ISO and what it means
How your Nikon’s sensitivity to light is measured, and when you should alter the ISO setting
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