back button focus

It might also be noted back-button AF alllows you to separate the action which triggers the camera's auto focusing from the button used to trigger capturing the shot (shutter button), moving the action to AF to one of a few buttons on the back of the camera.

(It sounds like the OP is confusing Manual Focus with buttons which allow the camera to determine focus.)

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I started in the 50's - my first picture was taken with a Leica and hooked me for life. I no longer use my Leicas, but I am still taking pictures. Some things never change.