ISO
The standardised measure of how strongly a camera amplifies the signal it captures, determining image brightness for a given exposure.
Formula / rule
ISO ∝ 1 / H_required
Measured in
ISO (arithmetic scale)
In depth
On film, ISO described the emulsion's inherent sensitivity to light. On a digital sensor it describes gain applied to the signal after the photons have been counted, which is why raising ISO amplifies noise along with signal rather than gathering more light. Doubling the ISO number always equals one stop of brightening.
Examples in practice
ISO 100 in daylight, ISO 3200 in a dim church, ISO 12800 at a night football match — each step buying shutter speed at the cost of visible noise.