a4 AF-on versus shutter/AF-on
(1) If the target is already in focus, you may not feel the movement of the focusing mechanism.
(2) Did you keep on pressing the AF-ON button to keep the focus? The AF-ON button is not a toggle button. I would prefer to have an option to set the AF-ON button as a toggle button, like the AE-L button. Under some situations, such an option would make life a bit easier for me to take a photo with myself in it.
When I select a4 AF-ON only mode in d800, I am not getting focus responses after using the camera for just a little bit. That is, the lens and image will not change focus after pressing AF-ON couple times. Switching back to Shutter/AF-ON makes it responsive again. What is happening here? Is my AF-ON defective? This is happening to both 50mm 1.8g and 70-210mm af-D lenses.