JPH033205B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is composed of a first group having a negative focal length and a second group having a positive focal length disposed on the image field side of the first group. This relates to a variable magnification optical system in which the air distance between the second group and the second group changes. FIG. 1 is a diagram showing the basic configuration of a variable magnification optical system to which the present invention pertains. It is a so-called reverse telephoto type consisting of a negative first group and a positive second group 2, and the first group By changing the air distance l between the image plane 3 and the second group, and at the same time changing the air distance between the image plane 3 and the second group, the image plane 3 is
A so-called mechanical correction is performed to keep the value constant. This variable magnification optical system consisting of a negative first group and a positive second group is an inverted telephoto type, so it is advantageous for long backfocus and widening the angle of view.
It is suitable as an ultra-wide-angle variable magnification optical system for still cameras, movies, and televisions. In the variable magnification optical system described above, a method for performing focusing on a close-range object is as follows:
There is a method called front lens extension in which focusing is performed by fixing the second group and simply extending the entire first group. In this method, when zooming is performed while focusing on a certain object distance, a slight change in focus occurs during zooming because the first group serves both as a focus section and a zoom section. This amount of change is small and is normally used as is because it falls within the depth of focus, but this method is often disadvantageous in terms of aberration correction.
That is, in a wide-angle lens that generally consists of a negative first group and a positive second group, increasing the air gap between the two groups increases astigmatism and causes the image plane to become significantly oversized. Therefore, in the above-mentioned variable magnification optical system, in which the first group is extended for focusing on a close object, astigmatism occurs because the distance between both groups increases. There are large fluctuations. In addition, in general, in a wide-angle lens, when the object distance becomes close, astigmatism increases and the image plane tends to overshoot even if the entire lens system is extended and focusing is performed. To explain this using third-order aberration coefficients: spherical aberration coefficient; comatic aberration coefficient; astigmatism coefficient; spherical aberration coefficient; distortion aberration coefficients ^S , ^S ; The aberration coefficients ′, ′ at ′ are ′=−δ(V+ ^S )+δ ^{2 S} ′=−δ(V+ ^S )+δ ^{2 S} ′. Here, δ is a quantity related to the object distance, and becomes larger as the distance becomes closer. From this equation, it can be seen that the conditions for removing short-range aberration fluctuations are V+ ^S =0, ^S =0. In general reverse telephoto lenses, ^S is almost zero in most cases, V is generally positive, and ^S is generally negative, but since the absolute value of ^S is larger than V, δ is large. In other words, the closer the distance is, the greater the influence of this term becomes. Since δ<0, the numbers are '<,'<, and the image plane is over. The condition for v+ ^S = 0 is V
=0, ^S =0 is ideal, but the condition for ^S =0 is that when the incident angle of the pupil paraxial ray is the exit angle, =' must be satisfied. This means that the pupil plane and the principal plane coincide. However, in an inverted telephoto lens, the principal plane is behind the pupil plane and >', so ^S = 0 cannot be set. For the reasons mentioned above, deterioration of the short-distance performance of ordinary reverse telephoto lenses is unavoidable due to the construction of the lens, and this drawback becomes more significant as the back focus becomes longer.
In this case, various aberrations other than astigmatism and curvature of field are also affected, but their amounts are relatively small. For the above reasons, in a variable magnification optical system that consists of a negative first group and a positive second group and performs zooming by changing the distance between the two groups, focusing with the first group is important in reducing aberrations at short distances. It was difficult to shorten the close range because of severe deterioration and vignetting of off-axis rays as the first group moved out. SUMMARY OF THE INVENTION An object of the present invention is to provide a variable power optical system comprising a negative first group and a positive second group, which improves the above-mentioned drawbacks in focusing. In order to solve this problem, the present invention changes a certain air interval within the first group as the first group is extended in a state that has a substantially linear relationship with the amount of extension of the first group. be. By reducing a certain air gap within the first group in proportion to the amount of extension of the first group, deterioration of imaging performance for close-range objects,
