I recently acquired a Canon FD 28/2.8 S.C. lens. I notice that when I release the shutter, the viewfinder becomes dim for a second or two. It would appear that I am seeing the effect of the lens diaphragm close down and then reopen. None of my other FD lenses do this (whether earlier versions of later versions). Should I be concerned about what I have observed? I have yet to process any film shot with this lens.
I don't know which FD Body you are using it with , but It's easy to check by pressing the stop down lever,releasing it, and see how quickly the diaphragm reacts, as the other guys say it probably needs a CLA.
I use the F-1N, A-1, and AE-1P. I agree: the lens is worth the CLA. The New FD 28/2.8s are as common as dirt. The 28/2.8 S.C. is harder to find and tested better. I prefer the breech mount lenses anyway.
I think we should remember all Canon FD equipment is more than twenty years old, and has had no maintenance, the majority of it could benefit from a CLA, I think rather than buy another one I would get a CLA done so at least I would know it was right.