While shutter speed and aperture control the exposure, i.e. how much light reaches the sensor, the ISO setting controls how the light received by the sensor translates to (numerical) brightness. This means that shutter speed, aperture and ISO all have an influence on the brightness of an image.
As with shutter speed and aperture, double the ISO value to get one stop brighter. For example, all these settings will result in the same brightness:
- 1/125 s, f/8, ISO 100
- 1/250 s, f/8, ISO 200
- 1/125 s, f/5.6, ISO 50.