I find myself increasingly using the rear AF button while having the shutter set to release only. My question is how long will the subject remain in focus once I take my finger off the button if I am using the AF-C setting? Lets say that I have focused on the subject using the rear AF button, and they then move. I know that the subject should remain in focus. However, if they are now closer or further away do I need to re-focus or will the camera continue to keep the subject in focus? I assume that after a certain amount of time one has to re-acquire focus, but I don't know how long it takes before I have to touch the rear AF button again.
Graham From St. Augustine, FL. "I like photographers, you don't ask questions." Ronald Reagan to White House Press Photogs
You'll need to press the AF-ON button again. When configured to use the AF-ON button only, the camera only tries to focus while that button is pressed; it will stop focusing the instant you release the button.
>So if you keep the button pressed it stays in focus even if >you are panning the shot?
That's a more complicated question, and the answer depends on your other AF settings. For example, in Single-Point AF, you'd need to keep the selected AF sensor over the desired subject as you pan. Basically, the camera will keep focusing on whatever the AF sensor is pointing at, for as long as you keep your thumb on the AF-ON button.