Back-Button AF: What it is and how to use it?

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One of the most frequent questions I have received is about Back Button Focus AF and how to use it on Canon and Nikon DSLR cameras. In this article you’ll learn what is Back Button Focus AF, and how to Back Button Focus AF for Canon and Nikon cameras.

For many years I used the shutter button to AF the subject. Some time I missed shot because my camera is in one shot AF Mode and subject (bird) moved very fast to capture. In shutter button AF mode you have to switch back and forth from One Shot AF to AI Servo AF depending on your subject.  Naw I use full-time Back-Button AF from last five years.

Canon is the  world’s first camera maker to incorporate such a feature, launching it back in 1989 with the EOS 630.

What is Back-Button AF?

The camera usually focuses when the shutter button is pressed halfway down, and then the photographer takes the picture when the button is pressed in fully. Many SLR cameras have offered photographers an option to change the way autofocus is activated. Often referred to by pros as “Back-Button AF”, this feature lets the user customize the camera so that focusing is performed by pressing a rear button on the top right back of the camera with the photographer’s right thumb. The shutter button still wakes up the camera with a half-press, and fires the shutter with a full press downward. You need to change a custom function or two to set up rear focus. And with some systems you set up rear focus via the camera’s menu. You can consult your camera body user manual to learn to set up rear focus.

Why would anyone want to remove AF from the shutter button?

This is a question many users ask me when Back-button AF is first explained to them. The standard method of operation, press the shutter button half-way down to focus, and then press fully to shoot it works perfectly. But back-button AF offers some significant advantages, especially for the experienced photographer.

  • Use AI-Servo for all time:

    The two main types of autofocus on your camera are (on a Canon) One-Shot and AI-Servo. On a Nikon they are called AF-Single (AF-S) and AF-Continuous (AF-C). One-Shot means that when you press your AF-ON button (or the shutter halfway down) your camera will set focus one time based on where the subject is at the given time. If your subject moves or if you move, you have to reset your focus. This method wouldn’t work very well if you were photographing subjects moving very fast, i.e. wildlife, birds or sports and action.

    That’s why there’s another method of focusing called AI-Servo (AF-C on Nikon). This method blew me away the first time I discovered it. Servo focusing actually tracks focusing on moving subjects. You can place a single focus point on subject in Servo mode and fire off as many shots as your camera can take before the buffer runs out. Assuming that you have a fast enough shutter speed, you should have a very high percentage of sharp, in focus images.

    So the thing I started to notice about having the camera in Servo mode is that I could use Servo just like One-Shot. You see, to use Servo focusing with Back-button AF you have to continuously hold down the AF-ON button to track focus. So if you have a still subject you can simply tap the AF-ON button and focus will stop adjusting when you let go of the button. I’ve found that if you have a still subject and hold down the AF-ON button in Servo mode, the camera will keep trying to work to find focus. So the subject will start going slightly in and out of focus because the camera thinks it should be looking for a moving subject. But again, if you simply tap the AF-ON button and let go when you see that your subject is in focus, you are good to go. When you practise this, it’s a very fast process that really takes no brain power or time at all.

    Aside from possibly better AI Servo AF tracking for Birds flight photography there are other advantages to using rear focus. In short, you have the best of all worlds all the time. When using rear focus you always have AI Servo (Continuous for Nikon) set. You will never need to switch back and forth from AI Servo to One Shot. To photograph flying birds or to focus track walking, running, or swimming birds (or perched birds that are changing their posture or head position almost continuously simply press and hold the rear button to focus track that you have set for AF and press the shutter button when you want to create an image.

  • Easier to lock focus:

    If you are shooting something like a series of portraits of a person, and you want them composed off-center, Back-Button AF makes it easy to take as many pictures as you want. Focus on your subject by pressing either the rear AE Lock button or if your camera is equipped with the rear AF-ON button. Once in-focus, take your thumb off the rear button. Re-compose the shot to move your subject off-center. Shoot as many pictures as you like. With focus activation removed from the shutter button, you now can fire any time you like, and remove your index finger from the shutter button after a shot is taken. The camera Will not re-focus when you press the shutter button half-way down again.

  • Easier timing of shots with a very active subject:

    Even with a very active subject i.e. birds, wildlife etc. that may be moving around, you can have your camera’s focus set to AI Servo AF (to track any movement), and just keep your right thumb on the back button to keep focus active, while your index finger can be ready to shoot with no worries about also preserving focus.

  • Less risk of focus errors with moving subjects:

    For wildlife, bird, sports or action photographers taking action pictures, back-button AF lets you stop focus whenever something might interfere with the moving subject you’re tracking without requiring you to stop shooting. In wildlife, for instance, it’s common that some another subject to come between the camera and a subject being photographed. With back-button AF, it’s easy to momentarily pull your thumb off the rear button, and you can still keep shooting by pressing the shutter button fully. The camera instantly stops focusing when your thumb comes off the back button. Once the obstruction is out of your way, you can immediately pick-up your primary subject by pressing your thumb on the back button again.

  • Easier over-riding of AF with full-time manual focus:

    Some lenses have a feature called full-time manual focus. Even if the lens’s AF/MF switch is in the AF position, these lenses allow the shooter to instantly adjust focus manually by simply turning the focus ring on the lens. There’s no need to first move the switch to MF. With back-button AF, this becomes a nearly top feature. Use the autofocus whenever you like by pressing the rear button with your right thumb. Shoot whenever you like by pressing the shutter button. And if you want manual focus, or override what the AF is doing, just pull your thumb off the rear button and turn the focusing ring. No matter how many pictures you shoot, pressing the shutter button will not auto focus.

  • Easier macro and close-up focusing:

    Many times, you’ll find that it’s actually easier to get consistently sharp close-up pictures of small objects by pre-focusing, and then moving yourself forward or backward until you see the critical sharp focus appear in your viewfinder. Once again, with back-button AF active, you can use the AF to get within general range (press the rear button with your thumb, then take your thumb off the button), and move a little bit to get things critically sharp. Most important, you can then shoot freely, without AF trying to re-focus each time you touch the shutter button. Finally, touching-up focus with the full-time manual focus feature on certain Canon lenses is simple and quick, and the autofocus never fights you by trying to un-do what you just adjusted.

I believes that when the shutter is released that AF tracking may be momentarily interrupted but when you use rear focus the camera continues to track well even when the shutter is released.

Activating back-button AF

You need to change a custom function or two to set up rear focus. And with some systems you set up rear focus via the camera’s menu. You can consult your camera body user manual to learn to set up rear focus. Remember, to use any Custom Function, your camera must first be in one of the P – Program, AV – Aperture Value /A –  Aperture Priority, TV – Time Value / S – Shutter Priority or M – Manual Mode.

Making the change from the shutter button to rear focus is first time hard and takes lots of getting used to, it takes a bit of retraining of the brain and the thumb to get used to rear focusing. But I have stuck with it for more than five years and will never go back.

Finally, remember that if you don’t like Back-Button AF , you can return the camera back to factory-default operation any time.

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