What is a "stop" of light in photography?
A stop is any doubling or halving of the light reaching the sensor. It's the one unit aperture, shutter speed and ISO all share, which is exactly why a change in any one of them can be exactly offset by an opposite change in either of the other two — the entire premise behind exposure equivalence.
How Each Variable Expresses a Stop Differently
Shutter speed is the most intuitive: halving the exposure time (1/125 to 1/250) halves the light, one full stop. ISO doubles or halves its numeric value for each stop (100, 200, 400, 800...), directly matching how much the signal is amplified. Aperture is the odd one out — its full-stop sequence (f/1.4 through f/16) doesn't look evenly spaced at all, because the f-number tracks a circle's diameter while light passing through tracks its area.
For exactly why that sequence is spaced the way it is, see f-stop.
Why 'Stop' Also Describes Non-Exposure Light Changes
The same unit shows up outside the core exposure triangle too — an ND filter is rated in stops of light reduction (an ND1000 filter blocks 10 stops), and flash power is typically adjusted in stops as well. All of these describe the same underlying doubling-or-halving relationship, which is why a photographer fluent in stops can reason about aperture, shutter speed, ISO, filters and flash power using one shared mental framework instead of five separate scales.
See stops in action, converting between any two of aperture, shutter speed and ISO, in the Exposure Triangle Solver.
Frequently Asked Questions
Why doesn't the aperture stop sequence look evenly spaced like shutter speed and ISO?
Because the f-number is a ratio of focal length to the aperture's physical diameter, and light gathering depends on the opening's area, not its diameter — doubling area (one stop) needs the diameter to grow by the square root of 2, which produces the specific, non-round f/1.4, f/2, f/2.8 sequence rather than simple doubling.
Is a 'third of a stop' a real, meaningful unit?
Yes — most modern cameras adjust exposure in 1/3-stop increments for finer control than full stops allow, and the same doubling/halving logic still applies, just calculated to a smaller fractional exponent of 2 rather than a whole number.