JPS6278520A - Wide angle lens with aspherical surface - Google Patents

Abstract

PURPOSE:To satisfactorily correct the out-of-axis aberration such as astigmatic aberration, distortion aberration and coma aberration by composing of five lenses and specifying the position of a aspherical surface and its shape. CONSTITUTION:The titled lens has five lenses, any one of which has an aspherical surface where a negative refraction becomes weaker toward the periphery. Where the aspherical surface coefficient of said surface, the radius of curvature of the lens surface of the 3rd lens at an object side and the focal length of an entire system are phi, R5 and (f), respectively, inequalities 0.7<f<4>.phi/R5<22 can be satisfied. The aspherical surface of the 1st lens and the radius of curvature of the lens surface of the 3rd lens at the object side are set so as to satisfy said conditions, whereby the out-of-axis aberration such as the astigmatic aberration, distortion one, coma one and especially color coma aberration can be corrected with a good balance.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a wide-angle lens having an aspherical surface, and in particular, by specifying all the historical aspects of the aspherical surface, it is possible to improve the lens configuration while maintaining good optical performance. The present invention relates to a wide-angle lens having an aspherical surface suitable for simplified photographic cameras, video cameras, etc.

(Prior technology) Conventionally, a so-called retrofocus type camera has been adopted for shooting extremely wide angles of view, with two lens groups: a front group with negative refractive power and a rear group with positive refractive power. There are many things. Retrofocus type photographic lenses have the advantage of being able to maintain a long back focus, but because they have a lens configuration in which the light beam diverged in the front group is converged in the rear group, off-axis aberrations such as spherical aberration, astigmatism, and distortion occur. A large amount of aberration occurs. In general, it is very difficult to properly correct these various aberrations, as the lens structure is asymmetrical around the aperture, compared to a symmetrical, near-Icauss-type photographic lens. If an attempt is made to achieve this, many higher-order spherical aberrations occur, the field curvature increases, the flatness of the image plane of the entire screen collapses, and furthermore, the distortion aberration significantly increases in the negative direction.

In order to obtain good optical performance while keeping brightness and photographing angle of view constant, there are methods, for example, of increasing the number of lenses or weakening the refractive power of both the front group and the rear group. However, in all of these methods, the overall length of the lens is long, resulting in an increase in the size of the entire lens system. Back focus again? In order to obtain a sufficiently long distance, it is possible to increase the distance between the front group and the rear group, but if the distance is increased too much, the overall length of the lens becomes long, making it difficult to downsize the photographic lens.

Retrofocus type photographic lenses with an F number of z8, a photographing angle of view of 37 to 38 mm, and a relatively small number of 5 lens elements are arranged in Japanese Patent Laid-Open No. 54-12728 and Japanese Patent Laid-Open No. 57-163212. etc. have been proposed.

However, since the photographic lenses proposed in these publications have fewer lenses, chromatic comatic aberration and astigmatism remain toward the middle of the screen, and as the angle of view increases, chromatic aberration increases. ing.

(Problems to be Solved by the Invention) In order to achieve a 17i3 lens system, the present invention is constructed with approximately t-5 lenses. The object of the present invention is to provide a wide-angle lens having an aspherical surface that can satisfactorily correct off-axis aberrations such as aberrations, astigmatism, and distortion by using an aspherical surface.

(Means for solving the problem) A meniscus-shaped negative first lens with a convex surface facing the object side in order from the object side, a second lens with both V lens surfaces convex, an aperture,
The first lens has five lenses: a third lens whose both lens surfaces are concave, a positive fourth lens whose convex surface faces toward the image plane, and a positive fifth V lens whose convex surface is fully thickened toward the image plane. At least one lens surface of the lens is an aspherical surface having a shape in which negative refraction weakens toward the periphery, and the shape of the aspherical surface is an X-axis in the direction of the original axis, an H-axis in the direction perpendicular to the optical axis, and a gold ball in the direction of light propagation,
R is the paraxial radius of curvature, A.

When B, C, D, and g are each aspherical coefficients, + DH8
+ EH” is the radius of curvature t of the lens surface of the object law of the third lens.
-R5, the focal length of the entire system is 1/, the refractive index of the medium before and after the aspherical surface 'iN, N', the aspherical coefficient φ is φ-8・(N-N'
HB-A3) When 0.7 (f'・ψ/R5<2.12...
(1) The following condition must be satisfied.

The features of this shrub invention are described in the implementation column.

(Example) FIG. 1 is a cross-sectional view of a lens of a numerical embodiment 1 of the present invention, which will be described later.

In this implementation array, as mentioned above, five lenses of a predetermined shape are arranged, and at least one lens surface of the first lens is made an aspherical surface, thereby correcting various aberrations, especially off-axis aberrations, in a well-balanced manner as a whole. There is.

