@phototex wrote:
On my 50D, sometimes I look through the viewfinder and it looks like I haven't removed the lens cap; at other times, it works normally; once in a while, I can see only through a small slit at the bottom of the viewfinder. It's like the mirror hasn't reset completely after taking my previous shot. When it happens, I can sometimes "break it loose" by shooting several frames in sports mode, and sometimes it works properly after turning the camera off and back on. What could be causing this, and how can it be prevented or fixed?
It sounds like something has entered the camera and contaminated the shutter/mirror mechanism. You might try gently brushing out the body "cavity" with a lens brush while the camera is held upside down. (It might require you to hold it overhead while you work.) You don't want the mirror to be up because you do not want to brush the camera sensor. You could also fire off a shot or two with the camera continuing to be held inverted.
These aren't "fixes" as much as they are opportunities for any loose grit to fall out.
Do not blow air into the camera as this could potentially move dirt/dust/etc further into the camera mechanism and make any contamination worse.
Good luck.
