What is ISO?
The camera’s ISO setting is its sensitivity to light. The higher the ISO, the more sensitive it is. This is measured according to international standards, so ISO100 on one camera will be exactly the same as ISO100 on another.
Each ISO setting is double the one before: if you increase the ISO from 100 to 200, you double the camera’s sensitivity; and if you increase it from 200 to 400, you double it again. This carries on through the ISO scale.
This is deliberate. The ISO settings are designed to double (or halve) the exposure in the same way that the lens aperture settings and shutter speed settings are, and this is why the lens aperture, shutter speed and ISO are often described as the ‘exposure triangle’.
For example, if you want to use a faster shutter speed without changing the aperture, you could increase the ISO instead.
This relationship between lens aperture, shutter speed and ISO could quickly get complicated, but there are drawbacks to changing the ISO which mean that in practice you tend to change the ISO only when you have to.
When you increase the ISO setting, you’re not really making it more sensitive to light, you’re simply amplifying the light values it’s managed to capture.
The problem with this is that all digital images have some background noise. Usually, you don’t see it because it’s faint compared to the light falling on the sensor, but when you increase the ISO, you amplify it, and it shows up as a kind of random speckling. The higher the ISO, the worse the noise.
The ‘signal-to-noise’ ratio is one of the things we measure in our camera tests. Low ISOs offer a high signal-to-noise ratio (lots of signal, not much noise), but higher ISO settings bring a lower signal-to-noise ratio, which means that this random noise is making up a larger part of the picture.