Reciprocity in photographic exposure is the principle that exposure depends on the product of light intensity and time. Doubling the light or doubling the exposure time produces the same recorded brightness on the sensor or film. This relationship underpins the entire exposure triangle: equivalent exposures across different aperture and shutter speed combinations rely on reciprocity holding true.
On digital sensors, reciprocity holds well across the practical exposure range. A 1-second exposure at f/8 produces the same brightness as a 4-second exposure at f/16 (two stops of aperture closed, two stops of shutter compensation). Departures appear only at very long exposures, where thermal noise rises and read noise becomes proportionally significant, or at extremely short exposures with electronic shutter where banding from artificial light sources interacts with the rolling readout. For ordinary work, photographers can rely on reciprocity holding without compensation.
Film, by contrast, exhibits a phenomenon called reciprocity failure at very long or very short exposures, where the linear relationship breaks down and the film records less density than the simple intensity-times-time calculation would predict. The cause is rooted in the photochemistry: at low light levels, individual photons must combine with multiple silver halide grains to form a stable latent image, and at very short flash durations, the chemistry cannot keep pace with rapid arrival of photons. The film loses sensitivity, requiring additional exposure beyond what calculations suggest.
Each emulsion has its own reciprocity curve, published by the manufacturer. Kodak Portra and T-Max are relatively well-behaved at long exposures, while older films like Kodak Tri-X 400 require a one- to two-stop compensation for a 30-second metered exposure. Fujifilm Provia 100F holds reciprocity better than its slide-film peers. Ilford publishes detailed reciprocity correction tables for its black-and-white films. Long-exposure landscape and astrophotography on film demands consulting these tables and adding exposure accordingly; ignoring reciprocity failure produces images that look underexposed by a stop or more.
Color shifts compound the problem for color film. Different layers in a color emulsion (cyan, magenta, yellow) fail reciprocity at different rates, so a long exposure on color film often shifts hue noticeably, typically toward green or blue. Filtration corrections are published alongside exposure corrections; many landscape film photographers carry a small notebook or a phone app with reciprocity tables for their preferred emulsions.
The practical workflow on film for long exposures is: meter the scene, calculate the indicated exposure, then apply the reciprocity correction from the data sheet. A 4-second indicated exposure on a film with a stated reciprocity failure of 1 stop at 4 seconds becomes an 8-second actual exposure. Repeat the calculation for any time well beyond 1 second. Digital cameras simplify the problem entirely, which is one of the small luxuries that justifies the shift from film for many long-exposure specialists.