Photography with your phone

What you'll learn: what the lenses on your phone are for, why digital zoom loses detail, and how much your phone edits a picture before you ever see it.

Rosa is a retired primary school teacher who goes to her town's Saturday farmers' market every week. This autumn she has set herself a small project: photograph the market over a few Saturdays and make a set of ten prints to give to the stallholders at the end of the season. She has no camera except her phone, a two-year-old model with three lenses on the back. We will follow her from her first disappointing pictures to that finished set.

Her first attempt went the way many people's do. She stood at the end of the aisle, zoomed in on the cheese stall from ten metres away, and got a picture that looked soft and smudgy when she viewed it at home. Nothing was wrong with her phone; she just did not yet know what it was doing.

The lenses on the back of your phone

Most recent phones have more than one camera, each a separate lens with its own small sensor, and the app switches between them when you tap the zoom buttons, labelled something like 0.5x, 1x, 2x, 3x or 5x. The exact set depends on the model: budget phones may have only a main camera and an ultra wide, while the more expensive iPhone, Pixel, Samsung and other models usually add a telephoto.

The main camera almost always has the biggest sensor and the best lens on the phone. A bigger sensor gathers more light, which means cleaner, sharper pictures, especially indoors or in the evening. If you only remember one thing from this lesson, remember that 1x is your default.

Some phones also offer a 2x button that uses the middle of the main camera's very high resolution sensor. That crop holds up well because the sensor has pixels to spare.

Optical zoom and digital zoom

This explains Rosa's smudgy cheese stall. When you tap one of the lens buttons, you switch to a different piece of glass that genuinely sees a narrower view. That is optical zoom, and it keeps full detail. When you pinch or slide beyond the highest lens number, the phone has no more glass to switch to. It takes the picture from its longest lens and enlarges the middle of it. That is digital zoom.

Digital zoom is like cutting a printed photo with scissors and then enlarging the small piece you kept on a photocopier. The image gets bigger, but no new detail appears, so edges go soft and textures turn to mush. Clever processing hides a little of this, but the further past the lens's own number you go, the worse it gets.

The fix is usually free: walk closer. Rosa's phone has a 3x telephoto. At 3x, her pictures are sharp. At 10x, she is looking at enlarged guesswork. Taking eight steps towards the cheese stall and using 3x would have given her a far better picture than standing still at 10x.

Your phone edits before you see the picture

When you press the shutter, a modern phone does not simply take one picture. It usually captures a quick series of frames, some of them from just before you pressed, and combines them. It lifts shadows, holds back bright skies, smooths grain, sharpens edges and adjusts colour, all in a fraction of a second. This is often called computational photography, and it is the main reason phone photos look as good as they do from such tiny lenses.

Different makers tune this differently. Some lean towards bright, punchy colour; others towards a flatter, more natural look. Recent models often let you choose a style or tone in the camera settings, though its name varies by brand. The point is that your phone has opinions, and this course will show you when to override them.

Habits that matter more than settings

Before any settings, four habits will improve almost every picture Rosa takes:

1. Clean the lens. A fingerprint on the lens makes bright lights glow and everything look hazy. Wipe it with a soft cloth before you start.
2. Hold it with two hands. Tuck your elbows against your body. A steady phone gives sharper pictures, especially indoors.
3. Press gently. Stabbing the on-screen button shakes the phone. Many phones also let you use a volume button as the shutter, which is steadier.
4. Take a second shot. Small changes in a face or the light often make it better.

Rosa's first lens walk

The following Saturday, Rosa tried an exercise you can copy. She picked one stall, the flower seller's, and photographed it from the same spot with each lens, then walked closer and did it again. At home, she compared them side by side.

At 0.5x from close up, the buckets of dahlias in front looked huge and the stall behind them looked far away. The buckets near the corners leaned outwards. At 1x, everything looked natural and sharp. At 3x from further back, the flowers filled the frame and the busy car park behind them disappeared. Each lens told a slightly different story about the same stall, and none of them involved digital zoom.

She now knows her tools. Next Saturday, she will learn to tell her phone what to focus on and how bright the picture should be.

Recap

• Most phones have several lenses; the main 1x camera has the biggest sensor and is the best default.
• Switching between the lens buttons is optical zoom and keeps detail; going past the longest lens is digital zoom and loses it.
• Walking closer usually beats zooming in.
• Your phone combines and processes several frames for every picture, and each brand tunes this differently.
• Clean the lens, hold the phone with two hands, press gently and take a second shot.

What you'll learn: how to tell your phone what to focus on and how bright to make the picture, and what HDR is doing when a scene has both very bright and very dark parts.

