When a shift lens is shifted, it is desirable (for stitching in particular) that the lens maintain perfect horizontal or vertical tracking so that the edges of the shifted frames match the edges of the unshifted frame (and each other) exactly. Perfect alignment is unrealistic on a camera like the Nikon D2X, with its 5 micron pixels, so misalignment is a matter of degree.
Checking this kind of misalignment is straightforward—shoot the lens unshifted, then shoot 4 additional frames, one for each direction of shift. Then, in Adobe Photoshop or similar program, match up the images pixel-for-pixel and observe any mismatched edges. (This test does assume that the subject is far enough away to eliminate parallax, which would make it impossible to match the images perfectly. Parallax, and how to eliminate it, are discussed in a forthcoming diglloyd article on shift-lens stitching).
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