Focus and exposure lock

Learning goal

You will be able to explain which two decisions change at once behind the scenes when you touch your phone's screen: where the sharpness will fall and how bright the photo will be recorded. You will learn to steer these two decisions by tapping, to fine-tune with the brightness slider that appears, to lock both at once by holding your finger down, and to rebuild the frame after locking; against the light, on a stage or in a moving moment, you will be able to replace the phone's decision with your own.

The question

You are at a school show. The hall is dark, and a single spotlight on the stage falls on your child. You raise your phone and shoot. On the screen the dark hall is nicely lit, and the chairs and the audience can be made out; but your child's face on the stage is a bright white blotch, and the colors of the costume are wiped out. Because the hall filled the frame, the phone made its decision based on the darkness of the hall and gave up the face that really mattered.

Why does touching the screen change everything? Because with a single tap you tell the phone two things: "Focus here" and "Set the brightness for this". When you tap your child's face, the phone moves the sharpness onto it and sets the brightness for the spotlight falling on it; the hall goes dark and the face comes out. In this lesson we will turn this small movement into a deliberate tool.

Context

As long as you don't touch it, the phone makes its decisions itself. Most of the time it looks for a face in the frame; if there is no face, it focuses on the center or on the most prominent object. It usually sets the brightness by looking at the whole frame. This automatic guess works well when most of the scene is evenly lit. But if a bright spotlight and a dark hall sit side by side in the scene, or if the main subject is a small part of the frame, the phone's guess parts ways with yours.

In the previous module you saw how to focus on a camera by half-pressing the shutter button and how to correct brightness with the compensation dial. On a phone, the equivalents of these jobs are on the screen, and most people use their phones without ever discovering them. Yet tapping, sliding and holding are the easiest tools for taking the phone's camera beyond auto. Without entering a menu, without memorizing the name of a setting, they give the decision back to you with nothing but your finger.

Simple explanation

What changes when you tap?

When you tap a spot on the screen, most phones show a small square or circle there. This marker is the focus point, and behind it the sharpness and the brightness change together. For sharpness, the phone moves the glass elements inside the lens to make the spot you tapped crisp. If you tap a flower in the foreground, the flower is sharp and the garden behind is soft; if you tap a tree in the back, the opposite. For brightness, the phone now measures the light not by the whole frame but largely by the area you tapped. You can also call this the metering point.

That is why the place you tap decides both how sharp and how bright the photo will be. If you tap a bright window, the phone darkens the whole frame to show the window correctly. If you tap a face in shadow, it brightens the frame to show the face correctly, and the window may turn white. So tapping is asking a question: what is the most important thing in this photo? This is a question the phone doesn't ask you in every frame, and you answer it with your finger.

How do you use the brightness slider?

After you tap, a small sun icon or a vertical line usually appears next to the focus marker. When you drag your finger up or down, the photo brightens or darkens. This exposure slider is the phone's equivalent of exposure compensation on a camera: it lets you fine-tune, one step further, the decision the phone made based on the spot you tapped.

The slider is most useful when the phone is looking at the right area but still gets it wrong. If a face under a spotlight is still a little too bright after you tap it, you pull the slider down gently. Here it helps to know that different skin tones behave differently. Light skin quickly slips into white in strong light; you may need to pull the slider down until detail remains on the cheek and forehead. Dark skin can still stay deep and detailed in the same light; there, pulling the slider down too far darkens the face for no reason. While sliding, look at the face on the screen and stop at the point where texture can be made out both in the bright areas and in the shadows.

What is the lock for?

After you choose the spot you tapped, if you move the phone a little or change the frame, the phone will often make its decision again and your choice is lost. To prevent this, you hold your finger down on the spot you tapped for a second or two. On many phones, a label or icon showing the focus and exposure lock (AE/AF lock) then appears on the screen. Now the sharpness and brightness are fixed; they don't change even if you move the phone.

The lock is most useful when recomposing: after tapping a face and locking, you can turn the phone slightly and move the face to the edge of the frame; the sharpness and brightness stay on the face. The only rule here is not to change your distance from the subject, or the focus will shift. It is also valuable while waiting: while you wait for a moment on stage, for a bird to land or for someone to laugh, the phone doesn't change its decision every second. To remove the lock, just tap somewhere else on the screen.

