JP2007225963A - Wide-angle lens system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a retro-focus type wide-angle lens system which is thinner (smaller), has satisfactorily corrected various aberrations and a viewing angle of about 70°, and exhibits high performance. <P>SOLUTION: The wide-angle lens system includes, in order from the object side, a front group that is negative and a rear group that is positive, wherein the front group includes, in order from the object side, a first 1a lens group that is negative and a first 1b lens group that is positive, and the rear group includes a positive lens and a cemented lens composed of a negative lens and positive lens, and the wide-angle lens system satisfies the conditional formula (1): -15<fF/f-3 (wherein fF is the focal distance of the front group and f is the focal distance of the entire system). <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 2001-13405 A Japanese Patent Laid-Open No. 2001-330771 JP 2002-303790 A JP 2003-121735 A

  It is an object of the present invention to obtain a high-performance wide-angle lens system that is a retrofocus type wide-angle lens system and is thinner (smaller) and has a field angle of about 70 ° with various aberrations corrected well.

The present invention is a wide-angle lens system including a negative front group, a stop, and a positive rear group in order from the object side. The front group includes a negative first lens group and a positive first b in order from the object side. It consists of a lens group and satisfies the following conditional expression (1).
(1) -15 <fF / f <-3
(2) -2.0 <f1a / f <-0.5
However,
fF: Focal length of the front group,
f: focal length of the entire system,
It is.

The wide-angle lens system of the present invention preferably satisfies the following conditional expression (2).
(2) -2.0 <f1a / f <-0.5
However,
f1a: focal length of the 1a lens group,
It is.

The wide-angle lens system of the present invention preferably satisfies the following conditional expression (3).
(3) 1.0 <fR / f <1.8
However,
fR: focal length of rear group,
It is.

  The positive rear group as a whole is composed of, for example, a positive lens, a negative lens, a positive lens, and a positive lens in order from the object side, and a positive lens, a negative lens, and a positive lens close to the stop are joined. It is preferable to do this.

  Specifically, it is desirable that the negative first lens group as a whole is composed of, for example, a positive lens and a negative lens sequentially located from the object side.

It is desirable that the 1b lens group satisfies the following conditional expression (4).
(4) 1.5 <f1b / f <2.5
However,
f1b; focal length of the 1b lens group,
It is.

  Specifically, the first positive lens group 1b as a whole is preferably composed of, for example, a negative lens and a positive lens positioned in order from the object side.

  The final lens on the image side in the rear group preferably includes an aspherical surface.

The wide-angle lens system of the present invention can apply a floating focus that performs focusing by moving the front group and the rear group by different amounts. When this floating focus method is used, it is preferable to satisfy the following conditional expression (5).
(5) 0.7 <X1 / X2 <0.9
However,
X1: the amount of movement of the front group by focusing,
X2: Movement amount of rear group by focusing,
It is.

  According to the present invention, it is possible to obtain a high-performance wide-angle lens system that is a retrofocus type wide-angle lens system and is thinner (smaller) and has a field angle of about 70 ° with various aberrations corrected favorably.

  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. The combined power of the three cemented lenses is negative.

  Conditional expression (1) is a conditional expression regarding the power of the negative front group. If the negative power of the front group increases beyond the upper limit of conditional expression (1), the angle of incidence of the off-axis light beam on the rear group becomes tight, which increases the variation in spherical aberration, which is not preferable. If the negative power of the front group becomes weaker than the lower limit of conditional expression (1), the spherical aberration and coma aberration that occur in the rear group cannot be corrected sufficiently. In this conditional expression (1), when the lower limit is −10 and the upper limit is −5 ((1 ′) − 10 <fF / f <−5), aberration correction can be performed more satisfactorily.

  In general, the retrofocus type lens has a negative and positive power arrangement before and after the stop S, and thus tends to generate various aberrations such as distortion and astigmatism. The negative power front group is preferably composed of a plurality of lenses including at least one positive lens in order to suppress the occurrence of distortion, astigmatism, and the like in the front group. In order to correct various aberrations (especially chromatic aberration) more favorably, it is preferable to dispose a pair of positive and negative lenses having a positive power as a whole. In addition, by disposing a relatively thick positive lens in the vicinity of the object side of the stop S, distortion can be corrected well.

