JPH09211321A - Retrofocus type lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a retrofocus type lens which has a small front lens diameter, consists of a small number of lens elements, and has aberrations well compensated and is compact. SOLUTION: This lens consists of a front group composed of a 1st negative meniscus lens L1 , a 2nd positive lens L2 , a 3rd lens L3 , and a 4th positive lens L4 which is larger in curvature on its object-side surface than on the image-side surface, and a rear group composed of a 5th lens L6 , a 6th lens L6 , and a 7th lens L7 in order from the object side and meets conditions of 2.2<ra +rb )/(ra -rb )<30.0 and L/f<5.8. Here, ra is the radius of curvature of the object- side surface of the most image-side lens, (f) is the focal length of the whole system, and L is the overall length of the lens system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus type lens, and more particularly to a rear focus type lens for high-performance wide-angle photography, which suppresses changes in field curvature and astigmatism that occur during focusing on a near object. Of retro focus lenses.

[0002]

2. Description of the Related Art Generally, a lens used for a television camera or the like requires a back focus several times as long as its focal length. Therefore, the front lens group has a negative refractive power and the rear lens group has a positive refractive power. A retrofocus type lens having the above arrangement is used. In focusing of this type of retrofocus type lens, it is generally performed to extend the entire lens. However, the method of extending the entire lens has a drawback that the weight of the moving group is heavy and the total length of the lens system becomes large during the extension. For this reason, in order to reduce the load on the lens drive unit, it is desirable to use a focusing unit with a small weight.With a retrofocus wide-angle lens, the front unit has a considerable weight, so only the rear unit of the lens is moved. Focusing is used. A wide-angle lens in which only the rear group of the lens is moved and focused in this way is disclosed in JP-A-2-167515 and JP-A-5-34592.

[0003]

However, in JP-A-2-167515, the front lens diameter is large because the lens closest to the object side in the front group is a positive lens.
In No. 92, it was found that there is a problem that the number of lenses in the rear group is large due to the aberration correction, and the load on the lens driving section becomes large.

The present invention has been made to solve at least one of the above problems, and provides a compact retrofocus type lens having a small front lens diameter, a small number of lenses, and well corrected aberrations. It is an object.

[0005]

[Means for Solving the Problems]

[1] In order from the object side, a negative meniscus first lens with a convex surface facing the object side, a positive second lens, a negative meniscus third lens with a convex surface facing the object side, and an object side surface It is composed of a front group consisting of a positive fourth lens having a strong curvature on the image side, a negative fifth lens, a sixth lens of biconvex lenses, and a rear group consisting of a positive seventh lens. The 6-lens is a lens system having a structure of 7 elements in 6 groups, which are cemented together, and 2.2 <(r _a + r _b ) / (r _a −r _b ) <30.0 (1) L / f <5. 8 (2) where r _{a is the} radius of curvature of the object side surface of the lens closest to the image side in the front group r _{b is the} radius of curvature of the image side surface of the lens closest to the image side in the front group f is the focal length of the entire system L : A retrofocus type lens characterized by satisfying conditional expressions (1) and (2) of the entire length of the lens system.

[2] A negative meniscus first lens having a convex surface directed toward the object side and a positive second lens in order from the object side.
A negative meniscus third lens whose convex surface faces the object side,
It is composed of a front group consisting of a positive fourth lens whose surface closer to the image side than the object side surface has a strong curvature, a negative fifth lens, a sixth lens of biconvex lenses, and a rear group consisting of a positive seventh lens, Fifth,
The sixth lens is a lens system of 7 elements in 6 groups cemented together, and 9.0 <(r _a + r _b ) / (r _a −r _b ) <20.0 (3) L / f <5 .6 (4) where r _{a is the} radius of curvature of the object side surface of the lens closest to the image side in the front group r _{b is the} radius of curvature of the image side surface of the lens closest to the image side in the front group f is the focal length of the entire system L: Total length of lens system A retrofocus lens characterized by satisfying the conditional expressions (3) and (4).

[3] In a retrofocus lens system including a front group including a diverging system and a weak convergent system in order from the object side, and a rear group, the rear group is moved to perform focusing, and f / | f ₁ | <0.4 (5) 0.31 <d / f <1.8 (6) L / f <5.8 (7) where f: focal length of the entire system f ₁ : focus of the front group Distance d: Distance between divergence system of front group and weak convergence system L: Total length of lens system (5), (6), (7) A retrofocus lens characterized by satisfying the conditional expressions.

