The advantages of AF-On with AF-C in Release priority
You're always ready for action photography.
The "Focus, then Compose" technique can still be easily used.
Switching to manual focus with telephoto lenses is easily accomplished at any time by simply releasing the AF-On button and manually turning the focusing ring.
D2X viewfinder with single AF area active
Note: To use the "Focus, then Compose" process with AF-On Only and AF-C focus mode, Release Priority (FPS Rate) must be used. If the AF-C focus mode is set to Focus Priority, then the shutter will not fire once the camera is shifted for desired composition (since the area now in front of active AF sensor would be out of focus).
Admittedly, it does take some time to get adjusted to using the AF-On button. Initially there will be some fumbling at times to locate the AF-On button with your thumb. This problem is compounded by the fact that the vertical release AF-On button is in a different (more vertical) position compared to the horizontal release AF-ON button. With practice, however, it will soon become second nature.
**Alternative Solution: One can also use the "Focus, then Compose" process in AF-C mode with Shutter Button activating AF (CSM a5 in default setting). Simply press the AE-L/AF-L button to lock AF and then recompose. To do the same when using the vertical shutter release, you must use CSM a8 to reprogram the vertical AF-On button to function as an AE-L/AF-L button. This method works, but I prefer to completely separate AF from shutter release by setting CSM a5 to AF-On only.
Some photographers question the accuracy of AF-C for routine photography since they find that the camera often continually adjusts focus on a "stationary" subject. They consider this a sign that the camera is "hunting" for the correct focus. In actuality the camera is simply doing what it is supposed to be doing—continually adjusting focus after subject or camera movement. Often it is the camera that is moving—slightly shifting the active sensor position horizontally and/or vertically across a three-dimensional subject. You can check this out easily for yourself by focusing on a two-dimensional, high contrast subject (such as a brick wall) with camera locked in position on a tripod. The camera will focus quickly and accurately, without hunting, whether in AF-S or AF-C mode.
Note: In low light with low contrast subjects, the D2X can be slow in acquiring focus. The focus indicator oscillations that are occurring in this situation may actually be due to "hunting" for correct focus rather than adjusting to subject/camera movement. The result may be out of focus images. In these situations, it may be best to use AF-S, which locks focus once acquired, or to manually focus.
Other photographers complain of a problem using the AF-On button for AF acquisition when using telephoto lenses with VR. Since the AF-On button does not activate vibration reduction, they feel that VR "kicking in" as the shutter button is depressed can blur images. I have not noticed this problem, but I tend to partially depress the shutter button as I get ready to take a photo, allowing VR to activate before fully depressing the shutter button.
More articles that might interest you