Analogue Gain
Amplification applied to the sensor's signal before it is digitised, which is the physical mechanism behind ISO settings.
Measured in
dB or ×
In depth
Applying gain before the analogue-to-digital converter amplifies the signal above the converter's own noise contribution, which is why raising ISO in camera can produce a cleaner file than brightening in software. The photon shot noise already present in the signal is amplified along with it, so gain never adds real sensitivity.
Examples in practice
Shooting at ISO 1600 rather than ISO 100 and lifting four stops in software gives visibly cleaner shadows on most cameras.