How do aperture, shutter speed and ISO work together?

The three controls are usually taught as a way of getting a correctly bright photograph. That is the least interesting thing they do. Each one changes the picture in a way that has nothing to do with brightness, and that is what you actually choose between.

Step by step

1. Understand what aperture really does.It is the size of the hole in the lens. A wide aperture — a small f-number like f/1.8 — lets in a lot of light and gives a shallow depth of field, so the subject is sharp and the background is soft. A narrow aperture like f/11 keeps more of the scene sharp.
2. Understand what shutter speed really does.It is how long the sensor is exposed. Fast speeds like 1/1000 freeze movement. Slow speeds like 1/15 let in far more light but blur anything that moves, including your own hands.
3. Understand what ISO really does.It amplifies the signal from the sensor. Low ISO is clean, high ISO is brighter in dim light but grainier and less detailed. It is best thought of as the last resort of the three.
4. See them as a trade, not a formula.Take light away with one and you must give it back with another to keep the same brightness. There is no single correct combination, only different side effects.
5. Choose which side effect you want first.Blurred background for a portrait means a wide aperture. Frozen action means a fast shutter. Everything else follows from that decision.
6. Use aperture priority for most situations.Marked A or Av. You set the aperture, the camera sets the shutter speed. It is what most experienced photographers use most of the time.
7. Use shutter priority when movement is the point.Marked S or Tv. You set the shutter speed and the camera works out the aperture. Right for sport, children, pets and moving water.
8. Set a minimum shutter speed you can hand-hold.A rough rule is one over the focal length — 1/50 second with a 50mm lens. Stabilisation buys a few stops, but people consistently overestimate how steady they are.
9. Let ISO float within a limit.Auto ISO with a maximum you are happy with, often 3200 or 6400 on a modern camera, keeps the other two controls where you want them. Slight grain is far better than blur.
10. Learn what a stop means.A stop is a doubling or halving of light. Every control moves in stops, so one stop less light from the aperture can be replaced by one stop more from shutter or ISO.
11. Practise deliberately.Photograph the same subject at f/1.8 and f/11, then at 1/1000 and 1/30. Seeing the difference on your own pictures teaches this far faster than reading about it.

Tips

• Choose the side effect you want, then balance the other two. That is the whole skill.
• Aperture priority handles most photography. Shutter priority when movement matters.
• Grain is recoverable, blur is not. Raise the ISO rather than dropping to an unsteady shutter speed.

Common mistakes

• Treating the three as only a brightness control — Any of them can make the picture bright enough, but only one of them gives the depth of field or motion effect you want. Choosing on brightness alone means the picture is a matter of luck.
• Refusing to raise ISO — Fear of grain leads to shutter speeds too slow to hand hold, and the result is a blurred picture. A slightly grainy sharp photo is usable; a clean blurred one is not.

If it doesn't work

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