A different way to handle autofocusing
Normally, when you press your shutter button half way down, the camera determines the exposure needed and, if autofocusing is switched on, it will focus the lens as well. You can opt to have the focusing done just once (One-shot) or have the camera continually refocus on whatever passes across its autofocus points (AI Servo).
Most photographers get by for years without ever thinking about changing how and when the lens is focused. This is understandable – if the normal method works, why change it?
However, focusing with the shutter button half pressed can be tiring if you want to focus initially and then wait for the right moment to take the picture. Your finger must remain static in order to stop the camera re-focusing when you press the shutter button and you may find that you accidentally press the shutter releasebefore you’re ready. Switching to manual focusing is a possible solution, but there is a way to use autofocus with a greater degree of control.
Many professional photographers change the way their autofocus operates by removing the focus from the shutter release button altogether. The task of autofocusing is instead transferred to the AF-ON button, which is conveniently located on the back of the camera under your right thumb. It has become known as ‘back button focusing’ (BBF).
Once set up, BBF allows you to quickly focus on a subject by pressing the AF-ON button and then recompose the shot without the camera altering the focus when you press the shutter release button.
If you set your focus mode to AI Servo, pressing the AF-ON button will focus as if you had selected One-shot mode. If you keep the AFON button pressed, it will track focus.
Sport photographers often use AI Servo focusing with BBF. They can just take their thumb off the AF-ON button if an unwanted subject wanders into frame during shooting (which might throw the focus off the main subject).
Custom Function settings allow you to control the function of the AF-ON button – some photographers set it up to activate focus while pressed and others like it to temporarily disable focus. Try it out and see what works for you.
The AF-ON button is usually used to initiate autofocus when in Live View mode, but can be set up to initiate focus when pressed. This may at first seem more complicated, but many photographers find that after a brief period of practice, they wouldn’t want to revert to the more standard method.
If you would like to try out back button focusing with AI Servo, you may want to store the settings on one of the custom modes. That way, you can easily switch back to the more familiar way of working while you’re experimenting.
No AF-ON button?
Use the AE lock (*) button to control focus if your camera lacks the AF-ON button.
Setting back button focusing
To move the focusing function away from the shutter release, first select the Custom Functions option from the menu. Scroll down to C.Fn IV and then select the option called ‘Operation/Others’.
Now select the option ‘Shutter button/AF-ON button’. The options displayed may appear rather cryptic at first but it’s not complicated. For each line, the text before the ‘/’ sign indicates what the shutter button will do and the text after it indicates what the AF-ON button will do.
Option ‘0’ is the factory default where both metering and autofocusing are controlled by the shutter release button. There are then four combinations from which to choose:
1 Autofocus starts when you press the shutter release button, but is held off as long as you keep the rear button pressed.
2 This is classic back button focusing.
3 Locks exposure when you press shutter release half-way and maintains this reading until you take your finger off the shutter release button again. Focus is still controlled using only the rear button.
4 Rear focus button is disabled. Use this setting if you’re worried about accidentally re-focusing.
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Back button focus
Want to use autofocus on your Canon EOS camera
with a greater degree of control? Then try back
button focusing which transfers control away from
the shutter button to the AF-ON button. Here's an
extract from an article in the July-September 2013
issue of EOS magazine which explains it all.
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