JPS5832685B2 - Low magnification variable angle of view lens system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a still camera lens with a trimming effect or an effect of widening the angle of view.

As a low magnification variable angle of view lens system, patent application No. 5268698
There is an invention in No. 1, but while the invention is a wide-angle lens or a standard lens with a trimming effect,
The present invention aims to impart a trimming effect to a lens system having an angle of view between a standard lens and a telephoto lens, and to broaden the uses of this class of lenses.

This class of lens system for Leica format often uses a Gaussian type or telephoto type optical system, but the present invention provides the above-mentioned trimming effect.
The main feature of the basic configuration is that the front group has a negative focal length, the rear group has a positive focal length, and a negative lens is placed on the image plane side. It is a retro focus type when viewed from the perspective of its relationship with the lens, but by using a new type called a telephoto type when looking only at the rear group, it has an angle of view that is intermediate between a standard lens and a telephoto lens, making it a compact fixed focus lens. This makes it possible to obtain a low magnification variable angle of view lens system that has performance that is as good as that of the previous one.

The present invention will be explained in detail below.

The present invention is a lens system comprising, in order from the object side, a front group with a negative focal length and a rear group with a positive focal length, and which changes the angle of view by changing the distance between the front and rear groups, The front group includes a negative lens and a positive lens with a strong convex surface on the object side, and the rear group includes a first positive lens group consisting of two or three positive lenses, a first negative lens, and one positive lens. It is composed of a second positive lens group consisting of a lens and a second negative lens whose surface on the object side is strongly concave, and where fw: the focal length of the entire system on the wide angle of view side; w: the front group on the wide angle of view side; Air distance between the rear group fTT: Focal length dl of the rear group: Air distance between the negative and positive lenses in the front group rT: Radius of curvature a of the object side surface of the positive lens in the front group, : Air distance between the first negative lens and second positive lens group in the rear group fw': Closed to the wide angle of view side Focal length from the front group to the second positive lens group in the rear group dTI': Rear This is a low magnification variable angle of view lens system that satisfies various conditions regarding the air distance between the second positive lens group and the second negative lens in the group.

The reason for arranging the positive lens on the image side in the front group is to prevent fluctuations and increases in the five aberrations that occur when the distance between the front and rear groups is changed.In the rear group, the first positive lens group on the object side The reason why the lens is composed of two or three positive lenses is to prevent various aberrations, especially spherical aberration, from changing and increasing when the angle of view of the entire system changes.

In addition, since the present invention is a lens system with a trimming effect that has an angle of view between a standard lens and a telephoto lens, a simple retrofocus type would have a long overall length. A negative lens is placed on the image plane side, making it compact.

Conditions (1) and (2) are related to the power arrangement of the lens system. If the upper limit of condition (1) is exceeded, the power of the rear group becomes weaker, which is advantageous in terms of aberration correction, but the distance between the front and rear groups This has the drawback of increasing the overall length and the diameter of the front ball.

The lower limit value means a performance limit; below this value, the power of each lens surface becomes too strong, which is disadvantageous in terms of aberration correction for a lens system having a moving part.

Condition (2), along with condition (1), is necessary for realizing a compact lens system.

If the upper limit value is exceeded, the overall length will increase, and the power of the rear group will also need to be weakened, resulting in an increase in the size of the entire system.

The lower limit value is necessary to ensure the amount of trimming; below this value, the effect of trimming is small, and if you try to ensure the amount of trimming, the power of each lens surface will become too strong, which is disadvantageous in terms of aberration correction.

Condition (3) is to keep the back focus above a certain level,
In addition to condition (4), this is necessary in order to maintain good fluctuations in aberration within the front group.

If the upper limit is exceeded, it is advantageous to lengthen the back focus, but the position of the marginal ray that enters the object-side surface of the final positive lens in the front group will be far away from the optical axis, causing spherical aberration and other problems. Various aberrations occur rapidly, and in order to correct them, the power of the valley lens in the front group must be weakened, which increases the overall length.