[4] In a retrofocus type lens system including a front group including a divergent system and a weak convergent system in order from the object side and a rear group, the rear group is moved to perform focusing, and −0. 26 <f / f ₁ <0 (8) 0.8 <d / f <1.2 (9) L / f <5.6 (10) where f: focal length of the entire system f ₁ : front group Focal length d: Distance between divergence system of front group and weak convergence system L: Total length of lens system (8), (9), (10) A retrofocus lens characterized by satisfying conditional expressions .

[5] The retrofocus lens according to [3], characterized in that the most object side of the diverging system of the front group is a negative meniscus lens having a convex object side surface.

[6] In the retrofocus type lens according to [5], the lens closest to the image side in the front group is 8.0 <(r _a + r _b ) / (r _a −r _b ) <30. 0 (11) where r _{a is the} radius of curvature of the object side surface of the lens closest to the image side in the front group r _{b is the} radius of curvature of the image side surface of the lens closest to the image side in the front group It is desirable to do.

[7] A meniscus-shaped negative first lens having a convex surface directed toward the object side, and a positive second lens in order from the object side.
A negative meniscus third lens whose convex surface faces the object side,
It is composed of a front group consisting of a positive fourth lens whose surface closer to the image side than the object side surface has a strong curvature, a negative fifth lens, a sixth lens of biconvex lenses, and a rear group consisting of a positive seventh lens, Fifth,
The sixth lens is a retrofocus type lens which is a lens system of 7 elements in 6 groups cemented together and in which at least the third lens and the fourth lens are plastic lenses.

[8] In the retrofocus lens according to [7], at least one of the refracting surfaces of the third lens and the fourth lens is moved toward the lens peripheral portion from the optical axis, and the refracting power of the lens is increased. It is a retrofocus lens characterized by having an aspherical surface such that

In FIG. 1 showing an example of a lens configuration and a focus movement mode of a lens system according to the present invention, the lens system is composed of a first lens L ₁ to a seventh lens L ₇ , and the front group is a fixed group G ₁ and G. _The fixed group G ₁ is a lens group having a negative composite focal length, the fixed group G ₂ is a weak positive lens, and the rear group is a lens group having a positive refractive power of the moving group G ₃ . When performing focusing, the moving group G ₃ is moved back and forth. When focusing from infinity to a close range, the moving group G ₃ is moved to the object side, and the fixed groups G ₁ and G ₂ and the moving group G ₃ are moved.
And reduce the air space between.

Further, in this focusing, in the present invention, the diameter of the front lens is kept small by using a divergent lens group for the fixed lens group G ₁ of the front lens group, and a weak convergent lens is used for the fixed lens group G ₂ of the front lens group. By using it, it is possible to correct various aberrations from infinity to a close range due to focusing.

The expression (1) defines the shaping factor in the fixed group G ₂ of the front group, and when it becomes smaller than the lower limit of the condition (1), an introverted coma occurs and the spherical aberration becomes excessive. become. Condition (1)
When the value exceeds the upper limit of, an outward coma occurs and spherical aberration becomes under. Further, if the upper limit of the condition (2) is exceeded, the lens becomes large, which is not preferable. Further, it is preferable that the conditions of the expressions (3) and (4) are satisfied.

Further, the expression (5) defines the power distribution between the front group, which is a fixed group, and the entire lens system. If the value is less than the lower limit of the condition (5), the amount of extension becomes large, If it is larger than the upper limit, the change in aberration during focusing becomes large. further,
Expression (6) is a conditional expression regarding the configuration of the fixed groups G ₁ and G _{2 in} the front group, and when the expression becomes smaller than the lower limit of the condition (6),
The powers of the fixed groups G ₁ and G ₂ are both strong, the image plane becomes under, and it becomes difficult to correct various aberrations. When the value exceeds the upper limit of the condition (6), barrel distortion becomes large, and the diameter of the front lens becomes large in order to correct various aberrations. Further, if the upper limit of the condition (7) is exceeded, the lens becomes large, which is not preferable.

