Base ISO
The sensitivity at which a sensor is read out with no amplification, delivering its maximum dynamic range and cleanest files.
Measured in
ISO
In depth
Base ISO is usually 100 or 64 on modern cameras, and it is the setting at which the full-well capacity of each photosite is used most efficiently. Every stop above base ISO trades highlight headroom for shadow brightness; every 'extended low' setting below it simply overexposes and pulls back, clipping highlights sooner.
Examples in practice
A landscape photographer on a tripod stays at the camera's base ISO 64 to keep the full fourteen stops of dynamic range for a high-contrast sunrise.