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This series has already covered two aspects of the exposure triangle: aperture and shutter speed. In my latest tip I’m going to address the final part – ISO – which will complete your knowledge of the reciprocal relationship between all three.

ISO tends to draw differing explanations of exactly what it does – most of which boils down to technicalities – so I’ll stick with what I believe is the clearest explanation.

What does ISO mean?

ISO is a short-form for the International Organisation of Standardization and, in relation to photography, historically referred to the standard ratings for film sensitivity. The term has since carried over into the digital age.

How is it measured?

Typical ISO ratings are: 100; 200; 400; 800; 1600; 3200; and 6400. Many cameras will go even higher (and some slightly lower).

What does it do?

Increasing the ISO increases the brightness of the output image. Each rating can be treated like a stop – just like with aperture and shutter speed – so an image at ISO 200 will be twice as bright than ISO 100.

It is correct that ISO is part of the exposure triangle but it is important to remember that it doesn’t influence the exposure itself; the amount of light entering the camera is controlled by the aperture and shutter speed. ISO instead influences how bright the output image will be.

When should I change it?

The most common need for increasing the ISO is in low light situations. If you are faced with a scene where light is limited, e.g. in the evening, and your shutter speed is too slow even with the aperture wide open, increase the ISO. Alternatively if you are shooting indoor sport and you need to keep the shutter speed at 1/500s but there is not enough light to allow this, increase the ISO.

Any drawbacks?

Using a higher ISO will introduce noise – a distracting digital fuzziness that also softens the image. Modern digital cameras are excellent at controlling noise and you should be able to shoot comfortably around ISO 1600 before you notice any serious image degradation.

Can my camera handle ISO?

If you’re not quite ready to dive in then use Auto-ISO. This is a mode whereby you can set the minimum / slowest shutter speed you want and, when conditions mean that the exposure value falls below that threshold, the camera will automatically increase the ISO until it is achievable.

Is there one golden rule?

Yes! When all is said and done you should always try to use the lowest ISO possible to achieve the aperture and / or shutter speed you want.

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