Originally posted by zapp Let's have a quick look on lens design. After designing a lens with a given focal length it is fairly straight forward to derive a lens with larger image circle and smaller f-stop or vice versa. Pentax did so when producing the 2.8/55 mm for 645 system. The DA* 1.4/55 mm came a little later for APS-C.
Sorry, but this is just plain wrong. The DA* 55/1.4 for APS-C is not a straight forward modification of the 55/2.8 for 645.
The design issues in creating larger image circles are different from the design issues in creating larger apertures because the different kinds of aberrations affecting the two. Moreover, in the context of DSLR lens design, the issue of flange focal length forces further differences in lens design for different formats. Sure, you can easily adapt any lens designed for the 645 to go on K-mount, but the result is an excessively long lens compared to what possible with a different design specifically for that mount. And there's no way that a 55mm f/1.4 for K-mount APS-C can be tweaked to operate on the deeper 645 mount with a larger image circle even if one sacrifices aperture.
The only fairly straight forward connection between designs for different image circles is direct scaling in which focal length and all the element sizes and element distances are all multiplied by some factor (with the result that the f-stop aperture remains constant). A 55/1.4 for APSC could be scaled by 1.833 to become a 100/1.4 for crop-645 although the result would be a huge expensive beast of a lens.