Why use the back button for AF?
Sports and action photographers often use back button auto focus in conjunction with SERVO AF to give them the ability to stop AF switching to a new subject if something comes in to the frame between the photographer and their subject. For example if you are tracking a footballer on the field and you follow him as he runs down the wing, chances are some line official will at some point come between you and the footballer. The camera AF will see the new closer subject and acquire focus on them. If you are using back button AF you can stop the AF for a fraction of a second and then resume it after the line official is out of view.
Canon USA has a great article on back button focus in the Digital Learning Center. Make sure to pay particular attention to how the exposure is locked or not for each of the different options.
Which button for back button focus?
For all the cameras with separate AF-ON and * buttons you have the option to switch round the function of the two, so * becomes the AF-ON and vice versa. Useful if you used back button AF on an older camera and have the muscle memory set in your fingers for back button AF on the * button.
Did you lift off the back button?
Sometimes i’ve seen some of my shots with back button AF be out of focus, though this is usually down to not pressing the back button for AF at the point the picture was taken. This is particularly the case when working with longer lenses and shooting sequences of frames. The longer lens tends to tip the camera body away from the thumb operating the back button and also sometimes when learning you just plain forget to press the back button, yet keep pressing the shutter!
Check to see if any AF points are shown as active in the pictures using Canon’s software applications like Digital Photo Professional or even the rear of the camera LCD. The screenshot from DPP above shows an example of no AF-ON button being pressed at the time that picture was taken.
Second screenshot from DPP shows that a single AF point was selected and that it had achieved focus with it, and that the AF-ON button was being pressed when the picture was taken.
You will need to check your camera’s custom functions for the one that controls back button AF and also for the option to switch round the AF-ON and * buttons, but here’s a list of some to check.
- EOS-1Ds Mark III – C.Fn IV-1 and C.Fn IV-2
- EOS-1D Mark IV – C.Fn IV-1 and C.Fn IV-2
- EOS 5D Mark II – C.Fn IV-1 and C.Fn IV-2
- EOS 7D – C.Fn IV-1
- EOS 60D – C.Fn IV-1
- EOS 600D – C.Fn IV-9
Most of the time I use back button focus for my pictures, but do you use back button AF, share your opinions in the comments?