  Conditional expression (2) relates to the power of the negative first-a lens group 11 in the front group 10, and is a condition for correcting various aberrations satisfactorily while achieving both a reduction in size and a back focus. If the negative power of the first-a lens unit is strong beyond the upper limit of conditional expression (2), it is advantageous for miniaturization and securing of the back focus, but it becomes difficult to correct astigmatism and curvature of field. If the negative power of the 1a lens group is weak beyond the lower limit of conditional expression (2), the total length tends to be long in order to ensure the back focus. If an attempt is made to shorten the total length while exceeding the lower limit of the conditional expression (2), the 1b lens group must also be given a negative power, and if the 1b lens group has a negative power, the 1a lens group Astigmatism and curvature of field that occur in the lens cannot be corrected, and sufficient performance cannot be obtained. When the lower limit value of the conditional expression (2) is −1.5 and the upper limit value is −1.0 ((2 ′) − 1.5 <f1a / f <−1.0), further excellent aberration correction is achieved. Is possible.

  The negative first lens group 11 preferably has a two-lens configuration including a positive lens and a negative lens in order from the object side. If the 1a lens group is composed of one negative lens, the distortion cannot be corrected. In the negative and positive configuration in order from the object side, it is possible to correct distortion, but the lens thickness of the 1a lens group becomes large, which is disadvantageous for thinning.

  Conditional expression (3) is a conditional expression regarding the power of the positive rear group. When the lower power of conditional expression (3) is exceeded and the positive power of the rear group becomes strong, spherical aberration and coma aberration are greatly generated. In addition, when the floating method is used for focusing, the variation in spherical aberration increases. When the upper limit of conditional expression (3) is exceeded and the positive power of the rear group becomes weak, the effect cannot be sufficiently obtained even if the floating method is adopted. In this conditional expression (3), when the upper limit is 1.4 ((3 ′) 1.0 <fR / f <1.4), aberration correction can be performed more satisfactorily.

  The rear group of the positive power includes at least one negative lens that generates a negative spherical aberration in order to correct the spherical aberration generated in the entire system while keeping the influence on the off-axis aberration small. Is good. In order to suppress the occurrence of spherical aberration and coma, it is preferable to include at least three positive lenses. Further, by positioning the positive lens on both surfaces of the negative lens and bonding these three together to form a set of cemented lenses, higher-order spherical aberration can be favorably corrected. The most preferable lens configuration of the rear group is a mode in which a positive lens, a negative lens, a positive lens, and a positive lens are sequentially arranged from the object side, and three lenses, a positive lens, a negative lens, and a positive lens close to the stop are cemented. Generation of higher-order aberrations can be suppressed by joining three objects on the object side in a positive / negative / positive alignment. The configuration of the rear group is suitable for correcting spherical aberration and coma aberration over the entire screen.

  Further, by providing an aspherical surface on the last lens on the image side in the rear group, it is possible to correct spherical aberration and coma aberration more favorably. The aspheric surface may be either an object-side surface or an image-side surface, but when the aspheric surface is a resin aspheric surface (a hybrid lens), the object-side surface is preferable. If an aspherical surface is used for the front group, field curvature and astigmatism can be corrected. However, coma cannot be corrected, and the front group has a larger lens diameter than the rear group. The formation cost of becomes high.

  Conditional expression (4) relates to the power of the positive first-b lens group 12 in the front group 10. The front group 10 has a negative and positive power arrangement in order from the object side even in the front group 10, and the 1b lens group has an action of correcting distortion and astigmatism generated in the 1a lens group. When the upper limit of conditional expression (4) is exceeded and the positive power of the 1b lens group becomes weak, distortion and field curvature generated in the 1a lens group cannot be corrected well. When the lower limit of the conditional expression (4) is exceeded and the positive power of the 1b lens group becomes strong, spherical aberration and coma aberration are greatly generated. If the lower limit value of the conditional expression (4) is 1.7 and the upper limit value is 2.0 ((4 ′) 1.7 <f1b / f <2.0), further excellent aberration correction can be performed. .

  If the positive first lens group 12 is composed of negative positive lenses in order from the object side, it is possible to satisfactorily correct field curvature, astigmatism, chromatic aberration, and spherical aberration. By joining these negative positive lenses positioned in order from the object side, it is possible to suppress the generation of high-order spherical aberration and to reduce the lens diameter. That is, since the front group 10 has a negative power as a whole, in order to correct the aberration, the front group 10 includes a negative 1a lens group and a positive 1b lens group, and the 1a lens group includes distortion aberration. The positive / negative configuration is advantageous for the correction. Therefore, when correcting the astigmatism that is undercorrected with the positive 1b lens group, if the 1b lens group is only the positive lens, spherical aberration and coma aberration are generated, which is not preferable. By using a positive cemented lens, astigmatism is favorably corrected while suppressing the occurrence of spherical aberration and coma.

  Conditional expression (5) is a condition regarding the amount of movement of the front group and the rear group when the floating method is adopted for focusing. It is known that the floating focus method in which the amount of movement of the front group and the rear group is different during focusing is effective in correcting field curvature that occurs during close-up shooting. When the upper limit of conditional expression (5) is exceeded, correction of field curvature is insufficient, and when the lower limit is exceeded, overcorrection occurs. The floating focus can be either linear floating in which the ratio of the amount of movement of the front group and the rear group is constant or non-linear floating in which the ratio of the amount of movement changes.

