquid crystal plate and the projection lens, and a lens with a large back focus is also required here.

On the other hand, retrofocus lenses are conventionally known as lenses with a large back focus.

[Problems to be Solved by the Invention] However, conventional retrofocus lenses have a large number of lenses, making them heavy and expensive. Furthermore, even if the lens has a small number of lenses, the second positive lens group consists of a first negative lens group and a second positive lens group. Many of them use a lens type based on the triplet type, and the radius of curvature of each lens surface is sharp (therefore, the higher-order aberrations generated on each lens surface are also large, and the tolerance during manufacturing is also large). I also had to suppress it severely.

Although the present invention is a retrofocus lens with a long back focus, it is intended to achieve high performance with a small number of lenses.

[Means for solving problems]

The present invention includes a first lens group having negative power in order from the large conjugate side, and a second lens group having positive power separated from the first lens group by the largest air gap in the entire system. In a retrofocus lens composed of a lens group, the first lens group has one or more negative meniscus lenses, and the second lens group has two lenses including one or more cemented surfaces.
The negative meniscus lens in the first lens group has a shape factor of 81, and the positive lens in the second lens group closest to the first lens group has a shape factor of 82. ．． , and the shape factor of the positive lens provided in the second lens group at the position farthest from the first lens group is 82.
RsThe power of the cemented surface of the second lens group is Φ3. ．． When SI*S 2F+S 2. l+ and Φ26
K is -3<S, <-1(1) 7×S2・,<-0.2(2) −0.5<S1,,<0.5 (3) Φtx
By satisfying the conditional expression (4), a retrofocus lens with a considerably large back focus (and various aberrations well corrected) has been achieved.

Here, the shape factor S is defined as S = (R11+RA)/(R, -RA), where the radius of curvature of the large conjugate side surface of the applicable lens is RA%, and the radius of curvature of the opposite surface is R1. .

The aberration correction effect of the present invention will be explained below.

In the present invention, the first lens group is a negative meniscus lens that is convex toward the large conjugate side, and the second lens in the second lens group is a convex lens with a sharp curvature on the surface opposite to the large conjugate side. Since the third lens is a biconvex lens with approximately the same radius of curvature, the lens shape is basically thick so that off-axis aberrations do not occur.Also, the positive lens in the second lens group has a negative By providing a cemented surface with power, it is possible to correct not only spherical aberration, coma aberration, and astigmatism, but also negative distortion and lateral chromatic aberration that tend to occur in lenses with large back focus. The shapes of the second and third lenses themselves are lens types that produce a large amount of spherical aberration, but they are designed so that they can be effectively offset by the cemented surface of the positive lens in the second lens group and the concave surface of the first lens group. There is.

Conditional expressions (1) to (4) express the concept of such aberration correction.

A lens shape that exceeds the upper limit values of conditional expressions (1) to (3) will produce large off-axis aberrations, especially negative distortion.

A lens shape that exceeds the lower limits of conditional expressions (1) to (3) is advantageous for off-axis aberrations, but increases higher-order spherical aberrations and comatic aberrations.

In particular, when the upper limit of conditional expression (2) is exceeded, it becomes difficult to reduce spherical aberration, coma aberration, astigmatism, and negative distortion and chromatic aberration of magnification that tend to occur in lenses with large back focus. .

The retrofocus type lens described in the Examples is particularly effective when it has a back focus of the following condition: 1.5<BF<2.8 (5). In other words, a lens with a long back focus that exceeds the lower limit is effective. However, with a lens having a long back focus exceeding the upper limit, negative distortion and lateral chromatic aberration cannot be corrected to a small value.

Conditional expressions (6), (7), and (8) are conditional expressions that should be further considered when the back focus is set to be considerably thicker than the focal length.

-2<f, 7f<-0.7 (6) Q, 8<f, /f<1.7 (7) 1<E/
f <2.5 (8) However, the focal length of the first lens group is f', and the focal length of the second lens group is f2.
, the focal length of the entire system is f, and the principal point interval between the first lens group and the second lens group is E.

If the upper limit values of conditional expressions (6) and (7) and the lower limit value of conditional expression (8) are exceeded, the back focus cannot be large.On the other hand, the lower limit value of conditional expressions (6) and (7) If the upper limit of conditional expression (8) is exceeded, it becomes difficult to correct distortion and chromatic aberration of magnification unless the number of constituent lenses is significantly increased.

In order to correct distortion even better, a cemented surface may be introduced into the negative meniscus lens of the first lens group so that the cemented lens satisfies conditional expressions (9) and (10).

