Noise Floor
The level of background noise below which no real signal can be distinguished.
Measured in
e⁻ (electrons)
In depth
The noise floor is set principally by read noise and, in long exposures, by dark current. It defines the darkest tone a sensor can meaningfully record, and therefore the bottom end of its dynamic range. Pushing shadows in processing amplifies signal and floor together, which is why underexposed files reveal noise rather than detail.
Examples in practice
Shadow detail sitting one electron above a two-electron noise floor cannot be recovered no matter how far it is lifted.