JPH03145613A - Retrofocus lens - Google Patents

Abstract

PURPOSE:To obtain the lens which is free from a deficiency in the quantity of light by specifying the refracting power arrangement of a 1st lens group which has negative refracting power, a 2nd lens group which has the largest lens thickness and positive refracting power, and a 3rd lens group which has a 3rd lens group which has positive refracting power from a large conjugation side. CONSTITUTION:Inequalities I-III hold, where f1, f2, f3, and (f) are the focal length of the 1st lens group 11 which is positioned on the large conjugation side and has the negative refracting power, the 2nd lens group 12 which has the positive refracting power, the 3rd lens group 13 which is positioned on a small conjugation side and has the positive refracting power, and the whole system, l1 is the air gap between the 1st lens group 11 and 2nd lens group 12, and l2 is the air gap between the 2nd lens group 12 and 3rd lens group 13. Consequently, the quantity of off-axis light never decreases.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a retrofocus type lens suitable as a projection lens for a color liquid crystal projection TV.

[Conventional technology]

First, Fig. 7 shows a color liquid crystal projection T that projects an image formed on a general liquid crystal display element onto a screen.
The block diagram of a V receiver is shown. 1 is a white light source that emits collimated light. 2 (2a, 2b, 2c) are liquid crystal display elements, 3 (3a).

3b, 3c) are reflective mirrors, 4.5 are blue reflective dichroic mirrors, green reflective dichroic mirrors, 6
The main body A is composed of a dichroic prism. 7 is a projection lens, and 8 is a screen. In such a configuration, it is necessary to arrange an optical element such as a dichroic prism for color image synthesis between the final surface of the projection lens and the liquid crystal display element (between the back focuses), so a long back focus is required. must be ensured. In addition, when used as an ablation TV receiver in which the projected image is observed from the back side of the screen, it is necessary to downsize the entire device and obtain a viewing angle as wide as possible by G in order to obtain a large screen. .

On the other hand, as a wide-angle lens with a long back focus,
- Retrofocus lenses used as wide-angle lenses for eye reflex cameras are known.

[Problem to be solved by the invention] However, since the retrofocus type lens used in conventional single-lens reflex cameras has a configuration in which the incident angle and exit angle of off-axis light beams are tight, it is difficult to use color LCD projectors for TVs. When used as a lens, the off-axis light beam from the liquid crystal display element is emitted as a substantially parallel light beam with respect to the optical axis, resulting in a disadvantage that the amount of off-axis light is reduced.

In view of these problems, an object of the present invention is to secure a relatively long back focus, and to maintain a wide angle while maintaining a relatively long back focus.
It is an object of the present invention to provide a trophofocus type lens having good optical performance and configured such that an off-axis light beam is incident from a small conjugate side as a substantially parallel light beam to the optical axis.

[Means for Solving the Problems] Therefore, the present invention provides a first lens group having a large conjugate side lens group having a negative refractive power, with the second lens group being a positive lens having the largest lens thickness as a boundary, The small conjugate side lens group is composed of a third lens group having positive refractive power, and the focal lengths of the first lens group, second lens group, third lens group and the entire system are f1, f2, . fs, f, and the first
When the air distance between the lens group and the second lens group is 41, and the air distance between the second lens and the third lens group is 42, 0.5<f2/f3<0.9 (1) 0.9<l
f+ l/f<1.2 (2) 1.0< A +
/ f2□<1.5 (3) It is configured to satisfy the following condition.

[Example] The retrofocus type lens of the present invention will be described below with reference to the drawings.

1 to 3 are cross-sectional views of lenses corresponding to numerical examples of the present invention, which will be described later. Although not shown in the figure, it is assumed that there is a screen on the left side.

11 is a first lens group located on the screen (large conjugate) side and has a negative refractive power; 12 is a second lens group having a positive refractive power; in reality, it is a single lens or a cemented positive lens; 13 is not shown This third lens group is located on the original image (small conjugate) side of the liquid crystal, etc., and has positive refractive power. In this example, the first lens group has one positive lens and two negative lenses, and the third lens group has one negative lens and three or more positive lenses.

In order to achieve the above-mentioned purpose, conditional expressions (1), (2), (
3) is satisfied, and the meaning of the extreme value will be explained below.

First, under the condition of formula (1) that determines the ratio of the focal lengths of the second lens group 12 and the third lens group 13, if the upper limit of formula (1) is exceeded, the third lens group 13
Since the refractive power sharing becomes too strong, if an attempt is made to adopt a configuration in which an off-axis light beam is incident on a substantially parallel light beam from the small conjugate side, the lens outer diameter of the third lens group 13 generally tends to increase. , off-axis distortion seen on the small conjugate side occurs in a large barrel shape, making it difficult to correct it. On the other hand, if the lower limit of equation (1) is exceeded, the positive refractive power share of the third lens group 13 on the small conjugate side is too weak compared to the second lens group 12, so the retrofocus type structure becomes weak. This makes it difficult to maintain back focus for a long time.

