If a tall building is photographed
by pointing a camera upward, the
vertical lines of the building converge. Large format photographers have long known that it is possible to avoid this problem by shifting the lens vertically upward while keeping the image plane parallel to the building. To do this, the lens must have an image circle that is larger in diameter than the diagonal of the film format. While this is the norm for large format photography, in the realm of the 35-mm system a special lens called a shift lens is required. In addition to having a larger than normal image circle, shift lenses have a special mechanical structure that enables the shifting motion. This mechanical structure requires that the back focal distance be even greater than that normally required for 35-mm SLR lenses. Needless to say, the combination of a larger than normal image circle and back focal distance places great demands on the optical design, and, as a result, shift lenses tend to be a little slower and a lot more expensive than non-shift lenses with the same focal length.
by J. Brian Caldwell