With f5 set to AF-L you would first acquire focus by pressing the shutter until the green dot appears. Then you would press the AE/AF button and the green light will show which means the focus is now locked. Then press the shutter release button. If f5 is set to AF-on (many prefer to activate focus with this button), pressing the shutter would have no effect on focusing but pressing the AE/AF will now become the way focus is achieved. Once focus is correct (green dot) you press the shutter button. So it depends what you want the buttons to do on how they should be set. I really like having AF-on assigned to the AE/AF button.
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