A 5D Mark 111 Mystery Solved.
For the last 6 months, since buying the camera, I have been extremely frustrated by the camera acting up on random occasions. I would be shooting (in AV mode), and would have no problem until I would hold the camera up to my eye to take another photo (invariably the best shot of the day!), and the camera would not fire. So I did all the research I could, and interestingly did not find any/many examples of other people having the same problem. I tried all the usual suggestions -- don't use an Eye-Fi card, use only Canon batteries, etc. Nothing helped. The most depressing thing was the uncertainty of it all -- I had lost faith in the camera to fire every time I pressed the shutter, and my interest in using it was greatly diminished (I also have a Mark 11 and started using that in critical situations). So, I have been very depressed, because the focus system in the Mark 111 is outstanding, and I wanted badly to commit to using the camera with the full faith that it would work every time. I was depressed that is, until I made an inadvertent discovery today. I was shooting one photo after another with no problem, then suddenly it didn't fire. So I decided to just freeze my hand on the camera and inspect every aspect of what I was doing. And then I found what was causing the problem: I have long fingers, and when gripping the camera with my right hand, my "ring" finger (next to the pinkie) would occasionally fall right on the stop-down button, without my realizing it. So, with my firm grip on the camera, I would be depressing the stop-down button without knowing it, and the camera would not fire. Naturally, this is a huge relief to make this discovery, and if there is a way to disconnect the stop-down button, please let me know!