Rosa's lens walk gave her sharp, natural pictures of the flower stall in soft morning light. Then she turned to the bread stall and ran into a new problem. The baker works under a white canopy, and behind him the morning sun was blazing on the street. Her phone gave her a bright, cheerful street and a baker who was almost a silhouette. In another shot of the loaves on their white table, the bread looked fine but the white tablecloth came out a dull grey.

Both problems come from the phone guessing what matters and how bright it should be. This lesson takes those decisions back.

Focus: tell the phone what matters

Phones focus automatically and usually choose well, especially on faces, but a busy market offers dozens of possible subjects. If Rosa wants the sourdough in front sharp rather than the baker behind it, she has to say so.

The way to say so is to tap on the screen where the subject is. On almost every phone a small box or circle appears there, and the camera focuses on that spot. It usually sets the brightness for that spot too, which leads to the next idea.

One limit to know: every lens has a minimum focusing distance. If you hold the phone a few centimetres from a jar of honey and it refuses to sharpen, you are too close for that lens. Step back a little, or see whether your phone offers a macro option, which on many models uses the ultra wide lens to focus close.

Exposure: why your phone gets brightness wrong

Exposure is simply how bright the picture is. The camera measures the light and picks a brightness, and it does so with a built-in assumption: most scenes, averaged together, are a middle tone, somewhere between black and white.

That assumption works like a thermostat. A thermostat does not know whether you are wearing a jumper or expecting guests; it just pushes the room towards its set temperature. The camera does the same with brightness. Point it at something mostly white, like Rosa's tablecloth, and it pushes the picture darker towards that middle, so white turns grey. Point it at something mostly dark, like a black cat or the inside of a dark van, and it pushes the picture lighter, so black turns muddy grey. When the scene mixes a bright background with a darker subject, as with the baker, it averages them and the subject loses.

Face detection and modern processing make phones smarter than a thermostat, but the pull towards the middle is still there.

The exposure slider

When you tap to focus, most camera apps show a brightness control next to the focus box or along the edge of the screen: a small sun icon to drag on an iPhone, a brightness slider on Pixel and Samsung phones, and similar controls on others. Up brightens, down darkens, and focus stays put.

For the white tablecloth, Rosa taps on the bread, then drags brightness up a little until the cloth looks white again on screen. For the baker, she taps on his face, which alone often fixes much of the problem, and nudges brightness up if needed. The street behind him gets brighter too: a trade-off she decides on.

Locking focus and exposure

Tapping has one side effect. If Rosa taps on a loaf, then moves the phone slightly to put the loaf off-centre, the phone may refocus and remeasure on whatever is now in the middle. The solution is to lock it.

On many phones, pressing and holding on the subject for a second locks both focus and exposure; an iPhone shows the words "AE/AF Lock". Other phones show a padlock icon after a tap, and the details shift between brands and software versions. Once locked, you can recompose freely and the settings stay put until you tap somewhere else.

Here is the routine Rosa now uses for any tricky shot:

1. Frame roughly, using the lens you want.
2. Tap on the most important part of the subject, usually a face or the main object.
3. Look at that part, not the whole screen, and drag the brightness until it looks right.
4. Lock focus and exposure if you are going to recompose.
5. Recompose and take two or three shots.

HDR: when the scene is too contrasty

Our eyes cope with a sunny street and a shaded doorway at once; a single exposure often cannot. HDR, short for high dynamic range, is the phone's answer: it combines frames taken at different brightness levels so that both the bright sky and the dark shadows keep some detail.

On most recent phones, HDR is automatic and built into every picture, under names like Smart HDR on iPhone and HDR+ on Pixel. Many current models no longer even offer a switch to turn it off. With the baker, HDR is why the street did not turn completely white.

HDR can be overdone, though. When the phone lifts shadows too far, pictures can look flat and slightly unreal, with faces lit as evenly as the background. A small downward drag of the brightness slider often restores some mood. And no amount of HDR beats moving yourself or your subject into better light, which is the subject of lesson 4.

Rosa's results

With her new routine, Rosa's second picture of the bread stall shows the baker's face clearly, the loaves golden, and the bright street behind him pale but still recognisable. The tablecloth is white, and he is laughing.

Recap

• Tap the screen to tell the phone what to focus on; it usually sets brightness there too.
• Cameras pull every scene towards a middle tone, so white things come out grey, dark things come out grey, and dark subjects against bright backgrounds come out too dark.
• Use the brightness slider to correct exposure while looking at the subject, not the whole frame.
• Lock focus and exposure, often with a long press, before you recompose.
• HDR is usually automatic now; it helps with contrasty scenes but can look flat if pushed.