I have a 20D that has started malfunctioning and I was wondering if anyone had experienced a similiar error. During focusing the AF point (little red squares) has a double image of itself. Initially I thought it might be related to the Lens, but it shows up on different lenses. It then began to occur with a lot more frequency and finally I started to have periods that the camera would not focus and I had to power cycle. Then it would function normally for a little while. I sent it back to Canon Factory Service. They adjusted the focus to the best focusing point and checked it. They then checked all functions to factory specs. Needless to say I got my camera back yesterday and after focusing about a hundred times I recreated the problem. Clearly the problem is intermittant and a quick pass on their functonal test station may not catch the error. I called Canon and they asked me to send it back in. Has anyone else seen this type failure? Thanks.
It's just a guess, but the symptoms you describe could be something that has nothing to do with the camera itself...
Could it be that you wear glasses?
If "yes", check and adjust the "diopter" which is built into the viewfinder. There is a small "wheel" (top right of the viewfinder, behind the rubber frame that is around the viewfinder). Turn that wheel while looking through the viewfinder to find the "best" adjustment for the clearest view. Once you'll have adjusted it, your "double" black/red squares" may very well be "single", or very close to each other.
Also, the fact that this problem seems to "come and go" is probably due to the angle at/through which you look through the viewfinder. Try this the next time this problem will be back: change the position of your eye/glass on the viewfinder and see if the distance or relative position of the two "double" squares varies. If "yes, somewhat",then I am quasi certain that this is the source of the "come and go" problem -- depending on the viewing angle, your glasses "duplicate" (or not) these squares.
Conclusion: whenever you notice this problem, simply change your viewing angle through the viewfinder... and you probably will have a much better and clearer view of what's going on, but don't expect perfection! (Viewing through the viewfinder on that camera with glasses [especially tri-focal ones!] can be a bit challenging!)