-2<R1,X<-0.7 (9) 0.08<N, A-N1,<0.45 (10)
However, the negative meniscus lens in the first lens group has a concave cemented surface on the large conjugate side, and the radius of curvature of the cemented surface is R1,
Let NIA and N111 be the refractive index of the lens on the large conjugate side of the Xs cemented lens and the other lens.

If the lower limit values of conditional expressions (9) and (10) are exceeded, the effect of the joint surface becomes weak (and the introduction becomes meaningless.On the other hand, if the upper limit value of conditional expression (9) is exceeded, the under one direction of the joint surface A large amount of astigmatism occurs.

〔Example〕

FIG. 1 shows a first embodiment of the invention.

In this embodiment, a first lens group having a negative power in order from the large conjugate side, and a first lens group having a positive power separated from the first lens group by the largest air gap D3 in the entire system. It is composed of two lens groups. The first lens group is a convex lens and a concave lens cemented together when viewed from the large conjugate side.
The first lens is a cemented lens having a concave cemented surface on the large conjugate side. The cemented lens as a whole is a large negative meniscus lens convex on the conjugate side.

The second lens group consists of a concave lens and a convex lens cemented together when viewed from the large conjugate side, and the second lens is a cemented lens that has a convex cemented surface on the large conjugate side and has positive power, and the third lens is a biconvex lens. has been done.

In this example, the second lens in the second lens group is a convex lens with a sharp curvature on the surface opposite to the large conjugate side, and the third lens is a biconvex lens with relatively weak power, so off-axis aberrations can be reduced. This is a lens type in which the occurrence basically does not increase.

In addition, in this embodiment, the second lens in the second lens group has negative power and has a convex cemented surface on the large conjugate side, so that in addition to spherical aberration, coma aberration, and astigmatism, Negative distortion and chromatic aberration of magnification, which tend to occur in lenses with large back focus, are corrected to a minimum. The shapes of the second and third lenses are lens types that produce a large amount of spherical aberration, but the cemented surface of the second lens and the concave surface of the first lens group successfully correct it.

When a convex cemented surface is provided on the large conjugate side in the second lens group, the radius of curvature of the cemented surface is R! ,X
If it is set in the range of 0.9<R1, X/f<5, aberrations can be corrected better.

In other words, if the radius of curvature R2, Lateral chromatic aberration can be corrected to a smaller extent.

In this embodiment, in addition to the second lens in the second lens group, the first
The first lens in the lens group is also a cemented lens, and has a concave cemented surface on the large conjugate side, and the refractive index of the positive lens is thicker than the refractive index of the negative lens. The negative distortion that tends to occur in lenses is further corrected there.

Furthermore, the Abbe number of the positive lens is made smaller than that of the negative lens, thereby further correcting the negative chromatic aberration of magnification that tends to occur in lenses with a large back focus.

[Other Examples]

FIG. 3 shows a second embodiment of the invention.

In this embodiment, a first lens group having a negative power in order from the large conjugate side, and a first lens group having a positive power separated from the first lens group by the largest air gap D3 in the entire system. It is composed of two lens groups. The first lens group, which is a negative lens group, is made up of a convex lens and a concave lens cemented when viewed from the large conjugate side, and the first lens is a cemented lens having a concave cemented surface on the large conjugate side.

The cemented lens as a whole is a large negative meniscus lens convex on the conjugate side. The second lens group, which is a positive lens group, is made up of a concave lens and a convex lens cemented when viewed from the large conjugate side, and the second lens is a cemented lens that has a convex cemented surface on the large conjugate side and has positive power, and a large conjugate lens. The third lens is a convex lens with a sharp radius of curvature on the opposite side, and the fourth lens is a biconvex lens with substantially the same radius of curvature.

In an optical system that uses a dichroic mirror system for three-color separation for light projection, color unevenness occurs unless the chief ray of the large conjugate-side off-axis beam is made substantially parallel to the optical axis. In this example, the first
Although the angle of the principal ray of the off-axis light beam is made smaller than in the embodiment, the number of positive lenses in the second lens group is increased by one to reduce the deterioration of aberrations. The increased shape of the third lens increases its shape factor to 82. In this case, S 21M should satisfy the condition -3<SaoM<-0.5. If the lower limit value is exceeded, spherical aberration and coma aberration will be excessively large (or occur).On the other hand, if the upper limit value is exceeded, distortion will be excessively large.

FIG. 5 shows a third embodiment of the invention.

