What is Back Button Focus?
One of the best things I have learned to get tack sharp images!
Normally the camera focuses when you press the shutter half way down, you would then recompose and take the photo by pressing the button fully down. Back button focus allows you to assign another button on the back of your camera to activate focusing instead of your shutter button. On the canon the AF-ON will be your back button focus.
Well, to get tack sharp, crisp, and clear images!
By using back button focus, you focus on your subject once, you can then take as many photos as you want without having to worry about loosing focus as long as your distance from your subject does not change. However, if you have very active subjects, such as kiddos on the move, you would keep your finger on the bbf, allowing your camera to continuously autofocus and follow your subjects movement. When you hit the shutter, your thumb still needs to be pressing the bbf the entire time, both actions work simultaneously. I also use a dynamic autofocus mode (AI-SERVO), I keep it in this mode at all times so I can go from a posed photo to an action shot without having to change settings.
Just a word of caution it takes time to get used to the bbf but once you nail it you won't ever want to go back.
Where can I find my back button focus?
I am going to show you where to find it on the Canon 7D.
You can google or youtube where to find it on your camera.
You will need to select your menu button which is on the left of your camera as seen in photo to access this menu. Next make sure the camera icon is highlighted, scroll down to Operation/Others option and select it.
Make sure you scroll through on the top right of the screen until you get to the number 1 then select it.
Make sure the top option is highlighted and select it.
This is the last step to assign your AF-ON as your back button focus, make sure the middle option is highlighted and select.