Preferably, equations (8), (9),
It is desirable to satisfy the condition of the expression (10), and it is preferable that the most object side of the divergence system of the front group is a negative meniscus lens whose object side surface is convex. Further, it is desirable that the condition of expression (11) is satisfied. If the upper and lower limits of expression (11) are exceeded, various aberrations will occur as in (1).

Further, in the retrofocus type lens having the structure of the present invention, the radius of curvature of the image side surface of the third lens is small and the effective diameter of the lens surface is large, so that the image side of the third lens tends to be a deep concave surface. Difficult, and the third
The difference in radius of curvature between the object side and the image side between the lens and the fourth lens is small, and it is difficult to process the outer peripheral portion of the lens to match the optical axis, but the third lens and the fourth lens are injection molded. If such a plastic lens is used, the above-mentioned processing problems are eliminated and mass productivity is improved.

Further, since the third lens and the fourth lens have negative and positive refracting powers respectively, it is possible to reduce the focus shift due to the temperature change.

If at least one of the refracting surfaces of the third lens or the fourth lens is an aspherical surface, the refracting power of which becomes smaller from the optical axis toward the lens peripheral portion, the field curvature can be easily corrected. become.

[0021]

EXAMPLES Examples of the retrofocus type lens of the present invention will be shown below. The symbols in each example are as follows.

R: radius of curvature of refracting surface D: distance between refracting surfaces Nd: refractive index of d-line of lens material νd: Abbe number of lens material f: focal length of entire system f ₁ : focal length of the front group F _NO: F-number d: air distance r _a of the fixed group G ₁ and G ₂ of the front group curvature radius r _b of the object side surface of the most image side lens of the front group: most image side of the front group Radius of curvature of the image side of the lens of A: variable with the air distance on the image side of the lens closest to the image side in the front group B: variable with the air distance on the image side of the lens closest to the image side of the rear group M: magnification ω : Half angle of view L: Total length of lens system The surface marked with "*" is an aspherical surface, and the shape of the aspherical surface of the present invention has the optical axis direction as the X axis and the direction perpendicular to the optical axis as the Y axis. Then, it is expressed by the following equation.

[0023]

[Equation 1]

Here, r is a paraxial radius of curvature, and K and A _2i are aspherical coefficients.

Example 1 FIG. 2 is an optical sectional view of Example 1.
Shown in

Next, the numerical values of the embodiment will be shown.

F = 6.035, F _NO = 2.8, ω = 29.2 ° Surface number R D Nd νd 1 12.502 0.80 1.65844 50.9 2 7.757 1.50 3 23 .343 1.70 1.80518 25.4 4 −55.611 0.20 5 6.965 0.70 1.72000 50.2 6 3.436 6.06 7 −5.912 1.50 1.58913 61.28-5.127 A9 100.031 0.60 1.846666 23.8 10 6.788 2.60 1.77250 49.6 11 -12.521 0.20 12 19.019 1.40 1.78590 44.2 13 −89.125 B 14 ∞ 1.00 1.52000 65.0 15 ∞ 0.00 16 ∞ 2.69 1.54880 67.0 17 ∞ 0.20 18 ∞ 0.75 1.51633 64.1 19 ∞ 1.35 (r _a + r _b ) / (r _a −r _b ) = 14.06 L / f = 5.47 f / | f ₁ | = 0.19 d /F=1.00 M = 0, A = 4.02, B = 5.72 M = -0.065, A = 3.61, B = 6.14 FIG. Shown in. As shown in the figure, the aberration is well corrected.

(Embodiment 2) Next, numerical values of Embodiment 2 are shown.

F = 6.06, f _NO = 2.8, ω = 29.2 ° Surface number R D Nd νd 1 11.600 0.70 1.69680 55.5 2 6.279 2.503 − 95.000 1.50 1.846666 23.8 4 -19.028 0.20 5 * 5.229 1.20 1.49200 57.0 6 2.934 5.52 7 -4.694 1.20 1 .49200 57.0 8 -4.108 A 9 35.794 0.60 1.846666 23.8 10 7.421 2.40 1.69680 55.5 11 -12.325 0.20 12 19.93561. .30 1.69680 55.5 13 -54.469 B 14 ∞ 1.00 1.52000 65.0 15 ∞ 0.00 16 ∞ 2.69 1.54880 67.0 17 ∞ 0.20 18 ∞ 0.75 1.51633 64.1 19 ∞ 1.35 Fifth surface aspherical coefficient K = 0.530530 A ₄ = -1.12124 × 10 ⁻⁴ A ₆ = −1.41572 × 10 ⁻⁵ A ₈ = 5.22331 × 10 ⁻⁷ (r _a + r _b ) / (r _a −r _b ) = 15.0 L / f = 5.45 f / | f ₁ | = 0.20 d / f = 0.91 M = 0, A = 3.80, B = 5.95 M = −0.065, A = 3.38, B = 6.37 In this embodiment, the third and fourth lenses are plastic, The change in the focus position with a temperature rise of 30 ° C. is 0.
01 or less.

