How do you clean a focusing screen?Answer:
First I blow off the screen with compressed air. Screen is out of camera when this is done. Air compressor has 2 water traps one at each end of the line, I've never had trouble with water in the line with this setup. I suggest never blowing compressed air into the body, it can wreck the meter, in some cases knocking the galvanometer off its pivot.contributed by John Hermanson
Then, if there are specs on the screen, I brush the screen with a goose feather. If specks are stuck to the pentaprism I clean first with lens tissue and alcohol. Persistent specks are picked off with the tip of a dental hook. I look into the lens opening with a loupe and gently touch the hook to the speck. Surface "moisture" makes speck stick to the hook.