Fri 30-Nov-12 08:05 AM | edited Fri 30-Nov-12 08:07 AM by briantilley
I use AF-C with AF assigned to the AF-ON button for shooting theatre performances with a D3s. I find it works well, because I'm in control of exactly when the camera is autofocussing, and when it isn't. If there is not much movement, I focus on the main subject, release the AF-ON button, then re-frame and shoot when appropriate. If the performers are moving around more, I choose an appropriate AF point, keep it over the main subject, and keep AF-ON pressed while releasing the shutter.
To answer the specific question, with these settings autofocus only works while the AF-ON button is pressed. When you release the AF-ON button, focus will remain set at that same distance (whether the subject is static or keeps moving).