Also in conditional expression (2) that determines the ratio of the focal length of the first lens group 11 to the entire system, if the upper limit of expression (2) is exceeded, the negative refractive power sharing of the first lens group 11 on the large conjugate side becomes weak. This makes it difficult to maintain hack focus for a long time because the retrofocus type composition becomes too weak. In addition, in order to configure the refractive power of the first lens group in this way and maintain a long back focus, it is necessary to provide a large distance between the second and third lens groups, which have positive refractive power on the small conjugate side. Also, the total length of the lens becomes longer, and the outer diameter of the lens of the 1171st group increases accordingly, which is not good. On the other hand, if the lower limit of equation (2) is exceeded, the refractive power of the 1171st group becomes too strong, and a large amount of astigmatism occurs off-axis, making it difficult to correct it.

Conditional expression (3) is a condition that appropriately determines the ratio of the air spacing between the first lens group 11 and the second lens group 12 and the air spacing between the second lens group 12 and the third lens group 13. If the upper limit is exceeded, it is advantageous to maintain the back focus for a long time, but the outer diameter of the first lens group 11 becomes large, and large barrel-shaped distortion aberration occurs in the first lens group 11 on the small conjugate side. Furthermore, since outward coma aberration occurs off-axis, it is difficult to correct it. On the other hand, if the lower limit of equation (3) is exceeded, the retrofocus type structure becomes weak and it becomes difficult to maintain hack focus for a long time. Although the purpose can be achieved with the above configuration, the design becomes easier if the following conditions are met.

The lens thickness of the second lens group is 4', the radius of curvature on the large conjugate side of the second lens group is r, and the radius of curvature on the small conjugate side is r'
When, 1.8 < f 3/ f < 2.5
(4) 1.5<(I2.+42+43') /f<2
．． 0 (5) 1.5 < l r/r'l < l1/
l. By configuring the lens to satisfy the condition (6), it is possible to easily achieve a trofocal lens with a long back focus.

In the condition of conditional expression (4) which determines the ratio of the focal length of the third lens group 13 to the entire system, if the upper limit of the expression (4) is exceeded, the focal length of the third lens group 13 becomes too large.
In order to satisfy the condition of formula (1), the focal length of the second lens group [2] must also be increased, which is not a good idea since the refractive power of each lens group becomes weaker overall and the total length of the lens becomes longer. Conversely, if the lower limit of equation (4) is exceeded, the third
The refractive power of the lens group 13 becomes too strong, and the third lens group 1
3, a large off-axis distortion aberration seen on the small conjugate side occurs in a barrel shape, and furthermore, the image plane is largely curved toward the lens side, making it difficult to correct it.

Conditional expression (5) is a condition for appropriately determining the distance between the first lens group 11 and the third lens group.If the upper limit of expression (5) is exceeded, the total length of the lens becomes longer and the distance outside the lens of the 1171st lens group is exceeded. This is not good because the diameter becomes large and the 1171st group produces a large barrel-shaped off-axis distortion when viewed from the small conjugate side.

On the other hand, if the lower limit of equation (5) is exceeded, it is difficult to create a configuration in which the off-axis beam is incident on the small conjugate side into a substantially parallel beam, and in this case, the symmetry of the entire lens system is broken, resulting in comatic aberrations and non-linear beams. Point aberration occurs, and its correction becomes difficult. Conditional expression (6) is a condition that appropriately determines the ratio of the radius of curvature on the large conjugate side and the radius of curvature on the small conjugate side of the second lens group.
2, the spherical aberration that occurs becomes large, and it becomes difficult to correct it. On the other hand, if the lower limit of equation (6) is exceeded, the principal point of the second lens group 12 will be too close to the large conjugate side, making it difficult to maintain the back focus for a long time.

In order to maintain even better optical performance, the first lens group 11 is composed of a positive lens and at least two negative lenses from the large conjugate side, and the third lens group is composed of a negative lens and at least three negative lenses from the large conjugate side. It is best to use a positive lens. Here, the positive lens of the 1171st lens group and the negative lens of the 3rd lens group have the effect of generating small off-axis distortion in a string-shaped manner when viewed from the conjugate side, and the barrel-shaped distortion that tends to occur with wide-angle lenses. Corrects distortion aberration. Also, the 1171st
By having at least two negative lenses in the group, barrel distortion and astigmatism that tend to occur in the 1171st group are corrected. Further, by including at least three positive lenses in the third lens group, it is possible to easily construct a configuration in which an off-axis light beam is incident on a substantially parallel light beam on the small conjugate side while maintaining a wide angle of view.

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 larger conjugate side, Di is the thickness and air gap of the first lens, Ni and ν
i is a number corresponding to the refractive index of the i-th lens.

