I am having three autofocus-related problems with my D4 and am seeking advice.
Intermittent AF failure. I've had the camera since mid-February but about a week ago I started having intermittent AF failure, especially with the Sigma 150-500 at full extension. It has happened with other lenses though. I cleaned the contacts with a Q-tip and the problem seems to have improved with other lenses but is still there with the Sigma. I really don't want to send it in for warranty repair, but I'm missing some good shots. Any advice?
AF-ON button. The owners' manual at page 99 says, "In viewfinder photography, focus will remain locked after the AF-ON button is released, although the in-focus indicator will not be displayed." However, when I release the AF-ON button and recompose, and press the shutter button to take the photo, focus is not locked, and the camera refocuses. Is this a problem with my camera, or do I need to set custom setting A4 to "AF-ON only" to avoid having the shutter button refocus the photo? And if the latter is true, isn't the owner's manual incomplete or misleading?
Focus priority release. This simply does not work. Although I have it selected for AF-S, the camera nonetheless fires readily when the image is out of focus. (This happens even when I am not using the AF-ON button - I've read somewhere that focus priority does not work after pressing the AF-ON button, but that's not what I'm doing.) Am I missing something, or is this a problem with my camera?
As always, thanks very much for all the help I get here!