What are the key things to know about ISO?

Your camera’s ‘sensitivity’ control can allow you to keep shooting when the lights dim – but it’s actually much more versatile than you might think…

There are three key camera controls that determine the exposure of a picture: aperture, shutter speed and ISO. The size of the aperture (the hole in the lens) dictates how much light enters the camera, with the shutter speed regulating how long the imaging sensor is exposed to that light. But it’s the ISO that determines how much light is required to make an exposure.

ISO essentially controls the sensor’s sensitivity to light. I say ‘essentially’ because technically it doesn’t really make the sensor more or less sensitive: the light falling on it is converted into an electrical signal, and the ISO setting determines the degree to which this signal is amplified.

The camera will set an appropriate sensitivity when you’re using an automatic shooting mode, but once you move to more advanced modes such as Aperture Priority and Manual, you can opt to continue using Auto ISO or set the sensitivity yourself. Digital cameras typically have an ISO range of 100 to 6,400, with lower numbers making the sensor less sensitive and higher numbers increasing the sensitivity. The more sensitive a sensor is to light, the less of it you need in order to record an image, so you can carry on taking pictures when there’s not much of it available.

As a rule, you should keep the ISO as low as