FIG. 4 is an aberration diagram of the second embodiment. As shown in the figure, the aberration is well corrected.

(Embodiment 3) Next, numerical values of Embodiment 3 are shown.

F = 6.06, f _NO = 2.8, ω = 29.0 ° Surface number R D Nd νd 1 10.000 0.70 1.69680 55.5 2 5.725 2.503 − 30.496 1.30 1.846666 23.8 4 -14.955 0.20 5 * 4.099 1.10 1.49200 57.0 6 2.874 5.59 7 -4.362 1.20 1 .49200 57.0 8 -4.373 A 9 -75.395 0.60 1.80518 25.4 10 8.063 2.40 1.69680 55.5 11 -9.299 0.20 12 14.917 1.30 1.77250 49.6 13-93.140 B 14 ∞ 1.00 1.52000 65.0 15 ∞ 0.00 16 ∞ 2.28 1.54880 67.0 17 ∞ 0.20 18 ∞ 0.75 1.51633 64.1 19 ∞ 1.35 Fifth surface aspherical coefficient K = 0.265951 A ₄ = 6.05691 × 10 ⁻⁵ A ₆ = −2.275535 × 10 ⁻⁵ A ₈ = 1 .88678 × 10 ⁻⁶ (r _a + r _b ) / (r _a −r _b ) = − 794.1 L / f = 5.36 f / | f ₁ | = 0.23 d / f = 0.92 M = 0, A = 3.71, B = 6.12 M = -0.065, A = 3.28, 6.55 In this embodiment, the third and fourth lenses are plastic lenses. The aberration diagram of Example 3 is shown in FIG. As shown in the figure, the aberration is well corrected.

(Embodiment 4) Next, numerical values of Embodiment 4 are shown.

F = 6.09, f _NO = 2.8, ω = 29.2 ° Surface number R D Nd νd 1 10.423 23.70 1.69680 55.5 2 6.611 2.20 3 67 0.005 1.85 1.846666 23.8 4 -28.658 0.20 5 5.877 0.70 1.69680 55.5 6 3.271 5.37 7-6.364 1.50 1.72000 50.28-5.213 A9-44.582 0.60 1.805518 25.4 10 7.129 2.60 1.72000 50.2 11 -11.644 0.20 12 20.257 1. 30 1.72000 50.2 13-23.619 B 14 ∞ 1.00 1.52000 65.0 15 ∞ 0.00 16 ∞ 2.28 1.54880 67.0 17 ∞ 0.20 18 ∞ 0 .75 1.51633 64.1 19 ∞ 1.35 (r _a + r _b ) / (r _a −r _b ) = 10.1 L / f = 5.42 f / | f ₁ | = 0.09 d / f = 0.88 M = 0, A = 3.82, B = 6.37 M = −0.065, A = 3.42, B = 6.77 An aberration diagram of the fourth embodiment is shown in FIG. Show. As shown in the figure, the aberration is well corrected.

[0035]

Since the present invention is configured as described above, it has the following effects. According to the present invention, since the front lens diameter is small with only a small number of lenses and focusing is performed by moving only the rear group, the lens driving unit can be simplified and the burden can be reduced, and the aberration is good from infinity to the close range. It is a corrected high-performance wide-angle lens, and it is possible to provide a lens having excellent performance as a lens of an imaging optical system such as a video camera or a still video camera, which requires compactness and macro photography.

[Brief description of drawings]

FIG. 1 is a diagram showing a lens configuration example and a focus movement mode of the present invention.

FIG. 2 is an optical cross-sectional view of Example 1.

FIG. 3 is an aberration diagram for Example 1 at infinity (A) and at close range (B).

FIG. 4 is an aberration diagram for Example 2 at infinity (A) and at close range (B).