Condition (4), together with condition (3), is necessary to maintain a good aberration balance within the front group.If the upper limit is exceeded, excessive spherical aberration and coma aberration generated in the negative lens of the front group will be corrected. It becomes difficult to do so.

In order to prevent this, it is necessary to exceed the upper limit of condition (3) or weaken the power of each lens, which tends to make the lens system harsh, and if the lower limit is exceeded, it is not advantageous for making the lens 4 more compact. However, in order to increase the pack focus, it is necessary to increase the power of the negative lens in the front group, which has the disadvantage that various aberrations occur, especially spherical aberration on the narrow angle of view side, which rapidly increases.

Condition (5) is necessary to maintain good astigmatism, image curvature, and distortion.

In other words, by moving the second positive lens group in the rear group away from its front lens, the under astigmatism and image curvature caused by the first positive lens group in the rear group can be corrected in the opposite direction. However, if the upper limit is exceeded, the amount of correction becomes excessive, barrel distortion aberration increases, and the overall length of the lens system increases.

Furthermore, if the lower limit is exceeded, astigmatism and curvature of field aberration change in one direction, which is unfavorable in terms of aberration balance.

Conditions (6) and (7) are the most important conditions in the present invention.

In other words, since the purpose of the present invention is to add a trimming effect to a lens system having an angle of view in the intermediate class between a standard lens and a telephoto lens, a second negative lens is arranged on the image plane side of the rear group, and the lens system condition (6).

(7) relates to the power arrangement of the second negative lens.

If the lower limit of condition (6) is exceeded, the final lens will not have a negative power, and the total length of the lens system will become longer.The upper limit indicates the performance limit, and if the upper limit is exceeded, it is advantageous for miniaturization. However, the back focus becomes small, and in order to compensate for this drawback, it is necessary to increase the negative power of the second negative lens in the rear group, making it difficult to correct various aberrations.

Condition (7), along with condition (6), is advantageous for downsizing when the upper limit is exceeded, but back focus becomes small and astigmatism becomes under.

In order to correct this defect, it is necessary to increase the negative power of the second negative lens in the rear group, making it difficult to correct various aberrations.

Examples 1 and 2 will be described below.

Here, f is the focal length, ω is the half angle of view, r is the radius of curvature, d is the lens thickness or distance between lens surfaces, N is the refractive index of dline, and ν is the Abbe number.

[Brief explanation of the drawing]

FIG. 1 shows a lens system configuration diagram on the wide angle of view side corresponding to the first embodiment. Figure 2 ayb is a diagram of various aberrations of Example 1, where a is the wide angle of view side;
b shows the state on the narrow angle of view side. FIG. 3 is a configuration diagram of a lens system on the wide angle of view side corresponding to Example 2. 4a and 4b are various aberration diagrams of Example 2, where ri is the radius of curvature of each V lens surface, and di is the V7 lens thickness or distance in the lens direction. Further, A indicates an aperture.

Claims (1)

[Claims] 1. A lens system consisting of, in order from the object side, a front group with a negative focal length and a rear group with a positive focal length, and which changes the angle of view by changing the distance between the front and rear groups. The front group includes a negative lens and a positive lens with a strong convex surface on the object side, and the rear group includes a first positive lens group consisting of two or three positive lenses, a first negative lens, and a first negative lens. It is composed of a second positive lens group consisting of two positive lenses and a second negative lens whose surface on the object side is strongly concave, and where fw: focal length of the entire system on the wide-angle side 1w: focal length of the entire system on the wide-angle side Air distance between the front group and rear group fTI: Outstanding focal length d, : Air distance between the negative lens and positive lens in the front group r□: On the object side surface of the positive lens in the front group Radius of curvature d1□: Air distance between the first negative lens and second positive lens group in the rear group fw': Focal distance dTI' from the front group to the second positive lens group in the rear group on the wide angle of view side : A low magnification variable angle of view lens system that satisfies various conditions regarding the air distance between the second positive lens group and the second negative lens of the f group fM.