A T-shirt? Seriously?
Originally Posted by mporter012
Firstly, how did the mirror get dirty? A mirror shouldn't get dirty. Keep a lens or a body cap on the camera at all times and it won't. I use a Nikon F which I bought about 1994. It came to me with a clean mirror, and I've never had to clean it.
Second, the force you may have applied to the mirror when you stretched the T-shirt over your finger, reached into the mirror box, and 'cleaned' the mirror may have damaged the mechanism that moves or holds the mirror in the proper position. Such a procedure could also damage the focussing screen, which is just above the mirror.
Third, a T-shirt is for wearing, not cleaning optics (any optics, even cheap binoculars) with. A search of this site will turn up the correct procedure for cleaning lenses, but here's a hint - it's better to keep your lenses clean, than to keep cleaning your lenses. Use the lens cap, it's what it's for. Of course, the lens will eventually get schmutz on it. Then you clean the UV filter you put on the lens to protect it.