Back button focus vs shutter button focus for BIF
I have complained several times about poor focus (searching) when using BBF for BIF. This is with the Canon 5D3 and several lenses, from the 300 f/2.8 IS to the 600 II. (AI Servo, of course.) I think I have pinpointed the problem, at least in my case, and am curious for other input. I was using BBF and recently followed someone's advice here (sorry, I don't remember who) and switched back to linking focus to the shutter button and now I get focus holding much steadier. Before it would start wandering with no obvious reason -- not enough to see in the viewfinder and warn me to bump focus, but enough to see at 100% in LR.
Possibly I have a dirty connection with the back button. As I recall, I've tried both the AF-ON button and the * button. Or possibly I'm not managing to hold them down firmly enough but I doubt that. Going back to the shutter button gives much more reliable results, so I'll stay with that for BIF situations. Will still use one of the BB options for recomposing still subjects -- focus in that case has been extremely accurate, which does puzzle me.
Same results, comparing front button to back button, hold whether on a tripod or hand-holding. As I've said, the issue is not gross focus searching, but a slight softening at 100% that makes an image a reject, for me. Then the next frame will generally be sharp again. No logic to it by where the subject is relative to the focus sensor (I'm using the middle sensor for BIF, without the 4 surrounding points).
Just curious about others' experiences, and with other bodies. I could believe that it's some issue with my particular body, but maybe the 5D3 just has a limitation in that regard. I've tried to take as much load as possible off the processor -- I'm in M exposure mode, and turning IS off didn't seem to make any difference (not that I can see why it would).