Optical: systems and elements – Lens – Reverse telephoto
Reexamination Certificate
2002-07-11
2004-02-24
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
Reverse telephoto
C359S754000
Reexamination Certificate
active
06697202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a retrofocus imaging lens suitable for, for example, a 35 mm camera, a medium-format camera, a video camera, and the like. More particularly, the invention relates to a retrofocus imaging lens of a rear focus system for achieving focus by moving a part, particularly, a rear-side lens group of a lens system.
2. Description of the Related Art
Conventionally, as an imaging lens of a wide angle of view having backfocus longer than a focal length, a retrofocus imaging lens is known. The retrofocus imaging lens is generally constructed by disposing, in order from an object side, a negative lens group and a positive lens group and the principal point is behind the whole lens system.
In a lens of this kind, by disposing a lens having a strong negative refractive power on the front side, an image can be captured to a wide angle of view, and a peripheral light amount can be increased. However, on the other hand, the lens has a drawback such that a large negative distortion is caused.
A retrofocus imaging lens has, generally, an asymmetric lens configuration as a whole in which a lens group having a negative refractive power is disposed in the front part and a lens group having a positive refractive power is disposed in the rear part, so that an amount of occurrence of various aberrations such as spherical aberration, comatic aberration, distortion, and astigmatism is large. There is a problem such that it is difficult to satisfactorily correct the various aberrations in total. Further, in the retrofocus imaging lens, in order to widen the angle of view of imaging while maintaining long backfocus, the negative refractive power on the front side has to be enhanced. In this case, to correct various aberrations which occur accordingly, the number of lenses increases. As a result, problems such as increase in the size of lenses, increase in cost, and occurrence of ghost which is hazardous to imaging occur.
In order to solve occurrence of the negative distortion, it is sufficient to use a negative lens having a high refractive index as the negative lens in the front-side negative lens group. However, a glass material with a high refractive index causes large dispersion, so that a chromatic aberration of negative magnification easily occurs. In the retrofocus imaging lens, by assuring a sufficient airspace between the negative front group and the positive rear group, a clear difference can be created between an incident height of an axial ray and that of an off-axis ray in each lens surface. The retrofocus imaging lens has therefore an element that the flexibility in aberration correction is increased. However, clear separation between the front and rear groups causes increase in the size of the whole system and increase in the diameter of the front lens.
Examples of conventional retrofocus wide-angle lenses are disclosed in Japanese Unexamined Patent Application Publication Nos. Sho 62-35089 and Hei 9-113800. Each of the lenses disclosed in the publications has front and rear lens groups sandwiching a stop. The converging action of the rear-side lens group is stronger than that of the front-side lens group, and a negative distortion is not fully corrected.
As a focusing method of a retrofocus imaging lens, a rear focus system of moving a rear group in a lens system can be considered. Generally, an amount of extending a focus lens group (hereinbelow, simply called a “focus group”) in the lens of the rear focus system is smaller as compared with an overall focus system in which the whole lens is extended. The focus group is relatively small and light and focus can be achieved with a small driving force, so that the rear focus type lens is suitable for a camera having an automatic focus detecting device, or the like. Although there are such advantages, the rear focus type lens has a problem such that aberrations fluctuate when the focus group is moved more as compared with the overall focus system and it is therefore difficult to satisfactorily correct the aberrations throughout an object distance.
Conventional rear focus type lenses are disclosed not only in Japanese Unexamined Patent Application Publication No. Hei 9-11380 but also Japanese Unexamined Patent Application Publication Nos. Sho 59-216114 and Sho 61-140910. Each of the lenses disclosed in the publications has a configuration that relative importance is placed on the focus group in comparison with the lens overall length and the total number of lenses, which is not preferable from the viewpoint of driving at the time of focusing. Particularly, in the case of using the lens in a camera having a large image size such as a medium-format camera, the size and weight are increased and, in addition, the load on the driving system becomes heavy, so that it is not preferable. Japanese Unexamined Patent Application Publication No. Hei 5-119254 discloses a technique of achieving focus only by the rear side of a stop. According to the technique, however, the focusing mechanism is complicated and the number of lenses is large. Techniques enabling focus to be achieved only by moving a part of lenses for the purpose of improving drivability are also disclosed in Japanese Unexamined Patent Application Publication Nos. Sho 55-147607 and Sho 58-202414 and the like. These techniques disclosed in the publications have a problem such that correction of aberrations only by a focus group is not sufficient, and the aberrations fluctuate largely due to movement of the focus group.
In the case of using a lens shutter in any of the above-described lenses in the publications, there is no space to dispose a mechanism for the lens shutter around the position of the stop. There is a problem such that when the space around the stop is sufficiently assured, the various aberrations cannot be satisfactorily corrected.
SUMMARY OF THE INVENTION
The present invention has been achieved in view of the above problems. Its first object is to provide a retrofocus imaging lens in which various aberrations such as a negative distortion are satisfactorily corrected. A second object of the invention is to provide a retrofocus imaging lens having a space sufficient for using a lens shutter and, particularly, having a sufficient space from a mechanism viewpoint so that a focus group does not interfere with a lens shutter mechanism at the time of achieving focus in accordance with a rear focus system.
A retrofocus imaging lens according to a first aspect of the invention includes in order from an object side: a first lens group having a positive refractive power as a whole constructed by a front-side first lens group having a negative refractive power as a whole and a rear-side first lens group having a positive refractive power as a whole; and a second lens group having a positive refractive power as a whole. Each of the front-side first lens group and the rear-side first lens group includes one or more positive lens and two negative lenses. The second lens group includes, basically, one or more positive lens and one or more negative lens. Further, a conditional expression (1) is satisfied.
−1.25
<f
1a
/f
1b
<−0.75 (1)
where f
1a
denotes a focal length of the front-side first lens group, and f
1b
represents a focal length of the rear-side first lens group.
In the retrofocus imaging lens according to the first aspect of the invention with the basic configuration, desirably, the front-side first lens group is constructed of four lenses which are, in order from the object side, for example, a positive lens, two negative meniscus lenses each having a convex surface directed to the object side, and a positive lens, or constructed of three lenses which are, in order from the object side, two negative meniscus lenses each having a convex surface directed to the object side and a positive lens and, further, a conditional expression (3) is satisfied.
0.35
<f
n
/f
1a
<0.65 (3)
where f
1a
denotes a focal length of the front-side first lens group and f
n
de
Frommer & Lawrence & Haug LLP
Fuji Photo Optical Co., Ltd.
Ryan Matthew K.
Schwartz Jordan M.
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