Back-button focus

Learning goal

You will be able to separate autofocus from the shutter button and move it to a button on the back of the camera, and judge what this arrangement gains you and what it makes harder. In the menu you will take the task of starting focus away from the shutter button's half-press and assign it to a back button; with continuous focus you will learn to track by keeping the button pressed and to fix sharpness by letting go; you will see in which scenes taking frames in a row with the same sharpness is useful; and in a group with people of different skin tones, you will know how to focus once on the eye you chose and keep that sharpness.

The question

At a graduation ceremony, you are standing at the side of the stage. The students go up onto the stage one by one and receive their diplomas. Every time you half-press the shutter button, the camera searches for sharpness again; once it goes to the microphone on the lectern in front, once to the curtain behind. Just when the moment comes, the lens is still going back and forth, and the frame is missed.

The photographer next to you, though, presses a button on the back of the camera with their thumb and lets go, shoots in a burst with their index finger, and seems not to miss a single frame. Why do professionals separate focus from the shutter button? In this lesson we will talk about this arrangement, which gives focus to the thumb and the shot to the index finger, what it changes, and whether it is right for everyone.

Context

On most cameras the shutter button does two jobs by default: on the half-press it searches for sharpness, and on the full press it shoots. In Lesson 10.2 we saw that this half-press locks sharpness in single-shot focus and follows the subject in continuous focus. This arrangement is easy, because a single finger does everything. But tying two jobs to one finger has a cost too: with every shot, sharpness is searched for again, even when you don't want it to be.

Back-button focus unties this link. From the menu you remove the focus task from the shutter button's half-press and give this task to a button under your thumb. Now the shutter button only shoots, and your thumb manages sharpness. This looks like a small menu change, but it changes how you hold the camera and the decisions you make while shooting.

Simple explanation

How is the button assigned?

On the back of many cameras, where the thumb reaches comfortably, there is a button labeled AF-ON. On cameras without this button there is usually a button labeled AE-L or AF-L, used for locking exposure or sharpness, and it can be given a new task from the menu. The names and the menu path change from manufacturer to manufacturer; usually, in the custom settings or in a section often called button customization, there are rows where you can choose the shutter button's half-press and the back button's task separately.

The setup consists of giving one task and taking one task back: the back button is given the task of starting focus, and the focus task is removed from the shutter button's half-press, which is set to only meter exposure or to do nothing. If you only give the button a task and leave the half-press as it is, both fingers search for sharpness and the arrangement's gain is lost. After the setup, test the camera by trying it: the lens shouldn't move when you half-press the shutter button, and sharpness should be searched for when you press the AF-ON button or whichever back button you chose.

What does the thumb gain you?

The biggest gain of back-button focus is that it combines the two focus modes into a single mode. You set the camera to continuous focus. While the subject moves, you keep the back button pressed and the camera follows it. When the subject stops, or when you want to fix sharpness, you lift your thumb; sharpness stays at that distance, as if you had locked it in single-shot focus. Without going into the menu and changing modes, just by pressing and releasing your thumb, you switch between two behaviors.

Recomposing gets easier too. Once you find sharpness and lift your thumb, you can set up the frame as you like and press the shutter button as many times as you want; sharpness isn't searched for again with every frame. When shooting a landscape or a series of portraits with a camera on a tripod, someone walking past in front or a leaf can't pull sharpness to itself. While you wait for a bird to land or a child to laugh, sharpness is ready, and the shot happens the moment you press the shutter button.

What does it make harder?

This arrangement has a learning period. A hand that has shot with the half-press for years forgets to press the back button in the first days, and frames come out soft. Getting used to two fingers doing separate jobs can take a few days of shooting, and trying the new arrangement on an important job during this period can be risky.

Someone else isn't used to your camera either. If at a family meal you hand the camera to a relative and ask them to photograph you, they press the shutter button and don't understand why sharpness doesn't come. That is why many photographers undo the setting before handing the camera to someone else, or save the two arrangements as stored settings. Also, on some cameras in single-shot focus the shutter button holds off shooting until sharpness is confirmed; you need to test on your own camera how this behavior changes in the back-button arrangement, where your half-press no longer starts focus. This arrangement is a powerful tool for jobs that need constant tracking and waiting; it isn't a compulsory rule for everyone and every job.

How do you keep sharpness in a crowd and on different skins?

In a group photo, searching for sharpness again in every frame means the risk of going to a different face each time. In one frame the camera may choose the face nearest the lamp, in the next the shoulder in front. With the back button, you put single-point focus on the eye of the person you chose, find sharpness once with your thumb, and let go. Then, while waiting for everyone to open their eyes and smile, you take as many frames as you like; sharpness stays on that eye. This is focus lock done with the thumb.

