Are you just reading the distance off the lens barrel? Try a roll and see what the negs look like. There's not much (other than serious, obvious damage) that could change the lens-to-film distance, or the lens to focusing screen distance, for that matter. If the mirror is not returning completely, that might be a problem. Take off the lens and carefully lift the front edge of the mirror up just a bit. It should return to the stop with a definite click when you release it. Be careful not to touch the mirror surface. If it seems at all stiff or reluctant to return to the "down" position, that could be your problem. I'd still try a roll.
Originally Posted by noumin