I've been shooting Hasselblad for a couple of years now and haven't noticed any degeneration in the lenses, but the odd time my backs will double expose or overlap a couple of frames (it's getting more frequent). My question on preventative maintenance is: is there a rule of thumb on how often a lens should be CLA'd? maybe a second question is: does my film backs need to be overhauled or is it a problem with my camera body (with the overlapping film)? I know that there are a lot of different variables to this (storing with the shutter uncocked, frequency of usage, etc. but maybe someone could elaborate their variables and how often they keep them maintained. Personally I use my gear weekly (3 a12 backs, 40mm, 50mm, 80mm and 150mm for lenses along with a 500cm body) and haven't done anything for 2 years (and the previous owner for 2 years before that), but have no issues besides the film advancing. Is this normal or should I be sending them in soon? Anyone care to comment or answer the above questions?