[Wannabe]

Originally Posted by silkstone

You could try a Rocket blower (or similar) - bulb-type blower you squeeze. Don't let the nozzle touch anything, and don't use aerosol air-dusters.

If that doesn't remove it all, the next stage is to get a very soft artist's brush - something like a Daler-Rowney SY21 - which has very fine bristles and won't scratch anything. Wipe the mirror and focusing screen very gently and see how it looks.

Thank-you very much!