Erik: I don't believe I will encounter the problem of focus-shift for the specific purpose I had in mind when I posted my query. I am experimenting with the use of a Rokinon 85mm at its maximum aperture setting, i.e., f. 1.4. As at this aperture there is, of course, almost no depth-of-field, thus little or no margin for focusing error. The focusing has to be "right-on." I am trying this lens on both my Sigma SD15 DSLR and my Sony NEX 5n. On the Sigma, there's a mirror, and on the Sony, no mirror. The sensors of both cameras are the same size. I am trying to determine not only which is easier for me to focus, but which, if either, gives me the sharpest focus. Thanks for your quick, and informative response. Regards, Jim