This is especially useful in scenes where the light doesn't fall equally on the faces. In a frame where a light-skinned face shines in window light and a dark-skinned face stays in slight shadow, every half-press can carry sharpness to the more contrasty face. Locking sharpness once on the eye you chose means not handing the decision of which face will be sharp back to the camera in every frame. If the group changes position, or you come closer, you focus again with your thumb.

Visual evidence

You go back to the graduation ceremony. You have set the camera to continuous focus and focus to the back button. As one of the students going up onto the stage, a tall, dark-skinned young person, walks to receive their diploma, you keep the button pressed with your thumb and the focus point follows their face. When they stop to shake hands with the rector, you lift your thumb. Sharpness stays on their eye; as they smile and raise their diploma you take three frames, and in all three you see that their eyelashes and the glittering stripe at the edge of their gown are sharp.

After the ceremony, in the garden, six friends of different skin tones pose side by side. The sun comes from one side; the faces at the edges are bright, those in the middle in slight shadow. You focus once with a single point on the eye of the friend in the middle and let go, then take ten frames, waiting for the frame where everyone opens their eyes at the same moment. Not once does the camera carry sharpness to the bright face at the edge. The same people, the same light; what changed was in whose hands the decision about sharpness stayed.

Common mistakes

Summary

Back-button focus takes searching for sharpness away from the shutter button's half-press and gives it to a button under the thumb; the shutter button only shoots. With continuous focus, keeping the button pressed follows the subject and letting go fixes sharpness; this combines two behaviors in one hand without changing modes, and spares the photographer who waits, recomposes or works with groups from having sharpness searched for again in every frame. The learning period and someone else using the camera are the costs of this arrangement; in a frame with people of different skin tones, though, it can be a way of not handing the decision of which eye will be sharp back to the camera in every frame.

Mini assignment

In this assignment you will set up back-button focus and compare the two arrangements in the same scene.

1. In your camera's menu, assign starting focus to a back button and remove the focus task from the shutter button's half-press; test that the lens doesn't move when you half-press the shutter button.
2. With continuous focus, follow someone walking by keeping the button pressed; when they stop, lift your thumb and take three frames in a row.
3. Focus on a person and lift your thumb, then press the shutter button at a moment when someone walks past in front of them; look at whether sharpness stays in place.
4. Photograph a group of light- and dark-skinned people, where the light doesn't fall equally on the faces, first focusing with the half-press, then focusing and fixing with the back button, and compare which eye sharpness stays on.

Check yourself

• Does the shutter button's half-press no longer cause any lens movement?
• After you lifted your thumb, did someone walking past pull sharpness to themselves, or did sharpness stay in place?
• In the group photo, did sharpness stay on the eye you chose in every frame?

Glossary

Mini quiz

1. In back-button focus, what is the shutter button's task?

• A) Only searching for sharpness
• B) Only shooting; the back button manages sharpness
• C) Setting the white balance
• D) Stopping down the aperture

2. In the continuous focus and back-button arrangement, what happens when you lift your thumb?

• A) The camera shoots
• B) Sharpness goes to infinity
• C) Sharpness stays fixed at the distance where it was found
• D) The camera opens a menu to switch to single-shot focus

3. If the back button was given the focus task but focus on the half-press wasn't turned off, what is the problem?

• A) The shutter button also searches for sharpness, and sharpness can be carried again with every press
• B) The camera never shoots
• C) The exposure goes wrong
• D) The back button is disabled

4. In a group of people of different skin tones, where the light doesn't fall equally on the faces, what do you do to keep sharpness on the eye of the person in the middle?

• A) Half-press for every frame and wait for the camera to choose
• B) Switch to wide-area focus
• C) Lengthen the shutter speed
• D) Focus once on that eye with a single point, lift your thumb, and take the frames that way

Answers

1. B — The aim of the arrangement is to separate the two jobs: the index finger shoots, the thumb manages sharpness.
2. C — When the button is released, the camera doesn't search for sharpness again; sharpness stays in place as in single-shot focus.
3. A — If both fingers search for sharpness, every press of the shutter button carries sharpness again and the arrangement's gain is lost.
4. D — Once sharpness is fixed on the eye chosen once, it is prevented from sliding to the more contrasty face in every frame.

Summary card

• The shutter button shoots, the thumb manages sharpness.
• Setup in two steps: give focus to the back button, take focus away from the half-press.
• With continuous focus, press: it follows; let go: it fixes.
• The learning period and a camera handed to someone else are the cost of this arrangement.
• In a group, lock sharpness once on the eye you chose; on every skin, let the decision stay yours.

Bridge to the next lesson

You now know where to put focus, in which mode to keep it, when to find it by hand and how to fix it with your thumb. Still, some frames will come out soft. The real question is being able to look at a blurry photo and read its cause: has the softness spread over the whole frame, or is sharpness sitting in the wrong place? The next lesson talks about telling apart the traces of blur that come from a shaking hand, a moving subject, a wrong focus point, a wrong mode, shallow depth of field, a dirty lens and not enough light.