I adopted the use of AF-on function on the AE-L button on my d7000 and assigned Fn button to AE-L Hold. Very quickly it became my favorite way of controlling these two important parameters. It allows contolling both at the same time, faster and more sure. I added that setting to my standard setup on my D800 also. A less used function can be assigned to the AF-L button on cameras which have both dedixated rear buttons. Stan St Petersburg Russia
I am experimenting with my D7100 with AE-L/AF-L assigned to back focus + the Fn to AE-L Hold. So its thumb on AE-L first finger on shutter release & pinky on Fn. At first it was pretty awkward but after a week of nothing else but that method its working beautifully.
I have the feeling camera makers made the half press shutter button focus the norm to save us having to learn this 'pro' technique. Maybe if we had always had to do it it would be less awkward.
I never tried with small finger, does not fit with my larger hands but is the Fn button on the D7000 is perfectly positioned for the middle finger, with index finger on the release and thumb on the rear AE-L button. This is more natural, faster and almost impossible to hit the wrong thumb button that I did often on the D800 with both rear buttons next to each other. The thumb is not as agile or sensitive to tactile feedback as the index or middle finger so using the front and rear buttons is almost foolproof, with no missed button activation. I only wish the grip had the Fn button on it also. I still read where people insist that the D7100 is not usable since it does not have the two thumb buttons next to each other like the D300. That is silly, being so entrenched in a less than optimum habit keeping one from getting the vast performance and feature advantages of the D7100 rather than spend 15 minutes getting used to another, better, button arrangement. The many long threads about how Nikon is abandoning the enthusiast market by not making an identical camera to the D300 with better sensor is misplacing the blame. Nikon DID create a great enthusiast camera, it is called the D7100 and lower the price of entry to boot. Sometimes the strength of holding onto an old habit is stronger than the stated goals of better photos, in lower light. Stan St Petersburg Russia