I just picked up a Computar 12.5-75 as an improvement to the fixed 16mm 1.4 I'd been using. The first thing I noticed is that the lens seems to be highly varifocal. As in, if I set the backfocus properly at one focal length, by the time I adjust the zoom/focal length even by a small amount the whole focus range has shifted so far I need to readjust the backfocus again. I'm curious if that's normal with this lens on the 1.4. Seems odd to have to adjust backfocus every time you change the zoom.