In this embodiment, a first lens group having a negative power in order from the large conjugate side, and a first lens group having a positive power separated from the first lens group by the largest air gap D3 in the entire system. It is composed of two lens groups. The first lens group is composed of one large negative meniscus lens convex on the conjugate side. The second lens group includes a second lens which is a biconvex lens, and a third lens which is a cemented lens in which a convex lens and a concave lens are cemented when viewed from the large conjugate side, and has a concave cemented surface on the large conjugate side and has positive power as a whole. Consists of lenses. The cemented surface of the third lens has negative power. If the cemented surface provided in the second lens group is concave on the large conjugate side, aberration correction will be better if it is set in the range -1<R2, xc/f<-0.4, regardless of the lens. can.

In other words, if the radius of curvature R1,x of the cemented surface is looser than the upper limit value, the occurrence of higher-order spherical aberration and comatic aberration can be reduced, whereas if the radius of curvature R2xc of the cemented surface is tighter than the lower limit value, distortion aberration and magnification can be reduced. Chromatic aberration can be corrected to a smaller extent.

Figure 7 shows how the lens according to the present invention is applied to a color liquid crystal projection television in which three liquid crystal panels are illuminated with B, G, and R lights and projected onto a screen using a projection lens for viewing images. It is. In a color liquid crystal projection television as shown in FIG. 7, a guichroic prism P for three-color synthesis is provided between the liquid crystal plates L1 to L and the projection lens. 81-S are illuminators. If such a guichroic prism P is in the optical path, spherical aberration and astigmatism will occur in the over direction, and lateral chromatic aberration will occur in the under direction, but optimization within the above conditional expressions is not possible. This is possible using normal methods.

Table 1 Numerical example 1 R1= R2= R3= R4= R5 engineering R6= R7= R8= 203,035 -87.888 34, 503 o, oo.

117, 910 -76, 394 176, 859 166, 705 D1 duck 7.00 02 = 5.00 D 3 = 115.72 D4 = 4.50 D5 = 12.00 06 = 1.70 D7 = 8.0O N 1=1.66446 ν 1=35.8N
2= 1.51633 2=64.1N 3=
1.80518 ν 3=25.4N 4=1.
51633 ν 4=64.1N 5= 1.51
633 ν 5=64.1 AJ ・← Aberration coefficient L of Example 1 0.005459 0.012230 −0.017497 0.035360 −0.1108.95 0.031858 G, 013551 0.028448 −0.001485 0.010088 -0.016824 0.005445 0.026415 -0.027600 -0.002665 0.00821? -0.002537 0.000540 S^ 0.014644 0.170229 -6.551038 0゜557636 -5.421313 5.261854 0.395738 6.784163 1.211911 M 0.027059 -0.23 4176 2.038828 0 .416567 -1.349298 -0.440103 0.239959 -0.604915 0.093919 ^S 0.050001 0.322145 -0.634528 0.311185 -0.335824 0.036810 G, 145501 0.053938 -0 .050773 T 0.145580 0.049445 -0.730734 o, ooooo.

-0.066264 0.330027 0.142555 0.151239 0.021847 S 0.361406 -0.511178 0.424900 0.232462 -0.100075 -0.030682 0.174665 -0.018295 0. 533203 Table 3 Numerical Example 2 R1= 133.626 R2= -71,128 R3= 32.976 R4= -151,374 R5= 272.020 R6= -74,496 R7= 1171.955 R8= -118,255 R9= 270.798 RIO = -208,156 D1 = 11.00 02 = 3.50 D 3 = 114.95 D4 = 4.00 D5 = 12.00 D6:0.70 D7 = 7.00 D8 = 0.50 D9= 7.0O N 1= 1.65016 ν 1=39.4N
2 = 1.5097? C2=62.1N 3=
1.80518 ν3=25.4N 4= 1.5
1633 Si4=64, lN5=1.5163
3 ν 5=64.1 N 6= 1.51633 ν 6=64.1 Aberration coefficient of Example 2 -Nα M M ^S T S 0.007235 0.011655 -0.017227 -0.013791 0. 073619 0.00?639 0.022598 0.003510 0.023410 0.015435 0.006425 0.021663 0.025506 0.003250 0.007652 0.007093 -0.002336 0.0465 27 0.320625 5.703299 0 .036130 -2.416369 1.253386 0.179502 2.729871 -0.007167 0.041151 -0.424608 0.182608 0.179806 0.014484 -0.5<S2・R<0. ．． 1548960.0363
96 0.562313 -0.793365 0.089149 -. 0 290050 0.026605 0.180111 0.029271 0.045388 0.218731 0.058772 -0.759574 -0.218589 -0.028778 0.339084 0.021554 0.213611 0.09 3282 0.225648 -0. 822511 0.203325 -0.053278 -0.032234 -0.016637 0.006285 Q 000132 0.998353 0.034751 0.129668 0.059446 0.304420 Table Numerical Example 3 R1 = 327.52 6 22 people 37. 704 R3・ 200.684 R4= −81,132 R5= 369.955 R6・ −55,298 R7= −224,523 D1= 5.00 02= 125.25 D3= 12.00 D4= 5.00 D5 = 12.00 D6= 4.0O N 1= 1.49700 ν 1=81.6 N 2= 1.64769 ν 2=33.8 N 3= 1.49700 ν 3281.6N
4=1.84666 ν 4=23.9 Example 3
Aberration coefficient L A M S T S 0.001227 -0.013941 0.066453 0.130532 -0.008559 0.276774 0.100136 0.005012 -0.005654 0.064226 0.083559 0.0015 51 -0 .197193 0.053808 0.003751 -7.160670 6.060432 38.890549 -0.665725 -42.539764 8.085464 0.015329 -2.904065 5.857303 24.895462 1 〇、120635 -30.308273 0.066407 169263 0.151109 0. 562463 -0.741913 5.611328 10.431214 0.008072 -15.505456 -0.000926 0.002207 0.674024 0.705124 0.624033 -0.024853 [Effect of the invention] Retro according to the requirements Focus type lenses are
As a lens with a significantly large back focus, various aberrations can be well corrected even with a small number of constituent lenses, and a lens that is small in weight and inexpensive can be achieved.

