Back button focus

Back button focus is one of the functions on EOS digital SLR cameras that gives a new layer of control over the picture taking process. In simple terms it places the shutter release, exposure lock and focus lock or activation on up to three separate buttons. If you have only ever used the 'out of the box' configuration with the AF activated with the half press on the shutter and the AE lock occuring with the focus lock in evaluative metering and this is not suiting you then it may be something to experiment with - though it's an acquired skill.

Probably one of the best articles on the topic is actually on the Canon USA website in their digital learning center. The article highlights five often quoted advantages of back button focus;

1. Easier to lock focus
2. Easier timing of shots
3. Less risk of focus errors with moving subjects
4. Easier over-riding of AF with full-time manual focus
5. Easier macro and close up focussing

With the latest EOS cameras from the mid-range up providing separate buttons for AF-ON and AE lock '*' then there's also a function to switch round the operation of the two buttons. Great if you plan on using back button focus on an EOS 450D and an EOS 50D, as the EOS 450D doesn't have the separate AF-ON button and then the placement of the buttons may be out of sync with the two cameras.

Some photographers find that the use of back button focus eliminates the need to switch between AI Servo and One-shot AF modes. They simply use AI Servo all the time and switch the AF off when they have the right focus for their picture.

You might also find the previous article on blabpictures about AE lock and the metering pattern valuable further reading.

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