In a retrofocus type photographic lens, the larger the angle of view of the off-axis light beam is, the higher the angle of view of the off-axis light beam is incident on the negative lens located far ahead of the diaphragm. Therefore, the amount of off-axis aberrations such as astigmatism and distortion increases toward the periphery of the screen. Therefore, in this actual light array, one lens surface of the first lens has a shape that weakens the negative refractive power toward the lens periphery, that is, a shape that increases the curvature of the convex surface when applied to a convex surface, and a shape that increases the curvature of the convex surface when applied to a concave surface. Astigmatism and negative distortion are well corrected by using an aspherical surface with a shape that reduces curvature.

Furthermore, the burden of aberration correction by the rear group placed behind the aperture is reduced. In particular, the burden of comatic aberration is reduced by correcting it on the object-side lens surface of the third lens immediately after the aperture.

That is, in this implementation row, astigmatism and distortion are reduced by setting the aspherical shape of the first lens and the radius of curvature of the object-side lens surface of the third lens so that the above-mentioned conditional expression + satisfies l1,1. Comatic aberrations, especially extramarginal aberrations such as chromatic coma aberrations, are all corrected in a well-balanced manner.

If the conditional expression + exceeds the lower limit value of l1 and the amount of aspherical surface of the first lens decreases, the correction of negative distortion becomes insufficient. If the refractive power of the surface becomes too strong, coma aberration will be overcorrected.

The object of the present invention is achieved by satisfying the above conditions, but in order to further reduce the absolute amounts of spherical aberration and comatic aberration, the radius of curvature of the lens surface on the image side of the second lens can be reduced. When R4 is -6,0(R4// (-4,5...-......(
2) It is better to satisfy the following conditions. Conditional expression (If the lower limit of 21 is exceeded and the refractive power of the lens surface becomes weaker, spherical aberration will be insufficiently corrected; if the upper limit is exceeded and the refractive power of the lens surface becomes strong, the absolute amount of comatic aberration is getting bigger.

In addition, in order to reduce the diameter of the first lens while maintaining the effect of the aspheric surface applied to the first lens in this 7M series, the focal length of the first lens should be set to l1, from the aspheric surface to the aperture! ll
The distance to t is the focal point of the entire system. When the point distance is f, -t49 (f/f < -1,35...
...(3)1 <t□/no゛ <1.2
・・・・・・−・−・ (4) It is better to satisfy the following condition.

Lower limit Ii1 of conditional expression (3)! All 1! If the refractive power of all lenses becomes too strong, the back focus will become long, but the distortion will increase in the negative direction. Father's upper limit? First beyond
As the refractive power of the lens becomes weaker, the lens diameter increases and the back focus becomes shorter, making it more elegant to apply to photographic cameras and the like.

When the lower limit +ni' of conditional expression (4) is exceeded and the distance from the aspherical surface to the diaphragm becomes shorter, the height of incidence of the off-axis light beam on the aspherical surface becomes lower, and the effect of the aspherical surface becomes smaller. Furthermore, if the distance from the aspherical surface to the diaphragm V becomes too long beyond the upper limit @, the diameter of the first lens increases, which is not preferable.

Furthermore, in this embodiment, by setting the negative first lens and the positive second lens in front of the aperture at an appropriate distance from the aperture, distortion and astigmatism can be corrected in a well-balanced manner. That is, when the distance from the image side lens surface of the second lens to the aperture stop is t2 (Li2 < 12/l, < 0.28 ・
It is better to satisfy all conditions (5).

If the lower limit of conditional expression (5) is exceeded and the second lens approaches the diaphragm too much, the effect of the positive refractive power will be reduced and distortion will be insufficiently corrected. Furthermore, if the upper limit is exceeded and the second lens and first lens are brought too close together, astigmatism will be insufficiently corrected and the back focus will become short, which is not preferable.

Next, numerical implementation sequences of the present invention are shown. R in the numerical implementation sequence
i is the radius of curvature of the first lens surface from the object side, Di is the radius of curvature of the first lens surface from the object side, and air:;
The i distance, Ni, and νl are the refractive index and Abbe number of the glass of the if-th lens from the main side, respectively.