In addition, the retrofocus type lens that complies with each condition can have a relatively gentle radius of curvature of each lens surface. Therefore, the higher-order aberrations occurring on each lens surface are small, the manufacturing tolerances are relatively loose, and the lens is easy to mass-produce.

[Brief explanation of drawings]

1 is a cross-sectional view of a lens according to a first embodiment of the present invention, FIG. 2 is a diagram of its aberrations, FIG. 3 is a cross-sectional view of a lens of a second embodiment of the present invention, FIG. The figure is a sectional view of a lens according to a third embodiment of the present invention, FIG. 6 is an aberration diagram thereof, and FIG. 7 is an example of a system in which the lens of the present invention is used. In the figure, 1 is the first lens group, 2 is the second lens group, Ri is the radius of curvature, Di is the distance between lens surfaces, M is the meridional image surface, and S is the Santal image surface. Fig. 2 Correction and aberration #μ point Volcanic cliff distortion U and cliff failure LJ - Cheer [0 cough aberration l5fij Distortion Aberration Fig. 6 5t @ u S 1E Non-fjL aberration next - f! g blind

Claims (1)

[Claims] (1) A first lens group having a negative power in order from the larger conjugate side, and a positive lens group that is separated by the largest air gap in the entire system. 2nd with power
In a retrofocus lens composed of a lens group, the first lens group has one or more negative meniscus lenses, and the second lens group has two or more lenses including one or more cemented surfaces. The shape factor of the negative meniscus lens in the first lens group is S_1, and the shape factor of the positive lens provided closest to the first lens group in the second lens group is S_1.
2_・_F and the shape factor of the positive lens provided in the second lens group at the position farthest from the first lens group is S_2
＿・＿R, the power of the cemented surface of the second lens group is Φ_2_
・When _X, S_1, S_2_・_F, S_2_
- A retrofocus lens characterized in that _R and Φ_2_・_X satisfy the following conditional expression. -3<S_1<-1 -7<S_2_・_F<-0.2 -0.5<S_2_・_R<0.5 -Φ_2_・_X<0 Here, the shape factor S is the large conjugate side of the corresponding lens. When the radius of curvature of the surface is R_A and the radius of curvature of the opposite surface is R_B, it is defined as follows: S=(R_B+R_A)/(R_B-R_A). (2) In claim 1, a retrofocus lens whose BF satisfies the following conditions, where the air-equivalent back focus of the lens system is BF. 1.5<BF<2.8 (3) In claim 1, the focal length of the first lens group is f_1, and the focal length of the second lens group is f_2.
, a retrofocus lens where f_1, f_2, and E satisfy the following conditional expression, where f is the focal length of the entire system and E is the principal point interval between the first lens group and the second lens group. . -2<f_1/f<-0.7 0.8<f_2/f<1.7 1<E/f<2.5 (4) In claim 1, the first lens group The negative meniscus lens has a concave cemented surface on the large conjugate side, the radius of curvature of the cemented surface is R_1_・_X, and the refractive index of the large conjugate side lens and the other lens of the cemented lens is N
When _I_A and N_I_B, R_1_・_X,
A retrofocus lens where N_I_A and N_I_B satisfy the following conditional expression. -2<R_1_・_X<-0.7 0.08<N_I_A-N_I_B<0.45