Originally posted by pres589 The mirror and focus screens get dirty just like sensors do... not a lot of difference, really. Every time I've thought about cleaning a focusing screen in a film camera it's been due to mirror damper pad breakdown.
I also learned this hard and fast after getting stuff on the focusing screen of my K-5 II that the focusing screen should be treated like a fragile glue trap out of a nightmare. I wish they were made of glass.
The difference is, a dirty mirror or screen has absolutely zero effect on the pictures, while a dirty sensor makes it's presence felt in every picture. A little dust on a screen may be annoying, but it isn't going to cause any post processing work. Of course a scratched screen is also just annoying, made more so because of the extra effort involved in damaging it.
Dust on a mirror isn't even annoying, it's invisible and can be ignored. The old trope about if it's on the mirror it can transfer to the sensor is pretty much false, as the mirror is up while the sensor is exposed.
I've probably mentioned this a hundred times on this forum, but when I was in the game of selling cameras the number one cause of damage to cameras and lenses that I saw was damage from cleaning them.