Numerical implementation sequence 1 F-1FNO-1: λ82ω-74°R1-1,742D 1-0.07 N 1-1.6
03 to l1, Shi1-60.7R2-Flash D 2-
0.66 R3-1,157D3~0.21 N2-L 78
590 Shi2-44.2R4--4,882D 4-
0.32 85~(Aperture) D5-0.13 R6--0.690 D 6-0.09 N
3-1.84666 v 3”219■77-λ
503 D 7-0.01R8-I as l1, 3.
'120 D 8-0.12 N 4-1.79
952 v ←4Z 2R9--0,722D 9
-0. 00R1()-7,814DIO-0,10N5-1.
78590 5-44.2all--1,=l
1, 4 R2: Aspherical surface A-0,8787 B-0,5712 C-0,5094 D-0,8715 -Z244 Number 1 center of gravity ρ1''2 F-I FNO- to l1, 82ω-74°R1
-1,747Di-0,07Nl-1,58313 1
-59.41R2- η D 2-0.67R
3-1,133D 3-0.23 N 2-1.7
8590 Shi2-44.2R← -4,691D
4-0. 26R5-(aperture) D 5-0. 18Rfly-0,695D 6=0.07N
3-1.84666 ν 3-2λ9R7-14
23D7-0.02 R8--5SL 291 08-0.12 N
4-1.79952 ν 4-4Z 2R9--0
,736D 9-0.00 RIO-10,345DIO-α09 N S-1
,78590ν 5-44.2R l1, --L 30
2 R2: Aspherical surface A-0,9041 B-0,6106 C-0,6058 D-1,081 E-λ173 Numerical example 3 F-I FNO-1: Z8 2ω-74°R1-
1,470Di-0,10N1-1.58313 Si1-59.4R2-60D2-0.68 R> 1. l1, 2 D 3-0.17 N 2
-1.80610 Shi2-40.9R4--4,70
4D 4-0. 17R5-(aperture) D 5-0.17R6=-0
,738D 6-0.13 N 3-L 8051
8 v 3-25.4R7-1028D? -α02 R8--75,432D 8-0. l1, N 4
-1.77250 Shi4-49.6R9--0,69
9D 9-0. 0XRio-7,225pio-o, 10N
5-1.77250 5-49.6R l1, -
-L 775 1t2: Aspherical A-0,9402B-0,680
5 C-L172 D--0,9720 z-1,0151 Real light train 4 F-I FNO-1:Z8 2ω-74°Rl-1,902D 1-0.10 N! -1,5
8313Si1-59.4R2-Sen D 2-0.
66 R3= 1.061 D 3-0.26 N
2-1.74320 y 2-49.3R4--4
,832D←0.30 R5-(Aperture T)) D5-0.14R6--0.
6 to l1, D 6”0.07 N 3-1.72
825 v 3-28.5R7-Z 128
D 7-0.02R8=61.449 D 8-0.
10 N 4-1.71300 y 4-53.
8R9--0,694D 9-0. 00RIO-7,814DIO-α09 N 54.
71300 v 5-53.8R l1,--1,
129 R2: Aspherical surface A-0,9066 B-0,5943 C-0,5010 D-1,0368 E-Z3070 Number 1 direct implementation uJ5 F-I FNO-1: λ8 2ω-746
R1-■ DI-0,07N 1-1.583
13 Si1-59.4R2-0,599D 2-0
．． 66 R3-t, 084 D 3-0.19 N 2
-1.74400 Shi2-447R4--4,31
4D 4-0.24 R5- (aperture r)) D 5-0.19R6--0.
640 D 6-0.10 N 3-1.84
666 ν 3-23.9R7-3,00707
-0,02 R Sea 6,250 D 8-0.10 N 4
-1.79952 Shi4-4λ2R9--0,69
8D 9-0.0X RIO-9,775 l1, 10-α09 N 5
-L 78590 Shi5-442R l1, --1
, 184 R1: Non-spherical j (I A-0, 2421B-0,0
7827 C-0,07954 D-9,525 mowing 0-3 E--1,465X10-2 (Effect of the invention) According to the present invention, the position of the aspherical surface and the By specifying the shape of the aspherical surface, it is possible to achieve a wide-angle lens having an aspherical surface with readability and excellent correction of off-axis aberrations such as astigmatism, distortion, and coma.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a lens of numerical value M14J1 of the present invention, and FIGS. 2 to 6 are various aberration diagrams of numerical examples 1 to 5 of the present invention, respectively. In the figure, S is a sagittal image plane, and M is a meridional image plane. Watch applicant Canon Co., Ltd.

Claims (1)

[Claims] (1) A meniscus-shaped negative first lens with a convex surface facing the object side in order from the object side, a second lens with both lens surfaces convex,
It has five lenses: an aperture, a third lens whose both lens surfaces are concave, a positive fourth lens whose convex surface faces toward the image plane, and a positive fifth lens whose convex surface faces toward the image plane. At least one lens surface of one lens is an aspherical surface whose negative refraction weakens toward the periphery, the aspherical coefficient of the aspherical surface is φ, the radius of curvature of the object-side lens surface of the third lens is R5, and the total A wide-angle lens having an aspherical surface, which satisfies the following condition: 0.7<f^4·φ/R5<2.2, where f is the focal length of the system. (2) The focal length of the first lens is f_1, the distance from the aspherical surface to the aperture is l_1, and the focal length of the entire system is f_1.
A wide-angle lens having an aspherical surface according to claim 1, which satisfies the following conditions: -1.49<f_1/f<-1.35 1<l_1/f<1.2 lens. (3) Claims characterized in that the following condition is satisfied: 0.16<l_2/l_1<0.28, where l_2 is the distance from the lens surface on the image side of the second lens to the aperture. 2. A wide-angle lens having an aspherical surface according to item 2.