Stop
The fundamental unit of exposure change: one stop doubles or halves the amount of light reaching the sensor.
Formula / rule
1 stop = ×2 or ÷2 light
Measured in
stops (EV)
In depth
The stop is what lets three unrelated controls speak the same language. One stop is a full f-number step, a doubling of shutter duration, or a doubling of ISO, and any of them can substitute for any other. It is also the unit used for filter density, flash power, dynamic range and exposure compensation.
Examples in practice
Adding a three-stop ND filter means you can drop from 1/500 s to 1/60 s while keeping the identical aperture and ISO.