JPH0420162B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has an aperture ratio of about 1:2.8 and an angle of view of about 25°.
The present invention relates to a lens system for close-up photography in which aberrations are well corrected over the entire photographing range from infinity to the shortest distance at equal magnification. Traditionally, there are lenses called micro or macro lenses that are mainly used for close-up photography, but when designing such lenses,
In general, emphasis has been placed on correcting aberrations at magnifications of 1/5 to 1/10, and less importance has been placed on aberrations at infinity. Therefore, compared to other general photographic lenses, it could not be said that the aberrations near infinity were very good. Therefore, in this type of conventional lens, in order to maintain aberrations near infinity to a certain degree, the aperture ratio could only be made brighter to about 1:3.5 to 4. In addition, there are lenses with an aperture ratio of about 1:2.8 that employ a floating function to correct changes in aberrations due to changes in the shooting distance, but any lens with this floating function is not suitable for the lens system of the present invention. I have never seen a lens that can focus at the same magnification at the shortest distance from infinity. Therefore, when photographing at the same magnification with a lens having this floating function, an adapter such as a close-up ring is required, which is inconvenient. The purpose of the present invention is to have an angle of view of approximately 25° and a mouth ratio of 1:2.8.
To provide a lens system that is comparatively bright, capable of focusing from infinity to close distances of equal magnification, and with the lens body, and in which aberrations are well corrected over the entire photographing range. First, to explain the lens system of the present invention, the lens configuration of the present invention includes, in order from the object side, a first lens group L having a negative refractive power, a second lens group L having a positive refractive power, It is composed of a third lens group L〓 having negative refractive power, and a first lens group L〓 and a second lens group L〓.
While reducing the distance between lens group L〓, both the lens groups are
This is a close-up lens system that allows focusing from infinity to close range by extending it relative to the lens group L〓, and that it satisfies the following conditions. (1) 0.69＜f〓〓／f＜0.81 (2) 4.52＜｜f〓｜／f〓＜11.03 (3) 0.86＜S〓〓 _(-1) /S _(-1) ＜0.99 (4) 4.80 ＜P〓〓／P＜7.16 However, f〓〓 is the first lens group L〓 and the second lens group L〓
, f is the focal length of the entire system in the infinity shooting state, f〓 and f〓 are the first lens group L〓 and second lens group, respectively.
The focal lengths of the lens group L〓, S〓〓 _(-1) and S _(-1) are the Seidel values of the entire system from the first lens group L to the second lens group L under the same magnification and shortest distance shooting condition, respectively. The third-order spherical coefficients P〓〓 and P are the Petzval sums of the entire system from the first lens group L〓 to the second lens group L〓, respectively. According to such a lesbian configuration of the present invention, the first,
Since the amount by which the second lens group extends relative to the third lens group is approximately half that of the case where the entire lens group is extended, the structure of the lens frame body can be made relatively compact.
This is mechanically advantageous. Next, each of the above conditions will be explained. Condition (1) is the composite focal length f〓〓 of the first lens group L〓 and the second lens group L〓 in the infinity photography state.
This shows the range for the focal length f of the entire system, and the angle of view is approximately 1:2.8 with an aperture ratio of approximately 1:2.8 as in the present invention.
This is a necessary condition to achieve a 25° lens system.
If the lower limit of this condition (1) is exceeded, the amount of protrusion of the first and second lens groups L〓 and L〓 relative to the third lens group L〓 can be made shorter, but the negative refraction of the third lens group L〓 Since the force becomes too strong, the Petzval sum of the entire system becomes an extremely small value or a negative value, making it difficult to correct astigmatism and field curvature, and resulting in unsatisfactory imaging performance. On the other hand, if the upper limit is exceeded, correction of various aberrations becomes relatively easy, but
The amount of protrusion of the first and second lens groups increases rapidly, and the objective of the present invention cannot be achieved because the lens system becomes similar to a normal lens system using the entire protrusion method. Condition (2) is the second lens group of the first lens group L
It stipulates the ratio of the focal length to L〓, and various aberrations at each imaging magnification from infinity to near 1x are corrected by changing the distance between the first lens group L〓 and the second lens group L〓. It has an effect. If the lower limit of condition (2) is exceeded, the focal length f of the second lens group L, which is regulated by condition (1), becomes too short, making it difficult to correct spherical aberration, coma, etc. The brightness of the aperture ratio of about 1:2.8 cannot be maintained.
Moreover, if the upper limit is exceeded, the focal length of the second lens group f〓
becomes longer, and aberration correction becomes relatively easy, but the effect of correcting aberrations at each imaging magnification to the best condition is lost, so the object of the present invention cannot be achieved. Condition (3) is based on Seidel's third-order spherical coefficient S〓〓〓〓 _(-1) from the first lens group L to the second lens group L in the same magnification shooting state, and the third-order Desal of the entire system. It represents the ratio of the spherical coefficient S _(-1) , and indicates the range in which spherical aberration can be appropriately corrected in the shortest distance photographing state. If the lower limit of this condition (3) is exceeded, the second
Spherical aberration up to the lens group will be over-corrected, and conversely if the upper limit is exceeded, it will be under-corrected, so if only the third lens group L is corrected, it will be difficult to properly correct the spherical aberration of the entire system, which is not desirable. . Condition (4) expresses the ratio of the Petzval sum P〓〓〓 from the first lens group L〓 to the second lens group L〓 and the Petzval sum P〓〓〓〓〓〓〓〓〓〓〓 from the first lens group L〓〓〓〓〓〓〓〓> This is an important condition for correction. In the lens system of the present invention, it is desirable that the Pettuval sum P of the entire system is approximately 0.02 to 0.13 in order to maintain good imaging performance, but the third lens group L of the present invention has negative refraction. Because of this, the Petzval sum of the entire system tends to be very small or negative. Therefore, the ratio of the Petzval sum P from the first lens group to the second lens group and the Petzval sum P of the entire system must be within the range expressed by conditional expression (4). Incidentally, if the lower limit of this condition (4) is exceeded, that is, if the Petzval sum P〓〓 takes a negative or very small value, it becomes difficult to correct astigmatism at the periphery of the screen, resulting in a good result. Imaging performance cannot be expected, and if the Petzval sum P of the entire system becomes large, astigmatism cannot be well balanced, and the imaging performance, especially in the middle part of the screen, deteriorates significantly, which is not desirable. Conversely, if the upper limit is exceeded,
In other words, if the Petzval sum P〓〓 becomes large, the astigmatism up to the second lens group cannot be properly corrected, so even if the first lens group is floated, the aberrations of each magnification will be If optimal correction cannot be made and the Petzval sum P of the entire system becomes small, even if the first lens group is floated and aberrations are corrected at each magnification, astigmatism at the periphery of the screen cannot be corrected. This will not be satisfactory and the imaging performance will deteriorate. Hereinafter, data of examples of the present invention will be shown and explained.

