Reciprocity Failure Calculator

At long shutter speeds (for most films, above about 1 second), film becomes progressively less sensitive to light. The reciprocal relationship between aperture and shutter speed breaks down, so you need a longer exposure than the meter indicates. This is called reciprocity failure or the Schwarzschild effect. Below the onset time the meter reading is correct as-is, and the correction only ever adds exposure — it never shortens it.

No. Digital sensors do not suffer from reciprocity failure. This calculator is specifically for film photography. Digital sensors have their own long-exposure issues (sensor heating, noise) but these are different from reciprocity failure.

Ilford films use the published Schwarzschild exponents from Ilford’s Film Reciprocity Failure Compensation document (Tc = Tm^P). Kodak black-and-white films use Kodak’s published stop adjustments at 1, 10 and 100 seconds (e.g. Tri-X 400 needs +1, +2 and +3 stops), interpolated between those points. Fuji Acros 100 II follows its datasheet: no correction to 120 s, then +1/2 stop. The colour-negative figures are approximate fits, since Kodak publishes colour reciprocity as filter-pack data.

Fuji Acros 100 II is widely regarded as having the best reciprocity characteristics of any currently available film, requiring no correction up to 120 seconds (2 minutes) and only +1/2 stop to 1000 seconds. Kodak T-Max films also perform well thanks to their tabular grain technology.

For B&W film, reduce development by 10-15% for exposures corrected by more than 2 stops to prevent highlights from blocking up. For colour negative film, long exposures can cause colour shifts that may need correction in printing or scanning.