A stop is the fundamental unit of exposure in photography, representing a doubling or halving of the light reaching the sensor. One stop brighter means twice as much light; one stop darker means half. The term comes from the metal slides with apertures of fixed size, called stops, that were dropped into early lens barrels to control light. The concept underlies everything in exposure work, because aperture, shutter speed, and ISO all move in stops or fractions of stops, which is precisely what makes the exposure triangle work as a coherent system.
Aperture stops follow a geometric series because the area of the aperture doubles or halves between values. The full stop sequence runs f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, with each step being a factor of the square root of two in f-stop number. Shutter speed stops are simpler doublings: 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000. ISO stops likewise double: 100, 200, 400, 800, 1600. Most cameras let the photographer adjust in third-stop or half-stop increments for finer control.
Stops are also the language of light measurement and image quality. Dynamic range is specified in stops, with modern full-frame sensors capturing roughly 13 to 15 stops between just-readable shadow noise and clipped highlights. A neutral density filter is rated in stops of light reduction, a 6-stop ND being a common choice for daylight long exposures. Exposure compensation is dialed in stops or fractions, and bracketing sequences are usually defined as plus and minus some stop interval.
The relationship between stops and perceived brightness is logarithmic, which matches how human vision actually responds to light. Doubling the photon count from a dim shadow lifts it only one stop, which the eye perceives as a modest change, but doubling a bright highlight produces visually overwhelming light. This is why photographers learn to think in stops rather than percentages. A two-stop adjustment in shadow recovery is much more meaningful than a percent-based slider would suggest.
Equivalent exposure follows directly from the stop concept. Opening one stop on aperture and shortening shutter by one stop keeps overall exposure unchanged while altering depth of field and motion rendering. ISO can absorb a stop in either direction to free aperture and shutter. The exposure triangle is taught precisely because three variables in stop units sum to a single exposure value, and a working photographer needs to be able to trade among them instantly.
Common confusions include treating one stop and one EV as different (they are identical for a fixed scene luminance), and forgetting that the f-stop sequence is geometric rather than arithmetic, leading beginners to think f/2 is only slightly faster than f/2.8 when it is actually one full stop, or twice the light.