[MarkJerling]

Focus screen. If it's below, then you can blow it off with a Rocket blower. (Carefully, don't bump against the mirror, keep your Rocketblower outside the mirrorbox) Sometimes, the dirt gets between the focus screen and the prism, in which case it's much harder to shift. Some will remove the focus screen - I'm not that brave, so if it's there - then I leave it alone - it eventually, usually, works it's way out again.