How equivalent exposure settings work

Aperture, shutter and ISO trade against each other in stops. Change one and another must move to compensate.

Three controls determine exposure, and they are interchangeable in their effect on brightness while being completely different in every other respect. That is the whole point.

Step-by-step

1. Enter your current settings.
2. Change the one you care about.
3. Read what the others must become.

Stops are the common currency

A stop is a doubling or halving of light, and every control moves in stops.

• Shutter: 1/125 to 1/250 is one stop less.
• ISO: 100 to 200 is one stop more.
• Aperture: f/2.8 to f/4 is one stop less. The sequence is f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16 — each a factor of the square root of two, because area scales with the square of diameter.

Any stop given up on one control can be taken back on another.

What each one costs

Equivalent exposures are not equivalent photographs.

• Aperture controls depth of field. Opening up for light gives you less of the scene sharp.
• Shutter controls motion. Slower means blur — sometimes wanted, usually not.
• ISO controls noise. Higher means grain and less recoverable shadow detail.

Which to give up

Decide what the picture requires and treat that as fixed. If the depth of field is the point, set the aperture and let the others follow. If freezing motion is the point, set the shutter first.

ISO is the usual release valve, and modern cameras are far better at high ISO than the received wisdom suggests. A slightly noisy sharp photograph beats a clean blurred one every time.

Frequently asked questions

What is a stop?

A doubling or halving of light. All three controls move in stops, which is what makes them interchangeable in their effect on brightness.

Why is the aperture sequence so odd?

Because f-numbers describe diameter while light depends on area. Doubling the area means multiplying the diameter by the square root of two, giving 1.4, 2, 2.8, 4 and so on.

Which setting should I sacrifice?

Whichever matters least to the picture you want. Fix the control the image depends on and let the others adjust — usually ending with ISO, which modern sensors handle better than reputation suggests.