For example, would an 85mm lens on a crop body visually maintain the same compression when put on a FF camera even though the focal length would appear shorter?
a) If you take a head-shot photo with 85mm on aps-c, then swap the lens to an FF body and take the same photo from the same spot
, you'll get a wider field of view and the head will be smaller within the frame. Then if you crop the second photo, you'll end up with *exactly* the same photo as the aps-c (except less MP).
b) If you take a head-shot photo with 85mm on aps-c, then swap the lens to an FF body and move closer so the head is the same size in the frame
, then perspective will change and you'll get more 'wide-angle' or less 'compression' or whatever you want to call it.