The exposure triangle
Three controls decide how bright a photograph is, and each one changes something else as well. That trade-off is the whole of exposure.
Three controls decide how bright a photograph is. Any of them will brighten or darken it, and each one also changes something no other control can.
That second effect is why the choice matters. If brightness were all that mattered, you could pick any of them at random.
Everything is measured in stops
A stop is a doubling or halving of light. It is the unit that makes the three controls interchangeable.
| Control | One stop more light |
|---|---|
| shutter speed | 1/500 → 1/250 |
| aperture | f/8 → f/5.6 |
| iso | ISO 200 → ISO 400 |
Because they are the same size, they trade evenly. Give up one stop on the shutter and take it back on the aperture and the brightness is identical.
Reciprocal exposures
Because a stop given up on one control can be taken back on another, many different combinations produce exactly the same brightness:
| Aperture | Shutter | ISO | Brightness |
|---|---|---|---|
| f/2.8 | 1/1000 | 100 | identical |
| f/4 | 1/500 | 100 | identical |
| f/5.6 | 1/250 | 100 | identical |
| f/5.6 | 1/1000 | 400 | identical |
These are not the same photograph. The first has a soft background and freezes movement. The third has more in focus. The fourth freezes movement and keeps the background, at the cost of some noise.
Choosing between them is photography. The exposure is just the constraint.
What each one costs
- Aperture buys light, and spends depth of field. Wider is brighter and shallower.
- Shutter speed buys light, and spends the ability to freeze movement. Slower is brighter and more smeared.
- ISO buys light, and spends cleanliness. Higher is brighter and noisier.
There is no free option. Every stop of brightness is bought with something.
An order that usually works
- Decide what the photograph depends on — frozen movement, or a soft background.
- Set that control first, and do not compromise on it.
- Set the other of the two to bring the exposure close.
- Use ISO to cover whatever is left.
ISO last, because it is the only one that does not change how much light actually arrives — it just amplifies what did.
Try it
Full manual
Everything is yours now. Freeze the bob, soften the backdrop, expose it correctly, and keep the sensitivity low enough that the picture stays clean.
Loads an interactive 3D camera simulator.
Key takeaways
- Aperture, shutter speed and ISO all change brightness, and nothing else does.
- All three are measured in stops, so they trade evenly against each other.
- Many combinations give the same brightness but different photographs.
- Set the control the photograph depends on first, and use ISO last.
Common questions
Why is it called a triangle?
Mostly habit. There is nothing geometric about it — it is just three controls that add up. What the name usefully conveys is that moving one means moving another to compensate.
Which one should I change first?
Whichever the photograph depends on. If movement matters, set the shutter first. If the background matters, set the aperture first. Use ISO last, to make up whatever is left.
Next steps
Shutter speed
How long the sensor is exposed decides whether movement is frozen or smeared. It is the most visible of the three controls.
Aperture and depth of field
The aperture controls light and how much of the scene is sharp at the same time. It is the control that most changes how a photograph feels.
ISO and noise
ISO buys brightness when there is not enough light, and pays for it with noise. Knowing when that trade is worth it is the skill.