FIG. 5 is an aberration diagram for Example 3 at infinity (A) and at close range (B).

FIG. 6 is an aberration diagram for Example 4 at infinity (A) and at close range (B).

[Explanation of symbols]

1 1 surface 2 2 surface 3 3 surface 4 4 surface 5 5 surface 6 6 surface 7 7 surface 8 8 surface 9 9 surface 10 10 surface 11 11 surface 12 12 surface 13 13 surface 14 14 surface 15 15 surface 16 16 surface 17 17 Surface 18 18 Surface 19 19 Surface L ₁ 1st lens L ₂ 2nd lens L ₃ 3rd lens L ₄ 4th lens L ₅ 5th lens L ₆ 6th lens L ₇ 7th lens G ₁ fixed group G ₂ fixed group G ₃ moving group

Claims (7)

[Claims] 1. A meniscus negative first lens having a convex surface directed toward the object side, a positive second lens, a meniscus negative third lens having a convex surface directed toward the object side, and an object side surface in order from the object side. A front lens group consisting of a positive fourth lens having a stronger curvature on the image side than that; a negative fifth lens; a biconvex sixth lens;
A rear lens group composed of a positive seventh lens,
A lens system is a lens system of 7 elements in 6 groups, which are cemented together. 2.2 <(r _a + r _b ) / (r _a −r _b ) <30.0 (1) L / f <5.8 (2) where r _{a is the} radius of curvature of the object side surface of the lens closest to the image side in the front group r _{b is the} radius of curvature of the image side surface of the lens closest to the image side in the front group f A retrofocus type lens characterized by satisfying conditional expressions (1) and (2) of the entire lens system. 2. A negative meniscus lens having a convex surface facing the object side, a positive second lens, a negative meniscus lens having a convex surface facing the object side, and an object side surface in order from the object side. A front lens group consisting of a positive fourth lens having a stronger curvature on the image side than that; a negative fifth lens; a biconvex sixth lens;
A rear lens group composed of a positive seventh lens,
The lens is a lens system having a structure of 7 elements in 6 groups, which is 9.0 <(r _a + r _b ) / (r _a −r _b ) <20.0 (3) L / f <5.6. (4) where r _{a is the} radius of curvature of the object side surface of the lens closest to the image side in the front group, r _{b is the} radius of curvature of the image side surface of the lens closest to the image side in the front group f is the focal length of the entire system L: A retrofocus lens characterized by satisfying the conditions (3) and (4) of the entire lens system. 3. A retrofocus type lens system comprising a front group consisting of a diverging system and a weak convergent system in order from the object side, and a rear group, wherein the rear group is moved for focusing, and f / | f ₁ | <0.4 (5) 0.31 <d / f <1.8 (6) L / f <5.8 (7) where f: focal length of the entire system f ₁ : focal length of the front group d: Distance between the divergence system of the front group and the weak convergence system L: The retrofocus lens characterized by satisfying the conditional expressions (5), (6), and (7) of the total length of the lens system. 4. A retrofocus type lens system comprising a front group consisting of a divergent system and a weak convergent system in order from the object side, and a rear group, wherein the rear group is moved for focusing, and -0.26. <F / f ₁ <0 (8) 0.8 <d / f <1.2 (9) L / f <5.6 (10) where f: focal length of the entire system f ₁ : focus of the front group Distance d: Distance between divergent system of front group and weak convergent system L: Total length of lens system (8), (9), (10) A retrofocus lens characterized by satisfying conditional expressions. 5. The retrofocus lens according to claim 3, wherein the most object side of the divergence system of the front group is a negative meniscus lens having a convex object side surface. 6. A negative meniscus lens having a convex surface facing the object side, a positive second lens, a negative meniscus lens having a convex surface facing the object side, and an object side surface in order from the object side. A front lens group consisting of a positive fourth lens having a stronger curvature on the image side than that; a negative fifth lens; a biconvex sixth lens;
A rear lens group composed of a positive seventh lens,
The lens is a retro-focus type lens, which is a lens system of 7 elements in 6 groups cemented together, wherein at least the third lens and the fourth lens are plastic lenses. 7. The retrofocus lens according to claim 6, wherein at least one of the refracting surfaces of the third lens and the fourth lens has a refracting power that decreases toward the lens peripheral portion from the optical axis. A retrofocus lens characterized by having an aspherical surface such that