JPH06242370A - Large aperture lens - Google Patents

Abstract

PURPOSE:To obtain a compact and large aperture photographing lens whose various abberations are effectively corrected by arranging the 1st to 7th lenses and satisfying specific conditions. CONSTITUTION:The 1st meniscus-shape lens having positive refractive power, the 2nd meniscus-shape lens having positive refractive power, the 3rd meniscus- like lens having negative refractive power, the 4th lens having negative refractive power, the 5th lens having positive refractive power, the 6th meniscus-like lens having negative refractive power, and the 7th lens having positive refractive power are successively arranged from the front. When it is defined that (ri) is the radius of curvature of the i-th lens surface counted from the front, (di) is lens thickness or an air gap on the axis of the i-th lens counted from the front, (ni) is the refractive index of a cutting material for the i-th single lens counted from the front, (fi) is the focal distance of the i-th lens group counted from the front, and (f) is the focal distance of the whole system, conditions expressed by inequalities I to IV are satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Gauss-type lens which has a field angle of about 46 degrees, an aperture ratio of about 1: 1.4, a long back focus, and a compact size.

[0002]

2. Description of the Related Art In general, a Gaussian lens having a long back focus has a refracting power of a lens arranged on the image side of a diaphragm provided in a space sandwiched by negative lenses and a refracting power of a lens arranged on the front side. It is necessary to be considerably stronger than that.

[0003]

Therefore, when it is attempted to make the lens compact while maintaining a predetermined back focus, the curvature of the lens is increased in order to bear the strong refracting power of the rear lens portion, and Since it is necessary to take measures such as increasing the number of constituent lenses, it becomes difficult to shorten the overall length and obtain a compact lens system. Further, since there is a difference in refracting power before and after the stop, the symmetrical shape about the stop is significantly impaired. Therefore, it is extremely difficult to perform good aberration correction when the lens has a large aperture.

[0004]

Since the refractive power of the rear lens group is particularly strong, the Patzval sum has a large positive value, so that the astigmatic difference tends to increase in the middle of the screen.

An object of the present invention is to provide a compact and large-diameter photographic lens in which various aberrations are well corrected by using glass having a relatively high refractive index for the positive lens of the rear group.

A feature of the present invention is that in order from the front, a meniscus-shaped first lens having a positive refractive power and having a convex surface directed to the front, and having a positive refractive power and having a convex surface directed to the front. A second lens having a meniscus shape, a third lens having a negative refracting power and having a convex surface facing forward, a fourth lens having a negative refracting power and a fifth lens having a positive refracting power, and a negative refracting power. And a meniscus-shaped sixth lens having a convex surface directed rearward, a meniscus-shaped sixth lens having a positive refractive power directed rearward, and a seventh lens having a positive refractive power disposed, r _i
From the front, the radius of curvature of the i-th lens surface, d _i
The lens thickness or air spacing on the i-th axis counting from the front, the i-th refractive index of the work material of the single lens to n _i counted from the front, the i-th a f _i counted from the front The focal length of the lens group, and when f is the focal length of the whole system,

[0007]

[Outside 2] To meet the following conditions.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a detailed understanding of the present invention, description will be given with reference to FIGS.

The first lens is a meniscus lens having a positive refractive power and has a convex surface facing forward. The second lens is a positive meniscus lens arranged with an air gap from the first lens, and has a convex surface facing forward.

The third lens is a negative meniscus lens arranged with a slight air gap from the second lens, and has a convex surface facing forward. The fourth lens is a negative lens which is arranged with a space including a diaphragm with respect to the third lens and has a concave surface facing forward. The fifth lens is a positive lens and the fourth lens and the fifth lens are cemented together. Constitutes a negative meniscus lens,
The convex surface is facing backward. The sixth lens is a positive meniscus lens arranged with a slight air gap from the fifth lens, and has a convex surface directed rearward. The seventh lens is a biconvex lens that is arranged with a slight air gap from the sixth lens. F ₁ and f ₂ are the focal lengths of the first lens and the second lens, respectively, and n ₅ , n ₆ and n ₇ are the fifth, sixth and seventh positive lenses respectively from the object side. work material refractive index of, d ₁ to d _10, the lens thickness or air spacing on the i-th axis in order from the object side, r ₆ is a curvature of the surface after the third lens radius, the r ₇ fourth lens front surface Where r ₄ is the radius of curvature of the rear surface of the second lens, and f is the focal length of the entire system, the following conditions are satisfied.

[0011]

[Outside 3]

Next, the meaning of various conditions will be described.

The condition (1) is a conditional expression that secures a long back focus and keeps distortion well.

