The present invention relates generally to a zoom lens, and more particularly to a wide-angle zoom lens adapted for use with single-lens reflex cameras for which increased back focuses are needed.
In recent years, camera systems using an image pickup device smaller in image circle than prior art silver halide 35-mm film have been developed in the form of digital single-lens reflex cameras. The objective of these systems is to diminish image pickup device size, thereby providing a smaller, relatively inexpensive single-lens reflex camera.
Even with an image pickup device of smaller size, however, it is still difficult to diminish the size of a shutter unit or the like congruously. Because an optical low-pass filter or the like unnecessary for silver halide single-lens reflex cameras must be located in front of the image pickup device, it is also still difficult to diminish the back focus necessary for interchangeable lenses at a similar ratio at which image pickup device size is more reduced than 35-mm system size. Accordingly, these interchangeable lenses must have a larger back focus than the focal length of those for the 35-mm system. Especially with a wide-angle lens having a short focal length, there is a problem that it is very difficult to ensure the necessary back focus.
So far, such systems as set forth in patent publications 1, 2 and 3 have been known as a superwide-angle zoom lens like one having an angle of view of greater than 100° at the wide-angle end.
However, they have only a back focus for prior art silver halide 35-mm film systems, and so it is difficult to apply them immediately to digital single-lens reflex cameras which incorporate an image pickup device of such smaller size as mentioned above and must have a back focus at a larger ratio relative to focal length.
Patent Publication 1
JP(A)7-261082
Patent Publication 2
JP(A)2000-221399
Patent Publication 3
JP (A) 2002-287031