Last week my two-year-old 80D started having focusing issues when I use the viewfinder. When using LiveView, it works fine, but when I use the viewfined, even though it shows green squares, the focus points come out blurry in the pictures. Tried differen lenses (70-200 2.8, 85 1.8, Sigma 50 Art), and all show the same problem (lenses work fine on my 6D Mark 2). Tried with auto and manual modes, same story. I installed the latest firmware, but it did not help. Any ideas what could have cause it and what I can do? I did not have any accidents with the camera (no drops, water, etc, so I am really puszzled). Thanks.
It sounds like it probably needs a trip to Canon but there is one thing to check. One of the options in the camera menu is to offset the focus adjustment for ALL lenses instead of individually. If you accidentally entered an offset there it could cause that issue, you want it set to 0. This probably isn't the issue but it is easy to check. With your 80D, this setting is in the custom function menu.
Have you tried to using a rocket blower to clean out the mirror box. The AF sensor could be dirty.
Would a dirty AF sensor work fine with LiveView? I would assume that it would always result in blur regardless of shooting with Live View or viewfinder. Or am I wrong? Don't want to accidentally make things even worse.
LiveView focuses directly using what the sensor sees. The AF sensor isn't even in the light path when using LiveView. You can't make things worse by using a blower to try and remove any dust that may be on the AF sensor.
This is one of those, I did something and it feels good but it didn't fix it, moments. I agree with Rodger you need to call Canon. Tell them the issue and ask for a C&C. 1-800-652-2666.
BTW, other things that don't, or very rarely, help are cleaning the lens contacts and, yes, updating/reinstalling the FW.
Could it be something a local camera repair shop can fix or does it need to go to Canon?
What is your definition of a camera shop? If it is a local store that sells cameras it's unlikely they are in a position to repair what appears to be an internal problem - assuming you've tried all the recommendations and it still doesn't work.
Since its local I guess you have nothing to lose by asking them, but be cautious - just because they say they can doesn't mean they are truly capable.
If it was my camera I'd be sending it to Canon.