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[Table] Here, numerical values corresponding to the above-mentioned conditional expressions of each of these examples are shown.

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In addition, in these embodiments, the third lens group L of the first lens group L and the second lens group L in the shortest distance photographing state
The amount of protrusion for the lens group L is as follows.

[Table] The configuration diagrams of the infinity shooting state of Examples 1, 2, 3, and 4 are shown in Fig. 1, Fig. 4, Fig. 7, and Fig. 1, respectively.
The aberration diagrams in that case are shown in the second figure.
5, 8, and 11, and the aberration diagrams at the above imaging magnifications are shown in FIGS. 3 and 3, respectively.
It is shown in FIG. 6, FIG. 9, and FIG. 12. In each of these examples, f, F _NO , ω, and f _B represent the focal length, aperture ratio, and half angle of view of the entire system when shooting at infinity, and r represents the curvature of each lens surface. radius, d is the lens thickness and air gap, nd is the d-line refractive index νd is d
Represents the Atsube number for a line. Also, F in the aberration diagram
is the aperture ratio in the shortest distance photographing state, and Y is the image height.

[Brief explanation of drawings]

1, 4, 7, and 10 are lens configuration diagrams in the infinity photography state of Examples 1, 2, 3, and 4 of the present invention, and FIGS. 2, 5, and 8 respectively. , 1st
Figure 1 is an aberration diagram of Examples 1, 2, 3, and 4 in the infinity photography state, Figure 3, Figure 6, Figure 9, and Figure 1.
FIG. 2 is an aberration diagram of Examples 1, 2, 3, and 4 when photographing at the shortest distance, respectively.

Claims (1)

[Claims] 1. In order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power,
It consists of a third lens group with negative refractive power, and by reducing the distance between the first lens group and the second lens group and extending both groups relative to the third lens group, A lens system for close-up photography that is capable of focusing from far to close distances and satisfies the following conditions. (1) 0.69＜f〓〓／f＜0.81 (2) 4.52＜｜f〓｜／f〓＜11.03 (3) 0.86＜S〓〓 _(-1) /S _(-1) ＜0.99 (4) 4.80 ＜P〓〓／P＜7.16 However, f〓〓 is the combined focal length of the first lens group and second lens group, and f is the focal length of the entire system in the infinity shooting state, and f〓 and f〓 are respectively the first lens The focal lengths of the lens group and the second lens group, S〓〓 _(-1) and S _(-1) are Seidel's 3 for the entire system, from the first lens group to the second lens group, and for the entire system, respectively, in the shortest distance shooting state at the same magnification. The following spherical coefficients,
P〓〓 and P are the Petzval sums of the entire system from the first lens group to the second lens group, respectively.