JP2007225960A - Wide angle lens system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high-performance retrofocus type wide angle lens system which is thinner (smaller in size), whose aberrations, especially, coma aberration, distortion aberration and chromatic aberration are satisfactorily corrected, and whose viewing angle is about 70°. <P>SOLUTION: In the retrofocus type wide angle lens system comprising a negative front group and a positive rear group in order from an object side, the front group comprises a negative first-a lens group and a positive first-b lens group in order from the object side, and the rear group includes a positive lens and a cemented lens comprising a negative lens and a positive lens. The wide angle lens system satisfies following conditional expressions (1) and (2). (1) -0.8<f1a/f1b<-0.2, (2) 0.7<f/fR<1.1, provided that f1a; focal distance of the first-a lens group, f1b; focal distance of the first-b lens group, f; focal distance of the entire system, and fR; focal distance of the rear group. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a wide-angle lens system suitable for a digital single lens reflex (SLR) camera.

As a wide-angle lens system for a single-lens reflex camera that requires a relatively long back focus with respect to the focal length, a retro-focus type with a negative and positive arrangement from the object side is widely used, its power arrangement, lens configuration, Various proposals have been made on dimensional configurations and the like.
JP-A-6-160706 JP-A-9-113800

  The present invention is a retrofocus type wide-angle lens system, which is thinner and more compact, and has a high-performance wide-angle lens with a field angle of about 70 ° in which various aberrations, particularly coma, distortion, and chromatic aberration are well corrected. The goal is to obtain a system.

According to the first aspect of the present invention, in the retrofocus type wide-angle lens system including the negative front group and the positive rear group in order from the object side, the front group includes the negative first a in order from the object side. It consists of a lens group and a positive 1b lens group, and the rear group includes a positive lens, a negative lens, and a cemented lens of a positive lens, and satisfies the following conditional expressions (1) and (2).
(1) -0.8 <f1a / f1b <-0.2
(2) 0.7 <f / fR <1.1
However,
f1a: focal length of the 1a lens group,
f1b; focal length of the 1b lens group,
f: focal length of the entire system,
fR: focal length of rear group,
It is.

In the first aspect, it is preferable that the following conditional expression (3) is satisfied.
(3) 0.1 <d / f <0.3

According to the second aspect of the present invention, in the retrofocus type wide-angle lens system including the negative front group and the positive rear group in order from the object side, the front group includes the negative first a in order from the object side. It consists of a lens group and a positive 1b lens group, and the rear group includes a positive lens, a negative lens, and a cemented lens of a positive lens, and satisfies the following conditional expressions (1) and (3).
(1) -0.8 <f1a / f1b <-0.2
(3) 0.1 <d / f <0.3
However,
d; axial air space between the front group and the rear group,
It is.

  In any aspect, the first a lens group and the first b lens group have a degree of freedom as long as they have negative and positive powers respectively. Preferably, the first a lens group has a convex surface on the object side in order from the object side. The first positive lens and the negative second lens are arranged, and the 1b lens group is constituted by a negative third lens having a convex surface facing the object side and a positive fourth lens in order from the object side. Is good. It is desirable to join the negative third lens and the positive fourth lens.

In any aspect, it is desirable that the following conditional expression (4) is satisfied.
(4) -0.6 <fR / fF <-0.1
However,
fF: Focal length of the front group,
It is.

  In any embodiment, it is preferable that the final lens in the rear group includes an aspherical surface.

  The rear group is composed of, in order from the object side, a positive lens, a negative lens, a positive lens having a convex surface facing the image side, and a positive lens, and it is desirable to bond three lenses on the object side.

  According to the present invention, the retrofocus type wide-angle lens system is thinner and more compact, and has a high performance with an angle of view of about 70 ° in which various aberrations, particularly coma, distortion, and chromatic aberration are well corrected. A wide-angle lens system can be obtained.