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, r is radius of curvature, d is lens thickness or lens interval, N _d represents the refractive index of the d-line, and ν represents 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 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 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 stop S is 2.000 ahead from the pole of the eighth surface (rear group 20).
Focusing employs a floating focus that focuses by moving the front group 10 and the rear group 20 by different amounts of movement, particularly in order to effectively correct curvature of field that occurs during close-up shooting.
(Table 1)
F = 1: 3.3
f = 21.60
W = 34.2
fB = 36.70
Surface No. r d N _d ν
1 37.707 2.80 1.70000 56.5
2 156.586 0.10--
3 24.702 1.10 1.83481 42.7
4 9.600 8.80--
5 40.676 1.00 1.71300 53.9
6 8.600 3.39 1.71879 29.0
7 -109.468 4.00--
8 -13.645 2.84 1.73258 50.5
9 -8.178 1.00 1.80482 25.4
10 ∞ 3.19 1.59851 59.7
11 -16.989 0.20--
12 * -134.928 0.10 1.52972 42.7
13 -94.797 2.78 1.77250 49.6
14 -15.434---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.47266 × 10 ^-4 -0.10789 × 10 ^-7
Data on floating focus;
Shooting magnification m -1/10 -1/4
Movement amount of front group X1 1.86 4.81
Rear group movement amount X2 2.16 5.41
X1 / X2 0.86 0.89

3 and 4 and Table 2 show Example 2 of the wide-angle lens 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 rear group 20 includes, in order from the object side, a positive meniscus lens (positive fifth lens) convex to the image side, a biconcave negative lens (negative sixth lens), and a biconvex positive lens (positive seventh lens). This cemented lens is composed of a positive meniscus lens (positive eighth lens) convex on the image side having an aspheric layer made of a synthetic resin material adhered to the object side surface. Other basic lens configurations are the same as those in the first embodiment. The stop S is 1.502 ahead from the pole of the eighth surface (rear group 20).
(Table 2)
F = 1: 2.9
f = 21.60
W = 34.2
fB = 36.70
Surface No. r d N _d ν
1 31.888 2.86 1.75593 50.7
2 105.492 0.25--
3 20.861 1.10 1.83481 42.7
4 8.408 6.37--
5 47.737 1.00 1.72679 53.7
6 8.603 3.47 1.74004 30.2
7 -103.730 3.34--
8 -16.632 5.00 1.77937 26.6
9 -7.600 1.00 1.84637 24.6
10 60.572 3.11 1.61999 63.7
11 -18.267 0.20--
12 * -222.196 0.10 1.52972 42.7
13 -226.265 3.41 1.71300 53.9
14 -15.007---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.44999 × 10 ^-4 0.84551 × 10 ^-7
Data on floating focus;
Shooting magnification m -1/10 -1/4
Movement amount of front group X1 1.88 4.85
Rear group movement amount X2 2.18 5.45
X1 / X2 0.86 0.89

5 and 6 and Table 3 show Example 3 of the wide-angle lens 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 in the second embodiment. The stop S is 1.501 ahead from the pole of the eighth surface (rear group 20).
(Table 3)
F = 1: 3.3
f = 21.60
W = 34.2
fB = 36.70
Surface No. r d N _d ν
1 31.807 2.77 1.76435 50.1
2 101.901 0.58--
3 20.842 1.10 1.83400 42.8
4 8.442 6.62--
5 50.370 1.00 1.71963 54.5
6 7.968 3.17 1.74397 31.2
7 -104.347 3.32--
8 -16.220 4.71 1.76211 27.3
9 -7.400 1.00 1.84700 25.2
10 46.490 3.31 1.61945 63.7
11 -18.228 0.20--
12 * -343.570 0.10 1.52972 42.7
13 -265.439 3.44 1.72882 53.3
14 -14.962---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 -0.44119 × 10 ^-4 0.86880 × 10 ^-7
Data on floating focus;
Shooting magnification m -1/10 -1/4
Front group movement X1 1.81 4.82
Rear group movement amount X2 2.17 5.44
X1 / X2 0.83 0.89

