Lesson 1 of 5 · 12 min · Free
The exposure triangle: aperture, shutter speed and ISO explained
The exposure triangle explained simply: how aperture, shutter speed and ISO control light, how stops and reciprocity work, plus the sunny 16 rule.
Every photo is a measured amount of light landing on a sensor. The exposure triangle is the name for the three settings that control that amount: aperture, shutter speed and ISO. Once you see how they trade against each other, your camera stops feeling like a mystery box.
The idea that ties them together is the stop. One stop means twice as much light, or half as much. All three settings can be measured in stops, and that is what lets you swap one for another.
Aperture: how wide the lens opens
Aperture is the size of the opening inside the lens. It is written as an f-number, like f/2.8 or f/8. Here is the tricky part: a smaller number means a bigger opening and more light. f/2 lets in far more light than f/16.
The full-stop sequence is f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. Each step lets in half the light of the one before it. Going from f/4 to f/5.6 costs you one stop. Going from f/4 to f/8 costs two.
Aperture also controls depth of field, which is how much of the scene looks sharp from front to back. At f/1.8, a portrait has a crisp face and a soft, blurry background. At f/11, a landscape can be sharp from the rocks at your feet to the hills on the horizon.
Shutter speed: how long the sensor sees light
Shutter speed is how long the shutter stays open, written in seconds or fractions of a second: 1/1000s, 1/250s, 1/30s, 2s. Doubling the time doubles the light, so moving from 1/125s to 1/60s is one stop brighter.
Shutter speed controls motion. Fast speeds freeze it and slow speeds blur it.
- 1/1000s freezes a running dog or a splash of water.
- 1/250s handles kids playing and people walking.
- 1/60s works for still subjects if your hands are steady and the lens is short.
- 1/2s or longer, on a tripod, turns a waterfall into smooth silk.
To avoid blur from your own hands, a good rule of thumb is to keep the shutter speed at least 1 over your focal length. With a 50mm lens, shoot at 1/50s or faster. With a 200mm lens, 1/200s or faster. Image stabilization lets you go slower on still subjects, but it can't freeze a subject that is moving.
ISO: how much the signal is amplified
ISO sets how strongly the camera boosts the signal from the sensor. ISO 100 is the base setting on many cameras. Each doubling (200, 400, 800, 1600, 3200) adds one stop of brightness.
The cost is noise, the grainy speckle you see in dim indoor shots. Recent cameras often look clean at ISO 1600 and beyond, but lower is always cleaner. Treat ISO as the setting you raise when aperture and shutter speed can't give you any more.
How the three trade off
Because every setting moves in stops, you can take one stop away from one setting, give one stop to another, and keep the same brightness. This is called reciprocity. All of these produce the same exposure:
- f/8 at 1/125s, ISO 200
- f/5.6 at 1/250s, ISO 200
- f/4 at 1/500s, ISO 200
- f/8 at 1/250s, ISO 400
The brightness is identical. What changes is the look: depth of field, motion and noise. Choosing an exposure means choosing which of those matters most.
A worked example
You are photographing a birthday party indoors. The camera suggests f/5.6 at 1/30s, ISO 400. The brightness is fine, but at 1/30s a running child will blur. You want 1/250s, which is three stops faster (1/30 to 1/60 to 1/125 to 1/250). Open the aperture from f/5.6 to f/2.8 for two stops, then raise ISO from 400 to 800 for the third. Your new setting is f/2.8 at 1/250s, ISO 800. Same brightness, sharp kid.
The sunny 16 rule
A classic check that needs no light meter: on a bright day with crisp, well-defined shadows, set the aperture to f/16 and the shutter speed to 1 over your ISO. At ISO 100, that is f/16 at 1/100s, or 1/125s if that is the closest speed your camera offers. From there, use reciprocity: f/8 at 1/500s and f/5.6 at 1/1000s give the same exposure. On an overcast day, open up a stop or two.
Which camera mode to use
You don't have to set all three by hand. Most cameras offer:
- Aperture priority (A or Av): you choose the f-stop and the camera chooses the shutter speed. Good for portraits and landscapes.
- Shutter priority (S or Tv): you choose the speed and the camera chooses the aperture. Good for sports and pets.
- Manual (M): you choose everything. Pair it with auto ISO for an easy middle ground.
If a shot comes out a little too bright or dark, use exposure compensation, the plus and minus control, to nudge it by a third of a stop or more.
What about phones?
Most phone cameras have a fixed aperture, so the phone balances exposure with shutter speed and ISO alone. The background blur in portrait mode is created in software rather than by a wide aperture. You still have control over brightness: tap your subject, then drag the exposure slider up or down. Lesson 4 covers phones in detail.
Practice
Set your camera to aperture priority (or use a phone with an exposure slider) and photograph the same still object, like a mug on a table near a window, five times: at f/2.8, f/4, f/5.6, f/8 and f/11, keeping ISO fixed at 400. Write down the shutter speed the camera picks for each. You should see it roughly double with every stop you close down. Then compare the background blur across the five shots.