While shutter speed and aperture settings control the exposure, i.e. how much light reaches the sensor, the ISO setting controls what appears to be the sensitivity of the sensor. With a higher ISO, the sensor becomes more sensitive, and the image gets lighter. However, from a technical point of view, things are a little different.
A modern CMOS image sensor of a digital camera consists of an array of millions of tiny pixels, each containing a light-sensitive element (a photodiode). When the sensor is exposed to light, a photon that hits the element causes an electron to be collected in an associated bin. The number of electrons in the bin (typically tens of thousands) is proportional to a voltage. This voltage is amplified, measured, and converted to a numerical intensity value using an analog to digital converter (ADC). The intensity value is stored in the image file.
The relation between the number of collected electrons and the resulting intensity value of a pixel is not fixed, but can be adjusted with the ISO setting. Basically, this can be implemented as a change in the analog electrical amplification (gain), as a digital multiplication, or as a combination of these.
Shutter speed, aperture and ISO all have an influence on the intensity of the image. As with shutter speed and aperture, double the ISO value to get one stop lighter. For example, all these settings will result in the same lightness of the image:
- 1/125 s, f/2.8, ISO 100
- 1/250 s, f/2.8, ISO 200
- 1/125 s, f/2, ISO 50