In particular, while preventing deterioration of astigmatism, reducing the combined thickness of the first group prevents an increase in the diameter of the front lens due to extension, and also prevents a decrease in the amount of peripheral light at short distances. The present invention will be explained in detail below. In the above-mentioned variable magnification optical system consisting of a negative first group and a positive second group, in order to maintain good image quality up to the periphery even in close-range shooting, spherical aberration, coma aberration, distortion, etc. have to be adversely affected. It is necessary to make a correction to return the image plane to an under level without causing any damage. This correction can be made by changing the air spacing within the lens system, and is a method that has been applied to many single cameras in the past. However, the present invention is a variable magnification optical system, and field curvature, which is the main cause of performance deterioration at short distances, increases as the angle of view increases. Therefore, in the present invention, there is a large correction effect on the field curvature on the wide-angle side,
It is necessary to provide a lens interval that allows correction such that the correction effect gradually decreases as you move toward the telephoto side. It is preferable for the lens interval to have such an effect that the absolute value of the height of the pupil paraxial ray at the interval is large on the wide-angle side and small on the telephoto side. However, as a correction for each focal length, it is desirable for the value of to change approximately linearly in proportion to the change in focal length when the variable power optical system changes magnification from the wide-angle side to the telephoto side. Furthermore, it is advantageous to have a lens spacing such that the light beam incident from the on-axis object point is nearly parallel to the optical axis of the lens. This is because changing the lens spacing like this does not affect axial aberrations, that is, spherical aberrations, but mainly affects aberrations of light beams with a large angle of view on the wide-angle side due to the variable magnification effect, and reduces field curvature and astigmatism. This method has advantages such as being advantageous for correction of , and that the focal length does not change with respect to focusing. It is possible to correct field curvature by changing an interval other than this, that is, an interval at which the light beam converges or diverges, but the height of incidence on the lens immediately after this interval changes, causing fluctuations in spherical aberration, and the screen This will reduce the image quality at the center, and the aberrations will fluctuate sharply due to the variable magnification effect. The present invention provides the above-mentioned lens spacing (hereinafter referred to as air spacing) in the first lens group, which is a negative lens, and the air spacing makes the first group consist of at least one negative lens and one or more positive lenses. A front group consisting of The amount is decreased in proportion to the amount of movement of one group.
In this case, the variable magnification optical system satisfies the following conditions (1) to (5). (1) −3.0＜f ₁ ／f _W ＜−1.17 (2) 0.54＜l _W ／f _W ＜1.5 (3) 0.04f _W ＜d＜0.3f _W (4) ｜f _1F ｜＞f _W (5 ₎ is | _W | > _T |, which _{monotonically} decreases as the focal length increases _. Distance f _1F between the first group and the second group in the first group; focal length d of the front group in the first group; air distance between the front group and the rear group when the object is at infinity; the pupil paraxial ray is in the first group The value at the air gap between the front and rear groups in the above, and the average value _W in the plane before and after the air gap; Value at the above wide-angle end _T : Value at the above telephoto end Conditional expression (1) and (2) are related to the power arrangement of the lens system. If the upper limit of condition (1) is exceeded, it becomes difficult to correct various aberrations including distortion. If it is below the lower limit, it is advantageous in terms of aberration correction, but the entire lens system becomes large, making it impossible to obtain a compact variable magnification optical system. The upper limit value of condition (2) was also determined to make the entire lens system smaller, while the lower limit value was determined to prevent the drawback that the amount of movement for zooming becomes too small and the zoom ratio becomes small. . Conditional expression (3) is intended to satisfactorily correct aberrations when there are an infinite number of objects. Since the first group is a negative divergence system, positive spherical aberration, negative distortion, and positive astigmatism are likely to occur. In order to correct these various aberrations, the effect of the positive converging system should be strongly exerted within this negative diverging system so that they cancel each other out.
That is, if d is less than the lower limit of conditional expression (3), the convergence effect of the positive lens provided closer to the object than the distance d cannot be sufficiently obtained. Therefore, the amount of aberrations generated within the first group increases, and aberrations cannot be sufficiently corrected in the entire system. On the other hand, when the value exceeds the upper limit, the aberration correction effect of the positive lens is too strong and becomes excessively corrected, resulting in deterioration of astigmatism, coma aberration, and spherical aberration. That is, when d is within the range of conditional expression (3), various aberrations can be maintained well when the object is infinite. Conditions (4) and (5) are intended to reduce aberration fluctuations when the object is close. Conditional expression (4) states that within