  The wide-angle lens system of the present embodiment is positive with the negative front group 10 divided by the stop S in order from the object side, as shown in the examples of FIGS. 1, 3, 5, 7, and 9. The rear group 20 is included. The front group 10 includes, in order from the object side, a negative first a lens group 11 and a positive first b lens group 12 that are divided at the maximum air gap. In all the examples, the 1a lens group 11 includes, in order from the object side, a positive first lens and a negative second lens having a convex surface facing the object side, and the 1b lens group 12 includes the object side The negative third lens and the positive fourth lens are arranged in order from the negative lens with the convex surface facing the object side, and the negative third lens and the positive fourth lens are cemented. The combined power of the third lens and the fourth lens is positive. The rear group 20 includes, in order from the object side, a positive fifth lens, a negative sixth lens, a positive seventh lens with a convex surface facing the image side, and a positive eighth lens. Three lenses of the seventh lens are bonded together. The eighth lens has an aspheric surface on the object side surface.

  In general, a retrofocus type lens obtains a back focus longer than the focal length of the entire system, and therefore, an asymmetrical power arrangement with respect to the stop S is adopted. Therefore, it is difficult to correct various aberrations such as distortion, astigmatism, and spherical aberration if an attempt is made to shorten the lens system while ensuring the required back focus. The front group having a negative power includes a plurality of lenses including at least one positive lens in order to suppress the occurrence of distortion in the front group. Since the front group gives negative power as a whole, it includes at least one negative lens, and astigmatism, distortion, and chromatic aberration can be corrected. In order to better correct various aberrations (particularly chromatic aberration), a set of cemented lenses of positive and negative lenses having a positive power as a whole is arranged. In addition, by arranging a relatively thick positive lens in the vicinity of the object side of the diaphragm S, the distortion is corrected well.

  For the rear group of positive power, at least one negative lens that generates negative spherical aberration is included in order to correct spherical aberration generated in the entire system while keeping the influence on off-axis aberration small. Yes. In order to suppress the occurrence of spherical aberration and coma, at least three positive lenses are included. Further, the positive lens is positioned on both surfaces of the negative lens, and the three lenses are bonded to form a pair of cemented lenses, so that high-order spherical aberration is corrected well. Further, by providing an aspherical surface on the final lens in the rear group, it is possible to correct spherical aberration and coma aberration better.

  Conditional expression (1) is a conditional expression regarding the power arrangement of the 1a lens group 11 and the 1b lens group 12 of the front group 10, and is a condition for correcting field curvature in particular. When the upper limit of conditional expression (1) is exceeded, the field curvature becomes under, and when the lower limit is exceeded, it becomes over. When the upper limit value of this conditional expression (1) is −0.25 and the lower limit value is −0.75 ((1 ′) − 0.75 <f1a / f1b <−0.25), a better image surface is obtained. The curvature can be corrected.

  Conditional expression (2) is a conditional expression regarding the power of the positive rear group. If the lower power of conditional expression (2) is exceeded and the positive power of the rear group becomes strong, correction of spherical aberration becomes difficult. If the upper limit of conditional expression (2) is exceeded and the positive power of the rear group becomes weak, astigmatism deteriorates and correction becomes difficult. If the upper limit of conditional expression (2) is 1.0 ((2 ′) 0.7 <f / fR <1.0), the overall length can be kept sufficiently short, which is more preferable.

  Conditional expression (3) is a conditional expression related to the interval between the front group and the rear group. If the upper limit of conditional expression (3) is exceeded, the entire length of the entire lens system becomes long, making it difficult to reduce the size. If the lower limit of conditional expression (3) is exceeded, it will be difficult to correct curvature of field and astigmatism. The upper limit of this conditional expression is more preferably 0.25 ((3 ′) 0.1 <d / f <0.25).

  Conditional expression (4) is a condition relating to the power balance between the front group and the rear group. When the upper limit of conditional expression (4) is exceeded and the positive power of the rear group increases, the front lens diameter increases, the spherical aberration deteriorates, and the back focus is insufficient. If the negative power of the front group increases beyond the lower limit of conditional expression (4), field curvature and astigmatism deteriorate and correction becomes difficult. By setting the lower limit of the conditional expression (4) to −0.55 ((4 ′) − 0.55 <fR / fF <−0.1), further excellent aberration correction can be performed.

