<AF ON> button - when to use?

Hi

Sorry for such a basic question.

I have a 7D which I use 99% for Aviation photography, take off, landing that sort of thing. I have the auto focus set to AI SERVO and drive to High Speed.

I usually keep the rear <AF ON> pressed throughout the shooting sequence. Is this the correct thing to do or should I use the button once to set the focus and then let go?

Advice great fully received

Thanks for looking and have a good New Year

Philip L Jackson

In AI Servo you have to keep the button held down to continue to AF; so, yes, you're doing it right. Using the button once and letting it go is a good way to simulate One Shot, without having to change away from AI Servo, though; so it's useful for stationary subjects as well.

Do you have your camera set up for back button focusing?

I agree with Eric, but make sure that AF is removed from the shutter button.

Make sure your shutter is set to Metering Start.

Hi

C.Fn IV AF-ON is set to AF

The shutter button is set to metering and AF Start so I will change to metering Start.

Thanks for the advice

Philip L Jackson

One more thing - my fingers are relatively short and I found it uncomfortable to reach over to the AF-ON button with my thumb, so I switched the * button to do AF and the AF-ON button to do AE Lock. AE Lock is not very commonly used at least for me, while I BBF all the time. The * button falls much more naturally under my thumb and is very comfortable.