During a session yesterday afternoon, I mounted an 85L to a 5DIII and encountered an odd problem. I use back button focus, had the camera set to one shot, and was using Case 2 for focusing. For some reason the camera would not focus when I pressed the * button. I confirmed the AF switch on the lens was in the correct position, switched it to MF and back to AF and tried again. Nothing. I confirmed the custom function for BBF and still nothing. However, when I tried the AF-ON button, the lens began to focus. I tried several times, even changed focus to Case 1, and the lens only focused with the AF-ON button.
I decided to switch lenses to my trustworthy 70-200 2.8II and it worked perfectly - with the * button.
After the session I tried to recreate the problem and haven't been able to. The lens seems to focus just fine with the * button now.
Any ideas what may have been going on? Camera problem or lens problem?
I purchased the lens used and have only had it about a week but this is the first time this has happened.