I shot another performance recently with the K-5 and the FA 50mm f/1.4, and whilst I got some excellent shots, I had a lot of trouble dealing with the camera totally missing the focus for quite a few separate moments during the performance. It generally focused about a metre and a half or so in front of the target for seemingly no reason. It was on 11 point auto, and I was trying to point it towards high contrast areas to help it in low light, but even in decent lighting it was missing some shots. You can see the focus point on the stage floor in front of the actors, but there is no reason it would lock focus on that point, as the floor is just solid black/dark grey. What could have caused this? And it's focused wrongly far too much for it to be a back focusing issue.
Twas really irritating, and got in the way of a lot of important shots. The K-5 autofocus sucks in my experience
I genuinely have better luck manual focusing a lot of the time in theatres, even in full lighting.
Also, a question for Canon/Nikon users, does AF-C (or the equivalent in your camera) constantly follow the subject, or does it adjust every half a second or so to catch focus again like the K-5 seems to do? Or is that just the K-5 being the K-5?