Back Button Focus – What is it and will it improve your photography?

"Back Button Focus" is a camera setting and photographic technique that separates focusing and shutter release onto two separate buttons.  Instead of using the shutter button to both focus and take the photo, the photographer assigns the focusing function to a separate button (usually on the back of the camera) to have more control over when and where the camera focuses.

This article provides a breakdown of the concept and discusses its pros and cons, and covers:

• What is back button focus?

• How to turn on and use back button focus.

• What are the considerations, advantages and disadvantages of back button focus?

• Should you use back button focus?

What is Back Button Focus?

To understand back button focus, you need to understand the different camera focus modes.  Let’s explore the main modes:

• Single Shot Autofocus - also called AF-S Mode, One-shot AF, or Single-shot AF - locks focus when you half-press the shutter button, allowing you to recompose the shot and capture it precisely.  It is ideal for stationary subjects and great for portraits, still life, architecture and landscape photography.

• Continuous Autofocus – also called AF-C Mode, AI Servo AF, or Continuous AF - continually adjusts focus as the subject moves.  It will continue to focus while you move the camera to recompose the shot, thus it’s best for tracking moving subjects and perfect for sports, wildlife, or dynamic scenes.

• Manual Focus, unlike autofocus, requires you to focus the camera yourself by adjusting a ring on the lens.  It’s necessary when autofocus struggles, such as low light conditions (the ability to focus manually is a must-have skill for astrophotography).

Note: Some cameras have a Hybrid Autofocus mode – also called AF-A Mode, AI Focus AF or Continuous AF - that automatically switches between single shot and continuous modes based on the situation.

All three of these methods have their uses, and you will likely encounter situations where you need to switch from one to the other.  Doing this requires scrolling through menus or changing a dial on your camera, which can be frustrating in the heat of the moment.

By default, the camera will focus using the settings above when you half-press the shutter button.  At that point, the camera will also determine the exposure (assuming that you are not using manual mode) depending on the metering and exposure modes you have selected.  Thus, the shutter button has multiple functions that you do not always want to trigger at the same time.

You can separate these functions by using a separate button to trigger focusing, which is referred to as Back Button Focus.  This button is usually located on the back of the camera, which explains the name, and is typically named “AF-ON”.

The back button enables you to use all three focusing modes without changing camera settings, giving you flexibility and avoiding wasted time when shooting.

How to turn on and use Back Button Focus.

Here’s how you turn it on:

1. Change your camera settings so that the shutter button doesn’t trigger focusing and the back (AF-ON) button starts autofocus.  You may have to consult your camera’s manual to find out how to do this.

2. Optionally, set your camera’s focus mode to Continuous Autofocus (AF-C).  This is useful when tracking moving subjects using the back button.

When back button focus is turned on, you take a shot as follows:

1. Press the back (AF-ON) button to focus

2. Recompose, if necessary

3. Half-press the shutter button to start light metering and determine the exposure.  This may not be necessary if you have set your back button function to also start metering.

4. Use exposure compensation, if necessary

5. Fully press the shutter button to take the shot

Here’s how you can easily replicate the three focusing modes with back button focus:

Single Shot Autofocus:  Press the back button until the scene is in focus, and then let go to keep the focus locked until you press the button again.  You can now compose the shot and press the shutter button to expose and take it.

Continuous Autofocus:  Press and hold the back button as long as you want, forcing your camera to continually adjust the focus until you take a picture.  Press the shutter button while holding the back button to expose and take the shot.  Note that you must have switched your camera to Continuous Autofocus for this to work.

Manual Focus:  Take your thumb off the back button and focus by rotating the ring on your lens.

Considerations

How to use back button focus with a touch screen.

Many modern cameras have a touch screen that can also be used to trigger the focus function that works with and without the back focus button.  Just press the area on the screen where you want the camera to focus.  Ideal for shooting on a tripod.

Does the metering mode of the camera make a difference?

The metering mode determines how the camera measures the quantity of light coming through the lens.  You can find out more about the different metering modes in another article.  However, there is one metering mode that is influenced by what you’re focusing at, which is the Evaluative Metering Mode (also known as Matrix or Multi-Pattern mode).  When using this mode, the camera will lock the exposure at the time focus is achieved (i.e. when the shutter button is half-pressed), because it assumes that the area you focused on is the most important part of the photo.  This could cause overexposure if this area is dark and the sky is bright.

Using the evaluative metering mode in combination with back button focus will not lock the exposure at the time focus is achieved.  Instead, it will start metering and setting the exposure when you half-press the shutter button, and it will continue to do so until you fully press or release the shutter button.  Thus, letting you recompose and enabling you to avoid the overexposure.

Note that the other metering modes (e.g. partial, spot and centre-weighted) work different, and the camera will continue to adjust the exposure until the shutter button is fully pressed and the shutter released (just like the back button technique for the evaluative metering mode).

Advantages of Back Button Focus

• Separation of functions:  Because the focusing and shutter release functions are on two separate buttons, you know what the camera will do when you press a button.

• Shoot in AF-C with the Benefits of AF-S:  You can use continuous autofocus (AF-C) while still enjoying the benefits of single autofocus (AF-S) when needed.  All you need to do is to press and hold your finger on the Back Focus Button to focus continuously and take it off again to stop focusing.

• Instant Manual Focus:  You can easily switch between autofocus and manual focus without changing camera settings.

• Ignore Obstructions:  You can recompose shots without the camera refocusing unintentionally.  Note that this can also be achieved using the Single Shot Autofocus (AF-S) mode, if you do not let go of the shutter button after you have focused.

• Metering Mode Consistency:  The Evaluative Metering Mode will operate similarly to the other metering modes and continue to meter while moving the camera.

Disadvantages of Back Button Focus

• Another Button:  You must get used to pressing another button to focus.  This will become second nature quickly though, and you’ll find that your thumb was already positioned on or close to the back button anyway, so it doesn’t change the way you hold your camera.

• Dirty Glasses.  If you wear glasses, they could become dirty with finger marks, streaks and body grease more quickly, because your fingers used to operate the back button will move often and potentially touch your glasses.  I wear glasses myself and have found no difference between using and not using back button focus – they get dirty regardless.

Should you use Back Button Focus?

The main problem that the Back Button Focus technique solves is unwanted change in focal point, exposure settings and metering.  By separating focusing and metering onto two separate buttons, you gain more control over taking a photo.

You decide when and where to focus, and you decide when the camera starts to meter and determine the exposure.  Giving you time in between to properly compose.

As a result, you’ll find that more of your photos will be in focus and be exposed correctly, especially when dealing with high contrast scenes.

However, it does require you to change your camera technique slightly in that it requires you to operate two buttons instead of one to take a photo.

As a landscape photographer who occasionally takes photos of wildlife, I use Back Button Focus all the time.  It has become second nature, and I like the control and consistency it has given me.

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