St Petersburg, RU
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Both the AF-On function and the default function of AE-L/AF-L button are very useful and most experienced photographers use one, the other or both.
When you switch the rear button to AF-On, as others have correctly stated, the AF operation is decoupled from the shutter release. That might seen awkward at first for users but after an hour, it becomes intuitive and automatic. The reasons are many why some prefer to separate AF function from the shutter release. The main one is for locking AF where you want to and pressing the shutter when you want without refocusing.
As an example of where this it helpful. Say there is a group of people you are shooting, and as usual, they will often naturally line up where the people on the ends will be closer to you than the person in the center of the group. If you used the default use of AF, composing on the center person means that person will be in focus and the outer people might not be. If the center person is on the focal plane, a little in front and a little in back of him will be in focus. To get everyone in focus, ideally you would want to focus on the person who is mid way between the most distant and closest persons, so all will be within the depth of field. If you are using the 1/2 press AF control, you would have to select a side focus point or crop later to center the group. An easier why is to use AF-On. Pressing the button focusing on what you want, in this case the person at mid depth, left go of the button and simply frame the scene as you wish and press the shutter release. No additional AF updates will occur when you do not have the AF-On button pressed.
Another situation; you are shooting some still scene, say a group of people so you want AF-S mode and take a shot, but suddenly a new scene that appeals to you, say, they are walking away but you are in AF-S mode that is optimum for still subjects but not useful for tracking movement. You have the choice of trying to AF and release on moving subjects or stopping and switching to the more effective AF-C mode.
That is simple with AF-on. Leave the camera in AF-C all the time. For still subjects, focus then release the AF-On button. It will be locked on the last AF capture point, just like AF-S mode. When something moves that you want to track and capture, just press AF-On and hold it on with your thumb for continuous updating AF up to and during your triggering the shutter release. AF will have been updating right up to capture. The only downside is AF Assist lighting does not function in any but AF-S mode. These cameras are pretty darn good in low light AF however so that is seldom needed. In both of these cases, having the flexibility to AF and lock whenever you choose becomes a feature that expands your shooting control options, in a simple almost automatic reflex way.
Changing the very often used rear button to AF-On, removes another very useful function, AE-L Exposure lock. I solve this by programming the Fn button the front of the camera to assume the AE-L function. When I need the metering to be independent of a focus target, a tap of the Fn button with my middle finger locks the exposure setting while I recompose. The location of the Fn makes more sense than in the more pro bodies like the D300s D700, D800 D4 which places dedicated AE-L/AF-L and AF-On buttons next to each other on the rear. With the AE-L function accessible with the middle finger, AF-On controlled by the thumb and index finger on the shutter release, all the important actions can be control at the same time without moving the camera or hand position, and all at the same time.
Stan
St Petersburg Russia Visit my Nikonians gallery.
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