Next, specific examples will be described. In the various aberration diagrams and tables, SA is spherical aberration, SC is a sine condition, chromatic aberration (axial chromatic aberration) diagram represented by spherical aberration, and d-line, g-line, and C-line are chromatic aberrations for each wavelength. , S is sagittal, M is meridional, W is half angle of view (°), F is F number, f is focal length of whole system, fB is back focus, y is maximum image height, r is radius of curvature, d is lens Thickness or lens interval, N _d is the refractive index of the d-line, and ν is the Abbe number.
A rotationally symmetric aspherical surface is defined by the following equation.
x = cy ² / [1+ [1- (1 + K) c ² y ² ] ^1/2 ] + A4y ⁴ + A6y ⁶ + A8y ⁸ + A10y ¹⁰ + A12y ¹² ...
(Where c is the curvature (1 / r), y is the height from the optical axis, K is the conic coefficient, A4, A6, A8,... Are the aspheric coefficients of each order)

1 and 2 and Table 1 show Example 1 of a wide-angle lens system according to the present invention. FIG. 1 is a lens configuration diagram, FIG. 2 is a diagram showing various aberrations thereof, and Table 1 is numerical data in an infinite photographing state.
The wide-angle lens system of the present embodiment includes a negative front group 10, an aperture stop S, and a positive rear group 20 in order from the object side. The front group 10 includes, in order from the object side, a negative first a lens group 11 and a positive first b lens group 12 that are divided at the maximum air gap. More specifically, the 1a lens group 11 includes, in order from the object side, a positive meniscus lens (positive first lens) convex toward the object side, and a negative meniscus lens (negative second lens) convex toward the object side. Consists of. The 1b lens group 12 includes a cemented lens of a negative meniscus lens (negative third lens) convex toward the object side and a positive meniscus lens (positive fourth lens) convex toward the object side. The fourth lens is the thickest lens in the entire system. The rear group 20 is located in order from the object side, sequentially from the object side, and has a positive meniscus lens convex to the image side (positive fifth lens) and a plano-concave negative lens (negative sixth lens) concave on the object side. Convex plano-convex positive lens (positive seventh lens) on the side and a positive meniscus lens convex on the image side with an aspheric layer made of a synthetic resin material adhered on the object side surface ( A positive eighth lens). The combined power of the three cemented lenses in the rear group is negative. The stop S is 2.00 behind the pole of the seventh surface (the 1b lens group 12).
(Table 1)
F = 1: 3.3
f = 21.60
W = 35.0
fB = 36.70
Surface No. r d N _d ν
1 32.486 3.62 1.69680 55.5
2 113.238 0.10--
3 20.902 1.10 1.80 400 46.6
4 8.178 6.75--
5 36.561 1.24 1.77250 49.6
6 8.178 5.67 1.69895 30.1
7 6403.341 4.00--
8 -20.013 2.59 1.72916 54.7
9 -8.600 1.00 1.74077 27.8
10 ∞ 2.37 1.62299 58.2
11 -19.142 0.10--
12 * -65.119 0.10 1.52972 42.7
13 -69.548 2.68 1.77250 49.6
14 -14.886---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.41055 × 10 ^-4 -0.12824 × 10 ^-7