7 and 8 and Table 4 show Example 4 of the wide-angle lens 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 rear group 20 includes, in order from the object side, a positive meniscus lens (positive fifth lens) convex to the image side, a biconcave negative lens (negative sixth lens), and a biconvex positive lens (positive seventh lens). And a positive meniscus lens (positive eighth lens) convex on the image side. Other basic lens configurations are the same as those in the second embodiment. The stop S is 1.743 ahead from the pole of the eighth surface (rear group 20).
(Table 4)
F = 1: 3.3
f = 21.57
W = 34.2
fB = 36.35
Surface No. r d N _d ν
1 35.754 2.78 1.69492 57.0
2 140.010 0.10--
3 23.976 1.10 1.83193 39.2
4 9.469 7.93--
5 46.987 1.26 1.68797 56.9
6 7.310 4.14 1.70198 31.3
7 -106.643 4.55--
8 -13.548 2.63 1.73800 52.3
9 -8.675 1.00 1.78640 27.0
10 75.642 3.61 1.59769 61.6
11 -15.465 0.20--
12 * -490.224 2.66 1.77247 49.7
13 -17.907---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 0.12172 × 10 ^-4 0.14920 × 10 ^-6
Data on floating focus;
Shooting magnification m -1/10 -1/4
Movement amount of front group X1 1.86 4.81
Rear group movement amount X2 2.17 5.42
X1 / X2 0.86 0.89

9 and 10 and Table 5 show Example 5 of the wide-angle lens 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 fourth embodiment. The stop S is 1.500 ahead from the pole of the eighth surface (rear group 20).
(Table 5)
F = 1: 3.1
f = 21.60
W = 34.2
fB = 36.32
Surface No. r d N _d ν
1 34.649 2.84 1.70000 56.8
2 133.094 0.10--
3 22.840 1.10 1.83400 37.2
4 9.168 7.83--
5 45.105 1.00 1.64999 59.2
6 6.297 4.08 1.67524 31.8
7 -109.031 4.14--
8 -14.382 3.08 1.69929 53.8
9 -7.688 1.03 1.77753 27.7
10 48.001 4.13 1.61173 63.7
11 -15.266 0.10--
12 * -2907.567 2.58 1.77250 44.1
13 -19.721---
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
Surface NO K A4 A6
12 0.00 0.97910 × 10 ^-5 0.11649 × 10 ^-6
Data on floating focus;
Shooting magnification m -1/10 -1/4
Movement amount of front group X1 1.88 4.72
Rear group movement amount X2 2.17 5.42
X1 / X2 0.87 0.87

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) -12.49 -5.37 -6.36 -7.85 -9.87
Conditional expression (2) 1.23 1.12 1.13 1.19 1.20
Conditional expression (3) -1.33 -1.26 -1.29 -1.35 -1.34
Conditional expression (4) 1.89 1.96 1.92 2.03 1.91
(Corresponding numerical value of conditional expression (5) is described in each example)

  Examples 1 to 5 satisfy the conditional expressions (1) to (5), and various aberrations are relatively well corrected as is apparent from the various aberration diagrams.

It is a lens block diagram of Example 1 of the wide angle lens 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 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 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 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 by this invention. FIG. 10 is a diagram of various aberrations in the configuration of FIG. 9.

Claims (9)

In order from the object side, in a wide-angle lens system including a negative front group, a stop, and a positive rear group,
The front group includes, in order from the object side, a negative first lens group and a positive first lens group,
A wide-angle lens system characterized by satisfying the following conditional expression (1).
(1) -15 <fF / f <-3
However,
fF: Focal length of the front group,
f: Focal length of the entire system. The wide-angle lens system according to claim 1, wherein the wide-angle lens system satisfies the following conditional expression (2).
(2) -2.0 <f1a / f <-0.5
However,
f1a: focal length of the 1a lens group. 3. The wide-angle lens system according to claim 1, wherein the wide-angle lens system satisfies the following conditional expression (3).
(3) 1.0 <fR / f <1.8
However,
fR: focal length of rear group. The wide-angle lens system according to any one of claims 1 to 3, wherein the rear group includes, in order from the object side, a positive lens, a negative lens, a positive lens, and a positive lens. A wide-angle lens system in which three lenses are joined. 5. The wide-angle lens system according to claim 1, wherein the first-a lens group includes a positive lens and a negative lens that are sequentially arranged from the object side. 6. The wide-angle lens system according to claim 1, wherein the wide-angle lens system satisfies the following conditional expression (4).
(4) 1.5 <f1b / f <2.5
However,
f1b: focal length of the 1b lens group. The wide-angle lens system according to any one of claims 1 to 6, wherein the first-b lens group includes a negative lens and a positive lens that are sequentially arranged from the object side. 8. The wide-angle lens system according to claim 1, wherein the final lens on the image side in the rear group includes an aspherical surface. 9. The wide-angle lens system according to claim 1, wherein focusing is performed by moving the front group and the rear group by different amounts of movement, and satisfies the following conditional expression (5).
(5) 0.7 <X1 / X2 <0.9
However,
X1: the amount of movement of the front group by focusing,
X2: The amount of movement of the rear group by focusing.

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