Visual evidence

Go back to the school show. Two children stand side by side on the stage: one with light skin, the other with dark skin. The spotlight falls on both of them, while the hall is dark. In the first frame you don't touch the screen at all. The phone looks at the dark hall and brightens the frame. The face of the light-skinned child has turned into a bright white blotch; their nose and cheeks can't be made out. The face of the dark-skinned child is in somewhat better shape, but the top of their forehead and nose is covered with a bright, whitish glow. The colors of the costumes have faded, and the curtain at the back of the stage has turned gray.

Now you tap the screen between the two children's faces and hold your finger down. The lock icon appears, and the hall goes dark. You pull the slider down a little and stop at the point where texture appears on the light-skinned child's cheek. In the new frame, the shadows of the smile and the eyebrows show on the light-skinned child's face. The dark-skinned child's face is a deep, warm tone, the shine on their forehead has shrunk to a small point of light, and their eyes sparkle. The red and yellow of the costumes have come alive, and the hall is pitch dark, but the hall was never what mattered. The same phone, the same moment; the only difference is that you told the phone with your finger what mattered.

Common mistakes

Summary

Tapping your phone's screen chooses, together, what the sharpness and brightness will be set for; the slider fine-tunes this decision, and holding down locks both at once. These three movements put your decision in place of the phone's guess, especially against the light and in dark settings such as a stage; after locking, you can rebuild the frame without changing your distance.

Mini assignment

In this assignment you try out the power of a tap.

1. Choose someone sitting in front of a bright window, or a plant pot on a windowsill, and photograph it without touching the screen at all.
2. From the same spot, touch your finger once to the window and once to the subject, and press the shutter button; watch on the screen how the brightness shifts.
3. After tapping the face, set the brightness with the slider, lock by holding your finger down, and shoot after turning the phone slightly to move the face to one edge of the frame.
4. In the locked frame, check whether the sharpness and brightness stayed on the face, and write the result in a single sentence.

Check yourself

• Were you able to observe how the brightness changed when you tapped the window and the face?
• Were you able to stop the slider at the point where detail showed on the face?
• Were you able to rebuild the frame after locking without changing your distance?

Glossary

Mini quiz

1. Which two decisions change at once when you tap the screen on a phone?

• A) Color temperature and file format
• B) Where the sharpness will fall and what the brightness will be set by
• C) Whether the flash fires and the resolution
• D) Which lens the camera will use and sound recording

2. If a face in front of a bright window comes out dark, where should you tap?

• A) The window
• B) The top corner of the screen
• C) The face
• D) Nowhere; turning on the flash is enough

3. Which movement after locking ruins the sharpness?

• A) Turning the phone slightly to the right
• B) Turning the phone slightly upward
• C) Looking at the brightness slider
• D) Moving closer to or further from the subject

4. If a light-skinned face slips into white under a spotlight, what should you do with the slider?

• A) Pull the slider down in small steps and stop at the point where detail appears on the cheek
• B) Pull the slider all the way up
• C) Pull the slider all the way down
• D) Not touch the slider at all

Answers

1. B — The tapped spot becomes both where the sharpness is moved and what the brightness is largely measured by; that is why a single tap changes two decisions at once.
2. C — When you tap the face, the phone sets the brightness for the face and brightens the frame; tapping the window would darken the frame even more.
3. D — The lock fixes the sharpness at a certain distance; turning the phone causes no problem, but when the distance to the subject changes, the focus shifts.
4. A — Pulling down in small steps brings back detail in the bright areas; pulling all the way darkens the face for no reason and makes the shadows grainy.

Summary card

• Tap: sharpness and brightness are set for that spot.
• Slide: fine-tune the brightness with the sun icon, stop where detail appears.
• Hold: focus and exposure are locked.
• After locking, turn, but don't move forward or back.
• Tapping somewhere else removes the lock.

Bridge to the next lesson

You have learned to manage sharpness and brightness. Now it's time to turn to the frame itself: is the horizon level, where does the subject stand? A simple tool that helps you answer these questions is hidden in your phone's settings. The next lesson asks: what are those grid lines for? There you will see how the grid keeps the horizon level and how it guides you as you build a frame.