3 and 4 and Table 2 show Example 2 of the wide-angle lens system according to the present invention. FIG. 3 is a lens configuration diagram, FIG. 4 is a diagram of various aberrations thereof, and Table 2 is numerical data in an infinite photographing state.
The 1b lens group 12 is composed of a cemented lens of a negative meniscus lens (negative third lens) convex to the object side and a biconvex lens (positive fourth lens), which is located in order from the object side. The combined power of the three cemented lenses in the rear group is negative. Other basic lens configurations are the same as those in the first embodiment. The stop S is 2.06 rearward from the pole of the seventh surface (first b lens group 12).
(Table 2)
F = 1: 3.3
f = 21.60
W = 35.0
fB = 36.70
Surface No. r d N _d ν
1 37.756 2.80 1.70469 55.3
2 157.205 0.10--
3 24.898 1.10 1.83481 42.7
4 9.600 9.00--
5 39.478 1.00 1.71300 53.9
6 8.945 3.31 1.71918 28.7
7 -131.430 4.10--
8 -13.854 2.79 1.71192 54.5
9 -8.792 1.01 1.80518 25.4
10 ∞ 3.05 1.60062 62.4
11 -17.049 0.27--
12 * -136.300 0.10 1.52972 42.7
13 -126.000 2.68 1.77250 49.6
14 -15.494---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.47288 × 10 ^-4 -0.91638 × 10 ^-8

5 and 6 and Table 3 show Example 3 of the wide-angle lens system according to the present invention. FIG. 5 is a lens configuration diagram, FIG. 6 is a diagram of various aberrations thereof, and Table 3 is numerical data in an infinite distance photographing state. The basic configuration is the same as that of the first embodiment. The combined power of the three cemented lenses in the rear group is negative. The stop S is 2.09 rearward from the pole of the seventh surface (the 1b lens group 12).
(Table 3)
F = 1: 3.3
f = 21.60
W = 35.0
fB = 36.70
Surface No. r d N _d ν
1 30.147 2.80 1.69680 55.5
2 103.268 0.10--
3 21.829 1.10 1.83399 42.3
4 8.521 7.22--
5 36.638 1.14 1.77617 48.6
6 8.104 6.00 1.70992 29.0
7 2239.858 4.17--
8 -19.436 2.70 1.72916 54.7
9 -8.327 1.00 1.72822 27.4
10 ∞ 2.30 1.69513 54.4
11 -21.099 0.10--
12 * -66.857 0.10 1.52972 42.7
13 -57.062 2.59 1.77250 49.6
14 -15.214---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.41950 × 10 ^-4 -0.10897 × 10 ^-7

7 and 8 and Table 4 show Example 4 of the wide-angle lens system according to the present invention. FIG. 7 is a lens configuration diagram, FIG. 8 is a diagram of various aberrations thereof, and Table 4 is numerical data in an infinite photographing state. The basic lens configuration is the same as that of the second embodiment. The combined power of the three cemented lenses in the rear group is positive. The stop S is 1.50 behind the pole of the seventh surface (the 1b lens group 12).
(Table 4)
F = 1: 3.3
f = 21.60
W = 35.0
fB = 36.70
Surface No. r d N _d ν
1 34.013 3.00 1.65123 50.7
2 140.405 0.10--
3 19.461 1.10 1.88300 40.8
4 8.178 6.91--
5 61.280 1.12 1.62216 57.4
6 8.991 3.81 1.68167 29.9
7 -70.997 3.50--
8 -17.683 4.64 1.71853 37.9
9 -8.600 1.00 1.80474 25.3
10 ∞ 2.71 1.60388 60.6
11 -14.186 0.10--
12 * -72.209 0.10 1.52972 42.7
13 -63.226 2.37 1.77250 49.7
14 -18.311---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.29180 × 10 ^-4 -0.79849 × 10 ^-7

9 and 10 and Table 5 show Example 5 of the wide-angle lens system according to the present invention. FIG. 9 is a lens configuration diagram, FIG. 10 is a diagram of various aberrations thereof, and Table 5 is numerical data in an infinite state photographing state. The basic lens configuration is the same as that of the second embodiment. The combined power of the three cemented lenses in the rear group is positive. The stop S is 1.60 behind the pole of the seventh surface (the 1b lens group 12).
(Table 5)
F = 1: 3.3
f = 21.60
W = 35.0
fB = 36.70
Surface No. r d N _d ν
1 40.909 2.95 1.71628 53.3
2 145.615 0.10--
3 17.264 1.10 1.88300 40.8
4 8.178 7.55--
5 72.599 1.00 1.69106 54.3
6 8.991 4.97 1.70950 29.3
7 -86.925 3.65--
8 -19.764 3.10 1.70629 40.6
9 -8.600 1.16 1.81072 25.8
10 ∞ 2.92 1.61693 60.0
11 -13.576 0.10--
12 * -61.836 0.10 1.52972 42.7
13 -111.342 2.57 1.77109 49.8
14 -19.155---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.23835 × 10 ^-4 -0.87028 × 10 ^-7

