Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-04-26
2003-11-11
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000
Reexamination Certificate
active
06646814
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens, and more particularly to a wide-angle zoom lens system suitable for use on video cameras, still video cameras and many others.
Recent zoom lenses currently in vogue for consumer-oriented video cameras are of the type which, as typically disclosed in JP-A 63-29718, comprise, in order from its object side, a positive, a negative, a positive and a positive lens group or four lens groups in all, with zooming occurring at the second lens group and correction of an image position upon zooming and focusing occurring at the fourth lens group. Many zoom lenses of this type have a field angle (2w) of about 50° at their wide-angle ends. On the other hand, JP-A 10-62687 discloses a zoom lens having a wider field angle, e.g., a field angle of about 66° at its wide-angle. This zoom lens comprises, in order from its object side, a positive, a negative, a positive and a positive lens group or four lens groups in all. While the second, third and fourth lens groups are all movable during zooming, a wide field angle is achievable by conforming to various conditions.
The zoom lenses set forth in JP-A 10-62687 all have a zoom ratio of about 3. Examples 1 and 2 are relatively fast as expressed by an F-number of 2 at their wide-angle ends. However, the back focus is small. Examples 3 to 6 have an F-number of about 2.8 at their wide-angle ends, but their back focuses are large.
There are strong demands for zoom lens systems that are ever higher in zoom ratios than those set forth in JP-A 10-62687. With pixel pitch reductions in electronic image pickup devices such as CCDs, a faster zoom lens system with well-corrected aberrations is increasingly demanded. In addition, it is desired to achieve a back focus large-enough for accommodation of an optical path splitter and a color separation prism for single-lens reflex cameras.
SUMMARY OF THE INVENTION
In view of such situations in the prior art as mentioned above, an object of the present invention is to provide a zoom lens system that has a wider field angle and a higher zoom ratio with well-corrected aberrations, is fast, and has a large back focus.
According to one aspect of the invention, this object is achieved by the provision of a zoom lens system which comprises, in order from an object side of said zoom lens system, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power and in which for zooming from a wide-angle end to a telephoto end of said zoom lens system, said second lens group moves toward an image side of said zoom lens system, said third lens group moves toward said object side and said fourth lens group moves toward said object side, wherein:
a lens located nearest to said image side in said third lens group is a negative lens concave with respect to an image plane of said zoom lens system and a lens located nearest to said object side in said fourth lens group is a negative lens concave with respect to an object.
According to another aspect of the invention, there is provided a zoom lens system which comprises, in order from an object side of said zoom lens system, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power and in which for zooming from a wide-angle end to a telephoto end of said zoom lens system, said first lens group remains fixed on an optical axis of said zoom lens system, said second lens group moves toward an image side of said zoom lens system, a stop remains fixed on said optical axis, said third lens group moves toward said object side and said fourth lens group moves toward said object side, wherein:
a lens located nearest to said image side in said third lens group is a negative lens concave with respect to an image plane of said zoom lens system and a lens located nearest to said object side in said fourth lens group is a negative lens concave with respect to an object.
Preferably in these cases, the fourth lens group should comprise, in order from its object side, a negative lens, a positive lens and a positive lens.
Why the aforesaid arrangements are used and how they work will now be explained.
The zoom lens system of the type which comprises, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power and in which for zooming from a wide-angle end to a telephoto end thereof, the second lens group moves toward an image side thereof, the third lens group moves toward the object side and the fourth lens group moves toward the object side is advantageous for achieving wider field angles, as explained with reference to JP-A 10-62687.
For the achievement of the object of the invention, the constructions of the third and fourth lens groups are of most important. To have a large back focus, the lens system must have a lens arrangement that enables the back focus to be easily ensured. However, the lens system also must be suitable for correction of axial aberrations, because with a large back focus yet a small F-number, the position of an axial light ray through the third and fourth lens groups becomes higher. To achieve high zoom ratios, the third and fourth lens groups in the lens system must move on the optical axis while aberration variations are reduced. To satisfy these conditions, the lens located nearest to the image side in the third lens group should be a negative lens concave with respect to the image plane and the lens located nearest to the object side in the fourth lens group should be a negative lens concave with respect to the object. The third and fourth lens groups, on the whole, comprise an arrangement approximate to a double-Gauss lens arrangement, and move for zooming while the separation between them is varied. It is thus possible to achieve the object of the invention.
In the second embodiment of the invention, where the stop is located is important to make a tradeoff between wide field angles and simplified lens barrel construction. For the downsizing of the first and fourth lens groups while wider field angles are achieved, it is favorable to locate the stop substantially at the center of the optical system. It is also favorable to fix the stop on the optical axis, because the construction of the lens barrel is not complicated. In view of the construction of the lens barrel, it is preferable to fix the first lens group in place during zooming.
In the first and second embodiments of the invention, the constructions of the third and fourth lens groups having zooming and image-forming actions are important to ensure the desired back focus and make correction for aberrations. This is particularly true of the construction of the fourth lens group. Since it is desired that the positive refracting power of the fourth lens group be shared by as many lens elements as possible, it is preferable that the fourth lens group comprises, in order from its object side, a negative lens, a positive lens and a positive lens.
According to the fourth embodiment of the invention where it is desired to reduce the number of lenses, the fourth lens group should preferably consist of, in order from the object side, a negative lens, a positive lens and a positive lens.
According to the fifth embodiment of the invention, the construction of the second lens group having zooming action is important to achieve higher zoom ratios. To make correction for aberrations due to zooming, it is desired that the negative refracting power of the second lens group be shared by as many lens elements as possible. In other words, it is preferable that the second lens group comprises, in order from the object side, a negative lens, a negative lens, a negati
Suzuki Masahiro
Uzawa Tsutomu
Olympus Optical Co,. Ltd.
Schwartz Jordan M.
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