I was just given a Nikon FM camera and I can see some trapped dust/dirt in the viewfinder. It does not appear to be in the mirror or screen, but possible further in the finder. What would be the best possible approach to cleaning it.
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I did two things to almost completely un-dust my FE's viewfinder:
First there may be dust behind the viewfinder cover. The black ring you put your eye to can be screwed off. Clean the glass and blow out dust that was stuck behind it.
Then there may also be dust on the focus screen, and between the screen and the prism. Just take out the screen and blow it clean. Or use a very clean, very gentle brush on the more stubborn particles. And blow out any other dust thats inside (mine coughed up a particularly nasty dust bunny that was apparently hiding behind the mirror).
Now, there may be more dust inside the prism housing, but I doubt thats what you're seeing.
It's easy, low risk, and the camera will feel like new again