the air gap in the first group of a variable magnification optical system consisting of a negative first group and a positive second group, the light flux incident from the on-axis object point is approximately aligned with the lens optical axis. The parallel air gap is variable, and when the first group is extended to focus on a close-range object, the air gap is shortened approximately in proportion to the amount of extension, thereby preventing image deterioration during close-range shooting. It is something. That is, when the range of conditional expression (4) is satisfied, only the curvature of field can be corrected, which is advantageous for aberration correction as described above. Conditional expression (5) ensures that aberrations are completely corrected at each focal length under the power arrangement of conditional expressions (1) and (2) in the variable magnification optical system. In other words, the absolute value of the height at the variable interval of the pupil paraxial rays (actually the average value of the heights on the planes before and after this interval) is greater at the wide-angle end and smaller at the telephoto end, and when the focal length is increased, On the other hand, the configuration is such that h monotonically decreases.
With such a configuration, by reducing the air gap for close objects and focusing with the first group,
Aberrations are kept good and no deterioration of aberrations occurs even when changing magnification. In this way, the air gap is reduced roughly in proportion to the amount of extension of the first lens group.
This is because the near-field aberration variation changes approximately linearly in proportion to the amount of extension. Therefore, the mechanism for extending the first lens group and the mechanism for changing the air gap can be easily linked. As is clear from the meaning of the conditional expressions mentioned above,
In the variable magnification optical system according to the present invention, if conditional expressions (3) and (4) are satisfied, aberration correction can be performed satisfactorily. Furthermore, it can be seen that if conditional expressions (1), (2), and (5) are satisfied, it is possible to downsize the variable power optical system. FIG. 2 shows that the increase in the effective diameter due to the feeding of the first group is suppressed by decreasing the air gap within the negative first group, that is, by substantially decreasing the composite thickness d of the first group. FIG. 3 is a diagram showing a state in which an increase in the effective diameter of the cylinder is prevented. Figure 2A shows how the axial light beam _L1 and the off-axis light beam _L2 are imaged on the film surface 3 by the negative first group 1 and the positive second group 2 when the object is infinite. It shows. As the object becomes closer, the first
As shown in FIG. 2B, in the method of performing focusing by simply advancing the group, the off-axis light beam, especially the light beam _L4 at the maximum angle of view, is eclipsed by the effective diameter of the first group, and the amount of peripheral light becomes extremely small. In this state, in order to increase the amount of peripheral light, the effective diameter of the first group must be increased, resulting in a loss of compactness. This effect becomes more pronounced as the angle becomes ultra-wide, becomes important in terms of the size of the optical system, and even affects operability. FIG. 2C is a diagram showing a state in which vignetting of the peripheral luminous flux is prevented by reducing the air gap according to the present invention described above. When the object is at a short distance, the combined thickness d of the first group decreases to d' at the same time as the first group is extended. For this purpose, the luminous flux at the maximum angle of view L ₆
The vignetting in the effective diameter of the first group is significantly reduced, and the amount of light is maintained almost the same as in the case where the object is infinite. Due to this, cine cameras and TV
When continuously photographing with a camera, etc., it is possible to prevent peripheral shadows. Examples of the present invention will be shown below. Figure 3 A, B, C
shows an optical path diagram when the object is infinite in the first embodiment of the present invention, where A is at the wide-angle end, B is in the intermediate state, and C is in the intermediate state.
indicates the telephoto end. Figure 4 A, B, and C are Figure 3 A,
It is an aberration diagram corresponding to B and C. Figure 5 A is the first
In the optical path diagram when the magnification is -0.0777 at the wide-angle end of the example, B and C show the optical path diagram in an intermediate state and at the telephoto end when the magnification is changed without changing the distance between the object and the image point. . The air distance d ₄ in FIGS. 5A, B, and C is reduced compared to FIGS. 3A, B, and C, which show the case where the object is infinite. Figure 6 A, B, C is Figure 5 A, B, C
It is an aberration diagram corresponding to . Figures 7A, B, and C show the case where the variable magnification optical system of the first embodiment shown in Figure 3 is simply extended with the first group without varying the air distance _d4 at a magnification of -0.0777. It shows. Figure 7A
As is clear from the figure, in this case, at the wide-angle end, the amount of peripheral light is significantly reduced. Figure 8A,
B and C are aberration diagrams corresponding to FIG. 7A, B, and C. Next, the specification values of the first embodiment will be shown. First example