Table 6 shows values for each conditional expression in each example.
(Table 6)
Example 1 Example 2 Example 3 Example 4 Example 5
Conditional expression (1) -0.32 -0.69 -0.34 -0.62 -0.51
Conditional expression (2) 0.99 0.81 0.96 0.85 0.88
Conditional expression (3) 0.19 0.19 0.19 0.16 0.17
Conditional expression (4) -0.53 -0.12 -0.50 -0.23 -0.33

  As apparent from Table 6, Examples 1 to 5 satisfy the conditional expressions (1) to (4), and as is apparent from the various aberration diagrams, various aberrations, particularly coma (astigmatism), Distortion and chromatic aberration are corrected relatively well.

It is a lens block diagram of Example 1 of the wide-angle lens system by this invention. FIG. 2 is a diagram illustrating various aberrations in the configuration of FIG. 1. It is a lens block diagram of Example 2 of the wide angle lens system by this invention. FIG. 4 is a diagram illustrating various aberrations in the configuration of FIG. 3. It is a lens block diagram of Example 3 of the wide angle lens system by this invention. FIG. 6 is a diagram illustrating various aberrations in the configuration of FIG. 5. It is a lens block diagram of Example 4 of the wide angle lens system by this invention. FIG. 8 is a diagram illustrating various aberrations in the configuration of FIG. 7. It is a lens block diagram of Example 5 of the wide angle lens system by this invention. FIG. 10 is a diagram of various aberrations in the configuration of FIG. 9.

Claims (8)

In the wide-angle lens system consisting of a negative front group and a positive rear group in order from the object side,
The front group includes, in order from the object side, a negative first lens group and a positive first lens group,
The rear group includes a positive lens, a negative lens, and a cemented lens of a positive lens,
A wide-angle lens system characterized by satisfying the following conditional expressions (1) and (2):
(1) -0.8 <f1a / f1b <-0.2
(2) 0.7 <f / fR <1.1
However,
f1a: focal length of the 1a lens group,
f1b; focal length of the 1b lens group,
f: focal length of the entire system,
fR: focal length of rear group. In the wide-angle lens system consisting of a negative front group and a positive rear group in order from the object side,
The front group includes, in order from the object side, a negative first lens group and a positive first lens group,
The rear group includes a positive lens, a negative lens, and a cemented lens of a positive lens,
A wide-angle lens system characterized by satisfying the following conditional expressions (1) and (3):
(1) -0.8 <f1a / f1b <-0.2
(3) 0.1 <d / f <0.3
However,
d: On-axis air space between the front group and the rear group. The wide-angle lens system according to claim 1, wherein the wide-angle lens system satisfies the following conditional expression (3).
(3) 0.1 <d / f <0.3 4. The wide-angle lens system according to claim 1, wherein the first-a lens group includes, in order from the object side, a positive first lens having a convex surface facing the object side and a negative second lens. The 1b lens group is a wide-angle lens system including, in order from the object side, a negative third lens and a positive fourth lens with a convex surface facing the object side. 5. The wide-angle lens system according to claim 4, wherein the negative third lens and the positive fourth lens are cemented. 6. The wide-angle lens system according to claim 1, wherein the wide-angle lens system satisfies the following conditional expression (4).
(4) -0.6 <fR / fF <-0.1
However,
fF: Focal length of the front group. 7. The wide-angle lens system according to claim 1, wherein the final lens in the rear group includes an aspherical surface. 7. The wide-angle lens system according to claim 1, wherein the rear group includes, in order from the object side, a positive lens, a negative lens, a positive lens having a convex surface facing the image side, and a positive lens. A wide-angle lens system in which three lenses are bonded together.

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