In Aperture Priority, Shutter Priority, Programme and Manual modes you can set the ISO yourself. Using a lower ISO gives you the highest quality images, whereas using a very high ISO setting results in some loss of image quality mainly due to noise.

Along with aperture and shutter speed, ISO is one corner of the exposure triangle.

If you select a lower ISO, you will have to select a larger aperture and / or slower shutter speed because the sensor is less sensitive to light.

Selecting a higher ISO means that the your will be able to use a faster shutter speed and / or a smaller aperture.

Here are some examples of when you might use certain ISO settings:

ISO 100 – on a bright sunny day when image quality is very important and your are photographing subjects that are not moving very fast

ISO 400 – for handheld shots on a cloudy day

ISO 800 – inside with artificial light

ISO 1600 – to freeze fast action with fast shutter speed on a cloudy day

ISO 3200 – in a dimly lit bar when you don’t want to use flash

As you can see, generally lower ISO is best used in bright conditions and high ISO when it is darker.

Using too low ISO in dark conditions will mean that you’re shutter speed will be so slow that it will cause camera shake. This should be avoided.