Marc speaks the truth.
I recently got a "wide angle" 6x6 lens to mount on my K20D, and sure enough - what is a "wide" 50mm lens on medium format is the same FOV as my FA50mm (but 4x larger and heavier). But, the image circle is comparatively gigantic! This huge image circle allows me to use a tilt or shift-adapter on a 35mm or APS-C frame, and move the lens all over without any vignetting:
And another thing to look out for is distortion. A 30mm "fisheye" from medium-format-land would be a "normal" perspective lens on APS-C, but with some serious barrel distortion for that crop factor, and even crazier effects if you tilt or shift it around the image circle! But then, that could be the fun part...
-Mark
Last edited by panoguy; 11-24-2009 at 04:04 PM.