When the camera lens is set to auto-focus, you may have noted that a half-press of the shutter release button focuses the lens on the subject. When we take multiple shots of the same object, the lens will try to focus each time we press the shutter button – half-press makes the lens acquire focus and full press grabs the image.
If we do not want the auto-focus feature to kick in each time, we have switch the camera to manual focus.
The ‘back button focus’ technique gives us a ‘best of both worlds’ combination while focusing. By using this technique, the lens will NOT try to change focus even when it is in auto-focus mode. Only when the back button is pressed, will the lens try to change focus.
This feature is very helpful when taking close-up pictures as well as for stacked focus images. This technique is also useful for portrait photography, where once we have set focus at one point, we wish to take multiple pictures at the acquired focus point.
When back button focus is enabled and the lens is in auto-focus mode, pressing the Blue ‘*’ button sets the focus. Pressing the shutter release button does not change focus. A full shutter press captures the image.
 select ‘camera settings (wrench symbol) 4’ menu
 select ‘Custom Functions’ (press < > keys to navigate custom functions)
 select custom function 6 (shutter/AE lock button)
 select from
 0 – AF/AE lock — default — half-shutter for focus, * button for Auto Exposure lock
 1 – AE lock/AF
 2 – AF/AF lock, no AE lock
 3 – AE/AF, no AE lock — choose this option — * button for focus lock, half-shutter for auto exposure lock
- 3 Reasons Why You Should Switch To Back Button Focus – http://digital-photography-school.com/back-button-focus
- Back Button Focusing – Easier than you think! – http://improvephotography.com/4552/back-button-focusing/
- Back Button Focusing with the Canon T5i (700D) – http://photography.marvincable.com/back-button-focusing-with-the-canon-t5i-700d/
- DSLR Tip of the Week: Back Button Focusing – http://photography.kinja.com/dslr-tip-of-the-week-back-button-focusing-1045526685
- How to setup and use back button focus – http://www.p4pictures.com/2014/08/how-to-setup-back-button-focus/
- Shooting with Focus Lock (EOS REBEL T5i / EOS 700D) – http://support-th.canon-asia.com/contents/TH/EN/8201775500.html
- Could someone explain back-button focusing for me? – https://www.reddit.com/r/photography/comments/47b2xm/could_someone_explain_backbutton_focusing_for_me/
- How many do the Back Button Focus thing? – http://www.dpreview.com/forums/thread/3687018