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The exposure triangle explained: aperture, shutter speed and ISO

Aperture, shutter speed and ISO all control how bright a photograph is, and each has a side effect. Exposure is not about finding "the right settings". There are dozens of right combinations. Choose the setting for its side effect, then balance the other two.

The three settings and their side effects

• Aperture: a wider opening (a smaller f-number) is brighter. Side effect: a shallower depth of field, so less of the scene is in focus and the background blurs.
• Shutter speed: a slower speed is brighter. Side effect: more motion blur, and below a certain speed, hand shake blurs everything.
• ISO: a higher ISO is brighter. Side effect: more noise, with a loss of fine detail and colour.

Stops

Photographers measure light in stops. One stop more is twice the light. One stop less is half.

• Shutter speeds, one stop apart: 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000.
• Apertures, one stop apart: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16.
• ISO, one stop apart: 100, 200, 400, 800, 1600, 3200.

A smaller f-number means a bigger opening and more light.

Trading one for another

Because each setting moves in stops, you can trade them and keep the same brightness. Open the aperture by one stop and make the shutter one stop faster, and the exposure is unchanged.

So pick the side effect you want. For a portrait with a blurred background, choose a wide aperture such as f/2.8 at the lowest ISO, and let the shutter speed follow.

How slow can you hand-hold?

The rule of thumb for a sharp hand-held picture is:

slowest safe shutter speed ≈ 1 ÷ focal length

With a 75 mm-equivalent lens, use 1/80 of a second or faster. Image stabilisation lets you go slower with a still subject, but it does nothing for a subject that moves.

Choosing ISO

• Use the lowest ISO that the light allows, usually 100 or 200 outdoors.
• Raise it when the aperture is as wide as it goes and the shutter is as slow as the subject allows.
• A noisy, sharp picture is better than a clean, blurred one.

Why the camera gets it wrong

The camera's meter assumes that every scene averages out to middle grey. For scenes that are mostly bright or mostly dark, that fails:

• a white wedding dress filling the frame comes out dull grey and too dark;
• a person against a bright sky has a face that is too dark;
• a black cat on a black sofa comes out grey and too bright.

Exposure compensation fixes this. It is the dial or button marked +/−.

• Beach, snow, white backgrounds or a white dress: +1 to +2.
• A backlit person against a window or the sky: +1 to +2.
• Dark backgrounds, or a spotlit performer on a dark stage: −1 to −2.

Decide what must be correct, usually the face, and expose for that.

Check the histogram

The screen looks bright in a dark room and dim in sunlight, so trust the histogram. A spike against the right-hand wall means highlights are clipped to pure white, and that detail is gone. Shadows can usually be brightened later, so when in doubt, protect the highlights.

Which mode?

• Aperture priority (A or Av): you set the aperture. Use it for portraits, products and landscapes.
• Shutter priority (S or Tv): you set the shutter speed. Use it for sport, children and events.
• Manual (M): you set both. Use it when the light is constant and every picture must match, as in a studio.
• Manual with Auto ISO: useful at events.

When the light is too harsh

A bright South African midday scene can hold more contrast than a sensor can record. Move the subject into shade, add a reflector, shoot early or late in the day, shoot RAW, or take several exposures and combine them.

This guide is based on Chapter 3 of Practical Photography for a Digital World.