Back button focus allows a photographer to separately control when the camera focuses and when it takes pictures.
By default all Canon EOS cameras have focus activated by pressing the shutter halfway down, just as you would to take a picture.
I’ve written about back button focus previously, but reader Kobus Pelser asked specifically about back button focus for the EOS 1200D / Rebel T5; can the entry level camera also do back button AF?
Well yes it can – in fact just about every digital EOS camera can be configured with back button focus.
C.Fn-8 Shutter/AE lock button
You need to set a specific custom function on the EOS 1200D. C.Fn-8 Shutter/AE lock button is used to assign the focus control to the AE lock (star button) on the rear of the camera.
The slash between Shutter and AE Lock in the title is the same notation used in the manual for describing which function is on which button. The default is listed as AF/AE Lock, so AF is on the half press of the shutter and AE Lock is on the AE Lock button.
There are four possible settings for C.Fn-8…
0 – The default, no back button focus. The AE-lock button still locks the exposure too
1 – Choose this to use the AE-Lock button to control focus. Press the button to drive focus, release to stop focussing. As you will also press the shutter then the exposure lock happens as you pass through the “half press” to take the picture. This will mean that each frame in a sequence taken using continuous shooting will have the same exposure settings.
2 – Back button focus with a difference, you press the AE-Lock button to stop focus. This can be used to pause the AF if you are using AI Servo AF.
3 – AF is still controlled by the AE-Lock button, but the half-press position of the shutter button doesn’t lock the exposure, so each frame in a sequence may have different exposure settings.
Back button focus is popular since it allows the photographer to use AI Servo focus mode all the time, even for static subjects, where you press to focus, then release the AE-Lock button and recompose the frame.Custom functions can only be set and used when the mode dial is set to one of the creative modes, P, Tv, Av, M