【table】

[Table] In the first embodiment, aspheric surfaces are used for the _R1 and _R20 surfaces in order to further compact the optical system. Equation of aspherical surface Then, R ₁ side r = 3.2115 A = 0 B = 8.3 x 10 ^-3 C = 4.8 x 10 ^-4 D = 0 E = 0 R ₂₀ side r = -0.9233 A = 0 B = 6 x 10 ^-2 C =2.0×10 ^-2 D=0 E=0 Next, a second example will be shown. 9A, B, and C show optical path diagrams when the object is infinite in the second embodiment of the present invention, where A shows the wide-angle end, B shows the intermediate state, and C shows the telephoto end. Figure 10 is A, B, C is 9th
Aberration diagrams corresponding to figures A, B, and C. Figure 11A is an optical path diagram at a magnification of -0.0750 at the wide-angle end of the second embodiment, and B and C are an intermediate state and a telephoto end when changing the magnification without changing the distance between the object and the image point. shows an optical path diagram. Figure 11 A, B, and C are compared to Figure 9 A, B, and C, which show the case where the object is infinite, with air spacing d ₄
is decreasing. FIGS. 12A, B, and C are aberration diagrams corresponding to FIGS. 11A, B, and C. 13A, B, and C show the case where the variable magnification optical system of the second embodiment shown in FIG. 9 is simply extended with the first group without changing the air distance d ₄ at a magnification of -0.0750. It shows. As is clear from FIG. 13A, in this case, at the wide-angle end, the amount of peripheral light is significantly reduced. Figure 14 A, B, C is Figure 13 A, B, C
It is an aberration diagram corresponding to . The specification values of the second embodiment are shown below. Second example

[Table] In the second embodiment, an aspherical surface is used for the _R1 surface in order to further compact the optical system. The aspherical coefficients at that time are: R ₁ -plane aspherical surface r = 3.2470 A = 0 B = 3.2 × 10 ^-2 C = 1.83 × 10 ^-3 D = 4.10 × 10 ^-3 E = 0

[Brief explanation of drawings]

FIG. 1 is a diagram showing the basic form of the variable magnification optical system of the present invention, FIG. Figures A, B, and C are diagrams showing the case where the object is infinite in the first embodiment, where A is the wide-angle end, B is the intermediate state, and C is the telephoto end. Figures 4A, B, and C are aberration diagrams corresponding to Figures 3A, B, and C, and Figures 5A, B, and C are graphs of the first embodiment when the magnification at the wide-angle end is -0.0777. Optical path diagrams in each variable magnification area, Figure 6 A, B, C are Figure 5 A, B, C
7A, B, and C are aberration diagrams corresponding to the conventional focusing method at a magnification of -0.0777 using the variable magnification optical system shown in FIG. 8, and FIG. B and C show aberration diagrams corresponding to Fig. 7 A, B, and C. Fig. 9 A, B, and C show optical path diagrams when the object is infinite in the second embodiment, and A is at the wide-angle end. , B indicates the intermediate region, and C indicates the telephoto end. 1st
Figures 0A, B, and C are aberration diagrams corresponding to Figures 9A, B, and C, and Figures 11A, B, and C are the aberration diagrams when the magnification at the wide-angle end of the second embodiment is -0.0750. Optical path diagrams in the variable magnification region; Figures 12A, B, and C are aberration diagrams corresponding to Figures 11A, B, and C; Figures 13A, B,
C is the optical path diagram of the variable magnification optical system shown in FIG. 9 at a magnification of -0.0750 in each variable magnification area using the conventional focusing method. Corresponding aberration diagram. 1...First lens group, 2...Second lens group, 3
...Film surface, d...Thickness of the first lens group,
L ₁ to _{L 6} ... Luminous flux, S ... Aperture, R ... Radius of curvature of lens surface, D ... Lens thickness or lens spacing.

Claims (1)

[Claims] 1. A first group having a negative focal length, and a second group disposed on the image field side of the first group and having a positive focal length,
The distance between the first group and the second group changes during zooming, and the first group includes a front group having at least one negative lens and one positive lens, and is arranged on the image field side of the front group. It consists of a rear group having negative refractive power,
For focusing from infinity to the nearest
When moving the group, the moving speed of the rear group is larger than that of the front group, and 0.04fw<d<0.3fw | f _1F | > f _w is | _W | > | _T | Monotonically decreases as the distance increases. However, f _1F ; Focal length of the front group f _W ; Focal length d of the entire system at the wide-angle end; Air distance between the front and rear groups when the object is at infinity; Pupil paraxial ray is the value in the space between the front group and the rear group in the first group, and satisfies the following conditions: the average value _W in the plane before and after the air gap; the value _T at the wide-angle end; the value at the telephoto end above. A variable magnification optical system featuring