In a single-lens reflex camera or the like, a distance of a certain value or more is required between the final lens surface and the image plane. In a general Gaussian lens, in order to obtain a back focus of 0.7 f or more, it is necessary to weaken the refractive power of the lens group in front of the diaphragm, and the distortion tends to show a negative value. In order to correct this distortion, the refractive power of the first lens located farthest from the diaphragm is adjusted to the second power.
It is corrected by making it relatively larger than the refractive power of the lens.

If the conditional expression exceeds the lower limit value, the distortion aberration becomes negative and is not corrected. When the value exceeds the upper limit, it is advantageous for correction of distortion, but the refracting power of the first lens increases, so that it becomes difficult to secure the back focus.

The conditional expression (2) is the fifth of the rear group, in which the refracting power becomes stronger because the refracting power of the front group (front of the diaphragm) is blocked.
It relates to the refractive index of the positive lens of the 6th and 7th lenses, and by using glass having a relatively high refractive index, it is possible to suppress the amount of aberration generation on each surface to be small.

When the value goes below the lower limit, it becomes necessary to form each surface with a strong curvature, spherical aberration becomes under, and Petzval sum increases, which is not preferable as a large-diameter lens.

Conditional expression (3) relates to the ratio of the axial thicknesses of the front group and the rear group, and is good when the optical path length of the diverging part of the front group is shorter than the optical path length of the converging part of the rear group and exceeds the lower limit. It becomes difficult to keep the back focus long with various aberration corrections.
On the other hand, when the upper limit is exceeded, it is advantageous to secure the back focus, but it is difficult to correct flare off-axis.

The conditional expressions (4) and (5) relate to a strong concave surface sandwiching a diaphragm peculiar to the Gauss type.

Due to the action of this surface, there is a drawback that coma aberration in the middle part of the screen and off-axis halos, especially halos of sagittal rays, are large. By making the curvatures of both surfaces gentle, these defects are corrected, but on the other hand, spherical aberration, astigmatism, and Petzval sum, which are responsible for most of the correcting action on this surface, are deteriorated. The conditional expression (4), together with the expression (5), is a condition for appropriately correcting these. If the lower limits of conditional expressions (4) and (5) are exceeded, coma and the halo of off-axis sagittal rays become significant, and if the values of conditional expressions (4) and (5) are exceeded, spherical aberration is undercorrected and Petzval sum increases, resulting in F1. It is not suitable for a large aperture lens of about 4. Conditional expression (6) is a condition for reducing the halo of the sagittal ray under the conditions (4) and (5), and when the lower limit value is exceeded, the refractive power of the ray passing through the high position on the surface is weakened and the sagittal halo is decreased. Will increase. When the value exceeds the upper limit, astigmatism becomes large, which is not preferable.

Next, numerical examples of the present invention will be shown. 1 to 4 are sectional views of Numerical Examples 1 to 4 in sequence, and FIGS. 5 to 8 show spherical aberration and astigmatism distortion with respect to an object point at infinity in Numerical Examples 1 to 4, respectively. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap from the object side, and Ni and νi are respectively from the object side of the i-th lens. Gaussian refractive index and Abbe number.

[0022]

[Outside 4]

[0023]

[Outside 5]

[0024]

[Outside 6]

[0025]

[Outside 7]

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of Numerical Example 1 according to the present invention.

FIG. 2 is a lens cross-sectional view of Numerical Example 2 according to the present invention.

FIG. 3 is a lens cross-sectional view of Numerical Example 3 according to the present invention.

FIG. 4 is a lens cross-sectional view of Numerical Example 4 according to the present invention.

FIG. 5 is a diagram of various types of aberration in Numerical example 1 according to the present invention.

FIG. 6 is a diagram of various types of aberration of Numerical example 2 according to the present invention.

FIG. 7 is a diagram of various types of aberration of Numerical example 3 according to the present invention.

FIG. 8 is a diagram of various types of aberration of Numerical example 4 according to the present invention.

[Explanation of symbols]

 ΔS sagittal image plane ΔM meridional image plane dd line gg line SC Limiting conditions

Claims (1)

[Claims] 1. A meniscus-shaped first lens having a positive refractive power and having a convex surface facing forward, a second meniscus-shaped lens having a positive refractive power and having a convex surface facing forward, and a negative lens. A third meniscus lens having a negative power and a convex surface facing forward, a fourth lens having a negative power and a fifth lens having a positive power, a negative surface having a negative power and a convex surface facing backward A sixth meniscus lens, a sixth meniscus lens having a positive refracting power and having a convex surface directed rearward, and a seventh lens having a positive refracting power are provided, and r _i is the i-th lens from the front. the radius of curvature of the lens surface, the refractive index of the work material of the i-th single lens lens thickness or air spacing on the i-th axis counting from the front of d _i, the n _i counted from the front, f _i Is the focal length of the i-th lens group counting from the front, and f is the focal length of the entire system, then A large-diameter lens characterized by satisfying the following conditions.