Originally posted by bychan Thanks all for the info and advice, it's fair to say that I won't be attempting to clean the screen now
Hi
It is true that the mirror and the focusing screen are probably the most delicate parts of the mirror chamber and can be easily ruined. It is also true dust can be removed from there without even the slightest chance of damage.
Please remember the piece of dust there was not attached to any of those surfaces with some sort of an applicator. This dust, at one stage, was actually floating around inside the chamber, floating because it is very light and small in volume. So if it floats like a feather it settles on the surface like a feather, do not forget this fact. The only thing it holds it there is surface tension and static and that can vary depending on the makeup of the dust. So if it is as light as a "feather" it can be removed with a "feather" if you understand what I mean.
The reason you cannot remove the offending dust with the rubber ball blower is twofold.
1) The strength of air coming off the end of the blower is actually very weak. You will need a concentrated jet of air pointing directly at the dust which is next to impossible because the nozzle end of the blower dances wildly in all directions as you squeeze the ball.
2) even if you are successful in hitting a jet of air at the fluff full on the velocity of that air may not be strong enough to shift it.
So how do you safely remove the crap. Simple. Buy from you art supply shop a very very soft artists brush. They have them, some are so soft you couldn't tickle a flea with it. Now gently without bearing down brush over the mirror or screen from left to right (or from right to left) in one pass and the offending matter will be gone. It will be stuck on the brush. There is no way there will be any damage left behind. It is simply not possible to do damage with this soft brush.
If the dirt is of an oily nature then the above method will not work, like Salami or Cheese. (Just kidding)
Greetings