Today’s Question: On my ancient Canon 50D I recently have accumulated significant artifacts in the viewfinder when I set up for a photo. Fortunately, the artifacts do not appear in any photos, which I think eliminates the sensor as the culprit. But I have thoroughly cleaned the viewfinder and the mirror (as well as my lenses), but they are still appearing and are very distracting. What am I missing in my cleaning chore?
Tim’s Quick Answer: It sounds like you are missing the focusing screen. While it may be possible to clean the focusing screen on many cameras, that focusing screen can be quite delicate, so you may want to opt for replacing the screen (http://timgrey.me/screen).
More Detail: Cleaning the optical viewfinder on a camera can be relatively straightforward, but doing so won’t always provide a completely clean view. I recommend using a cotton swab with lens cleaner solution to clean the eyepiece of the viewfinder.
Beyond that, you may want to have your camera professionally serviced if the view through the viewfinder shows obstructions that are distracting.
Cleaning the mirror yourself can be a little risky, as the mirror and related components are very delicate. I recommend only using an air blower to clean debris off the mirror, with the camera titled so the debris will fall out of the camera rather than remaining inside. Cleaning the mirror with a cotton swab and lens cleaner involves the risk you will use too much pressure and damage the mirror or other components.
If the viewfinder eyepiece and mirror are clean but you’re still seeing debris through the viewfinder, the focusing screen is the most likely culprit. With some cameras (including the Canon 50D) the focusing screen can be replaced. This is often the best solution, as the screen is rather delicate and can be damaged with cleaning. There is a special tool that enables you to safely handle the focusing screen, and this tool is generally included with a replacement screen.
You can get information about a replacement focusing screen (in this case for some Canon cameras including the 50D) here: