The Exposure Triangle

01What the app is

The Exposure Triangle teaches the three settings behind every photograph: the aperture, the shutter speed and ISO. You change them on a camera that behaves like a real one, and the picture answers straight away.

Two views show the result side by side. The viewfinder shows the photograph as the camera would take it, with its brightness, its blur and its grain. The side-on view shows the same scene from the side and draws the sharp stretch across it, so you can see why the backdrop went soft.

After every change, a short note says what just happened and why. It isn't a course you read from front to back: try something, watch what it does, and read the note.

02Before you begin

• A laptop or desktop computer. The camera needs a window 1280 pixels wide or more to show everything at once. At 1280 by 800, 1366 by 657 and 1440 by 900, the viewfinder, the side-on view and the controls all fit on one screen, and the page doesn't scroll. On the shortest of those, a running challenge's panel scrolls inside itself, and its hint and next-step buttons stay in sight. Between 961 and 1279 pixels it still works, and the bar tucks a few labels away; at 960 and below, the views stack in one column and the page scrolls. A phone layout comes later.
• A recent browser with WebGL2. WebGL2 is the graphics system that draws the picture, and every recent browser has it, as long as hardware acceleration is on. Every check so far ran in Chromium, the engine inside Chrome and Edge. Safari and Firefox checks come later.
• A good connection for the first visit. The camera opens on the portrait. A still of its opening view appears with the page, the live camera takes over as soon as its first photograph arrives, and the rest of its 56 MB loads behind the picture. The road landscape and the rainy street are bigger, at about 112 MB and 103 MB, and the lakeside is about 60 MB (see Troubleshooting). Once the scene you opened has every photograph, the app downloads the other scenes in the background, so switching to one doesn't mean waiting for its download. That's about 330 MB in all on a first visit; if your browser is set to save data, it skips this.
• Nothing else. There's no account, no sign-in and nothing to install.

03The viewfinder and the side-on view

The viewfinder

The left-hand picture is what the camera sees. The strip along its bottom edge reads like a camera's electronic viewfinder: the mode, the shutter speed, the f-number, the exposure meter, ISO and the focal length.

• The meter runs from three stops too dark (−) to three stops too bright (+). The pointer sits on the centre line when the exposure's right.
• The histogram, top right, shows how the picture's tones spread, from black on the left to white on the right.
• The focus box turns dashed when the subject falls outside the sharp zone.
• When you zoom out, a frame marks the framing you started from (“Your 85 mm framing”), so you can see how much wider the new view is.

Take a shot, and camera shake

Take a shot, at the right-hand end of the viewfinder's title, records the picture the way the camera would. If the shutter stays open too long to hold the camera steady by hand, the shot shows the shake as blur, and the live view wanders a little to warn you. The shot stays on screen with a Your shot label until you change a setting. With the viewfinder selected, Space or Enter does the same.

The side-on view

The strip on the right shows the scene from the side: the camera on the left, then what's in front of it, nearest first. On the portrait that's a reflector, a softbox, the model and the backdrop; on the road landscape, the gate, the car, the sheep and a cottage; on the rainy street, the café table, the lamp post, the cyclist, the walkers, the bus and the church; at the lakeside, the cascade, the figure on his bench, the heron and the far shore. The blue band is the sharp zone, the stretch of distance that comes out sharp, and the headline above it gives its depth, such as “Sharp zone 28 cm deep”. The drawing doesn't show the camera to scale, and the distance scale tightens beyond 12 m on the road landscape, beyond 10 m on the rainy street and beyond 6 m at the lakeside, so things far off still fit.

05Click to aim

Click anywhere on the picture and the zoom aims there, so zooming in closes in on that spot rather than the middle of the frame. A small cross marks it. Double-click the picture, or press Esc with it selected, to go back to the scene's own framing. With the viewfinder selected, the arrow keys move the aim.

06What just happened, and the light ledger

The cream panel under the side-on view explains each change once you let go of the control: what you changed, what the camera did in reply, and what that means for the picture. It never runs to more than three sentences.

Underneath, the light ledger adds up the exposure in three rows:

Detail adds the exposure value, and how much light a mid-grey pixel collects, counted in electrons, with its signal-to-noise ratio.

07Beginner and Detail

The Beginner and Detail switch at the top right sets how the app talks to you. Change it whenever you like: the last note rewrites itself at the new level.

• Beginner uses plain words. The dials keep the names photographers use, aperture, shutter speed and ISO, with a plain meaning under each. The first time the aperture or ISO comes up, the note adds a reminder in brackets: bigger f-numbers are narrower, and ISO is brightening.
• Detail uses photographers' terms and adds the numbers behind them: exposure value, the circle of confusion, the hyperfocal distance, the angle of view, the closest focus distance and how much light a mid-grey pixel collects. It also adds the Idealised lens switch, and the challenges' hints come without a wait.

09The scenes

Four scenes are live. Pick one from the scene buttons at the top.

• The studio portrait is where the camera opens: a model in front of a studio backdrop, with a 24 to 200 mm zoom that opens to f/1.4.
• The road landscape is a country road running towards mountains 8 km off, with a field gate 5.5 m away, sheep grazing beyond it and a white cottage, on a 14 to 600 mm zoom. It's the first scene with something moving: a car driving away from you down the road. Put the camera on the tripod and try a slow shutter speed on it. Because it's heading away rather than crossing the frame, it blurs far less than a car driving across the picture would.
• The rainy street is a wet city street at blue hour, looking up the road to a church spire 220 m away, on the same 14 to 600 mm zoom. A café table sits beside you, with a lamp post 3.5 m off. A cyclist rides away up the road at an angle, so part of his speed crosses the frame: at 24 mm, 1/30 s smears him and 1/500 s freezes him.
• The lakeside looks across a lake at dusk to a far shore 800 m away, with a bronze figure on a bench 5.9 m off and a small cascade tumbling over rocks at your feet, on a 16 to 200 mm zoom. Two things move: the water and a heron flying across, whose reflection streaks with it. At 20 mm on the tripod, 1/2 s turns the cascade into a silky veil and 1/500 s freezes its droplets. The lake smooths more slowly, because its ripples move far more slowly than the falling water.

One more scene is on its way: a bee on a coneflower. Until it arrives, the scene picker marks it soon and you can't choose it.

Every scene starts from AI-generated photographs, and none of them shows a real person. The Roadmap page shows how each scene is coming along.

11Troubleshooting

12Version history