Numerical Example F = 52.24 FN○ 1:4.6 2W = 82.4° R1 = 75.295 82 = 274.322 R3 = 52.902 R4 = 26.101 R5 = 91.299 R6 = 30. 253 R7= 166,015 R8= 128.580 89= -48,651 R10= -70,683 R11= 292.933 R12= -264,512 R13= -57,568 R14= -338,554 R15= -90 ,480 R+6= -1147,935 R17= -121,011 R18= 472.769 R19= -175,651 R20= ■ R21: ω DI= D2= D3= D4= D5= D6= 07= D8= 09= D10 = D11= D12= D13= D]4= D15= D16= D17= D18= D19= D20 Near, 15 0.20 3.00 6.68 2.00 25, 65 30, 00 10.20 23, 20 4 .00 3.12 8.55 0.20 7.05 0.20 7.35 20 8.10 10,00 70,0O N 1=1.51633 v 1=64.1 N 2=1.60311 v 2 =60.7 N 3=1.60311 ν 3=60.7 N 4=1.80518 N 5=1.59551 ν 4=25.4 ν 5=39.2 N 6=1.84666 C 6=23 .9 N 7 = 1.603] 1 v 7 = 60.7 N 8 = 1.51633 υ B = 64.1 N 9 = 1.51633 ν 9 = 64.1 NIO = 1.51633 S1O = 64. ]N
11=1.51633 C]1=64.1 Numerical Example 2 47, 52 N0 14.1 2W=78.54°R1= 66.913 R2= 273.967 R3= 42.517 R4= 24.795 R5 = 156.262 R6= 32.812 87= 114.570 R8= -55,433 R9= -72,075 81O= 217.095 R11= -185,067 812= -60,954 RI3=29376,059 R14= -105,022 R15= 323.245 R16= -138,138 R17= 00 R+8= ■ DI= 02= D3= D4= 05= D6= 07= 08= 09= D10= 011= D12= D13: DI4= D15 = D16= DI7= 9.79 0.20 4.00 8.33 3.00 26, ll 36, 69 19, +2 00 3.90 6.70 0.20 7.38 0.20 8.72 +0. 00 70.0O Nl=1.51633 N2=1.60311 N3=1.6968O N4=1.6031] N5÷1.84666 N6=1.7725O N7=1.6968O N8=1 .6968O N 9=1.51633 ν I=64.1 v2=607 C 3=55.5 v 4=60.7 C 5=23.9 ν 6=49.6 C 7=55.5 ν 8= 55.5 ν 9=64.1 Numerical Example 3 F =52.89 NO 1:4.5 7260°R1= R2= R3= R4= R5= R6= R7= R8= R9= R10= R11= R12= R13= R14= R15= R16= R17= R18= 65,066 270,068 39.590 23,032 169,785 37.461 112,605 -54.490 -63.518 223,189 -220.847 -67 , 794 3587, 852 +10.554 382, 818 -133.997 0 D1= D2= D3= D4= 05= D6= D7= D8= D9= D10= D1]= D12= 13- 014= D15= D16: D17 = 8.50 0.20 4.00 8.42 3.00 28, 46 40.11 24, 51 4.00 3.50 6.15 0.20 83 0.20 7.92 10, 00 70, 0O N 1=1.49171 N 2=], 6180O N 3=1.7725O N 4=1.61484 N 5=1.84666 N 6=1.7725O N7=1.6968O N 8=1.6968O N 9= 1.51633 v I=57.4 1 no 2=63.2 v 3=49.6 v 4=51.2 ν 5=23.9 υ 6=49.6 C 7=55.5 v 8=55 .5 ν 9=64.1 [Effect of the invention] As explained above, from the large conjugate side, the first lens group with negative refractive power, the second lens group with positive refractive power and the largest lens thickness, and the positive refractive power By specifying the refractive power arrangement of a retrofocus lens composed of the third lens group of refractive power,
When used as a projection lens for a color liquid crystal projection TV, it is possible to achieve a lens that has a long back focus, a wide angle of view, and does not have a shortage of light off-axis.

[Brief explanation of the drawing]

FIGS. 1 to 3 are cross-sectional views of lenses of numerical embodiments 1 to 3 of the present invention, and FIGS. 4 to 6 are lens cross-sectional views of numerical embodiments 1 to 3 of the present invention.
It is a diagram of various aberrations seen on the small conjugate side at a magnification of 1/15 times of ~3. FIG. 7 is a block diagram of the projector. 2 is a light source, 2 is a liquid crystal display element, 3 is a blue reflective dichroic mirror 5, a green reflective dichroic mirror 6 is a graphical prism, 7 is a projection lens, and 8 is a screen. FNO/46 inside, 41.2" W = 41.26 -3.1llllllL[Il]C%): 3θriF 匈O/41 Wpu 39.27°1-W; 39.27°3θri 6 legs Ill[]Cn;ノ30ノ

Claims (1)

[Claims] Starting from the second lens group, which is the positive lens with the largest lens thickness, the large conjugate side lens group is the first lens group with negative refractive power, and the small conjugate side lens group is the positive lens group. It is composed of a third lens group having refractive power, and the focal lengths of the first lens group, second lens group, third lens group and the entire system are respectively f_1, f_2, f_3, and f. The air distance between the two lens groups is l_1, and the second lens group and the third lens group are
When the air distance between the lens groups is l_2, the following conditions are satisfied: 0.5<f_2/f_3<0.9 0.9<|f_1|/f<1.2 1.0<l_1/l_2<1.5 A retrofocus lens that is characterized by