What are the Pros & Cons of back button autofocus?
It's easy to get stuck in a rut with your photography and camera technique and never try new things. If you do feel you have reached a bit of a plateau, and are having particularly problems with focussing on moving subjects, it may be a good time to try back button focussing.
This technique is very popular with news, sports and wildlife photographers, who see it as a faster and more fool proof way of focusing than the conventional method, where you half press the shutter button to get focus and then press it all the way down to take the actual picture.
Some 'pros' of back button focussing
Depending on who you talk to, there are two big advantages in decoupling focussing from the shutter button and assigning it to the AF button or similar on the back of the camera (that is why it's called back-button focussing).
First, it's reliable and fast. In the conventional focussing method, every time you half press the shutter button the camera refocuses, and it could end up focussing on the wrong thing. Also with conventional focussing you have to keep switching between single shot AF mode (AF-S) for a static subject, and continuous (AI Servo on Canon) for a moving object.
With back button focussing, you can keep the back button pressed down to keep tracking a moving object, or just hit it once to focus on a static object. It's this ease of tracking which explains the popularity of this technique with sports shooters, who obviously spend all their time carefully following fast-moving objects.
Here's another advantage – with static back button focussing, you only need set focus once and can then recompose, so long as the distance between you and the subject stays the same when using a shallow depth of field.
The biggest downside is that back button focussing can take some time get used to, and you may miss some shots at the beginning while you are getting your head around it. Also, if you forget to release the button when shooting a static object, your camera's autofocus can get seriously confused as it's expecting a moving subject – so again you might end up fluffing the shot.
It's fair to say that back button focussing is not the best technique for beginners, as it's adding an extra layer of complexity. Once you have cracked the basics of autofocus and AF points using conventional shutter-button focussing, however, it's well worth a try. You can always change back.
How do I do it?
Annoyingly, each camera make requires a slightly different set-up for back-button focussing. If you wish to give it a try, check your manual or look online – owners of most mainstream cameras will have explained exactly how to do it in a website or blog. It's usually the AF-On or AF-L button that is used for back button focussing, but every camera is different.
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