So, what is the rear af button?
Typically digital DSLR's will ship with certain factory set default settings.
One such standard setting is to link auto focus and exposure lock to the shutter button.
Depressing the shutter button halfway activates autofocus (based on whichever AF points have been activated) and sets the exposure. Fully depressing the shutter button then captures the image based on these settings.
For standard “snapshots” and many other types of general photography this system functions well and is possibly the most fail-safe way to grab a decent shot.
However, there are certain photographic themes where separating the autofocus and exposure lock from the shutter activation can really help get the desired end result.
Canon DSLRS such as 60D, 7D, 5D etc. provide the option to separate the auto focus activation from the shutter, by re allocating the function to the rear “AF” button located on the the back of the camera.
Access to this function is gained via the Custom functions menu – see your specific camera manual for details.
Listed below are some examples of when using the back AF button can be advantageous.
Fast paced wildlife/sports photography –
If you are tracking a fast moving object – it would normally be recommended to shoot in high speed burst mode. With the standard settings a problem can arise if the subject moves out of the selected AF point during the sequence, as the camera may try to refocus on a different object in the frame.
By using the rear AF button, the photographer can simply stop the camera refocussing by releasing the rear AF button - provided the subject remains more or less the same distance from the camera , it will remain sharp throughout the entire burst , irrespective of its position within the frame.
This also gives the photographer greater flexibility to adjust the composition during the burst sequence.
In portraits, use of the back AF button allows the technique of using the centre AF point (often the point with greatest focus detection) to be used to focus on the subject's eyes and then recomposed within the frame – without the need to maintain the shutter button pressed halfway.
If single shot AF mode is selected then holding the af button whilst recomposing the shot will also maitain the exposure settings - whilst this is effectively the same as holding the shutter button in its half pressed postion, the af button is less likely to result in accidental operation of the shutter.
In landscape photography, depth of field is often maximised by focussing at the hyper-focal distance.
A common technique with the standard settings is to use autofocus to focus on an object at or around the desired distance and then switch the lens to its manual focus mode before recomposing the shot.
Using the rear AF button allows this operation without the need to use the AF/MF switch on the lens – which can often be small and quite fiddly. It also avoids the pitfall of forgetting to return the switch to its normal AF position - which could result in a missed opportunity at a later date!
Being a custom function, the rear af button will only function when in one of the create modes that your camera offers (B, M, Av, Tv, P on the mode selctor dial). The scene or auto modes the camera will return to its default settings.
If you are accustomed to shooting in the creative modes, then you may well find (as I do) that using the rear af button is so useful that soon you will choose to use this as the primary method of activating autofocus when using your digital slr.
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