A couple of months ago I replaced my Canon 50D with a 5Dm3. My primary use lately has been relatively low-light (small club) rock concert photography...slightly better than dance floor lighting perhaps. No flash.
I'm using the 24-70 f2.8L almost exclusively, wide-open, high ISO, and typically about 5 feet from the nearest subject. In these conditions, grabbing focus on a contrasty area (performer's face, guitar, etc) can be a bit of a challenge - i achieve focus on maybe 65% of the time upon half-shutter press, the rest of the time, no focus, no actuation. Although I was hoping for better performance, that's not the problem I am worried about... a few times per night when I am trying to find a focus point and failing to do so in a fairly rapid-fire series of attempts, the shutter-button will simply "lock up" and not allow me to grab focus on anything, anywhere. The button simply dies. I am forced to restart the camera. After restart it will go back to it's "normal, but still disappointing self" and then eventually fail again at some point.
Anyone else experiencing this type of behavior with a 5D3? Does it sound like I'm doing anything wrong?
For AF mode, I have tried "One Shot" "AI Focus" and occasionally "AI Servo" (with Case 1 and Case 3). For point selection method, I have tried the various Spot-AF, Expanded-Spot AF, and Surround-Spot AF settings. Metering is tied to focus point. None of this seems to change the situation. When in One-Shot mode, I do have the camera set to Focus Priority over Release Priority (cause I do want the subject to be in focus).
I will call Canon soon, just wanted some of your insight.