13 March 2009

Back button focus

Exposure lockBack 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.


1 comment:

Anonymous said...

I've been using this technique for a while now on the EOS 30D, swapping the AE-lock for focus instead. It works very well and has improved my focus accuracy massively. (It much reduces the chance of the auto focus bouncing onto some other target.)

The slight problem comes with the 50D. The 50D, like the 40D, has the AF-on button, which is great, right up until you use it with a battery grip. No AF-on button on the grip. Okay, so then you go into the custom functions and select to swap the operation of the AF-on button and the AE-lock button, which at least gives you focus back on the grip.

Sadly this also messes up the zoom controls when reviewing pictures because it swaps all functions for the buttons. So now to zoom out of images on the 50D I have to press the AF-on button which is a nuisance. If I press the AE-lock button, the camera drops out of review mode, which is annoying.

The 30D didn't have this issue because if you assigned focus to the AE-lock button, it didn't change it's function in review mode. I wish the 50D had been the same - or Canon had put an AF-on button on the grip.

In the end, for the 50D and grip I'm persisting with separating the focus from the shutter release - it is a nice way of working. Just occasionally I'll drop out of review by mistake and wonder what on earth Canon were thinking.