Reciprocity Calculator — Film Long Exposure Correction

Reciprocity failure (also called the Schwarzschild effect) occurs when film loses sensitivity during long exposures. Below about 1 second, the relationship between exposure time and film response is no longer linear — you need more time than the meter indicates.

No. Digital sensors respond linearly regardless of exposure time. However, digital sensors accumulate thermal noise on long exposures, which is a different issue typically handled by long exposure noise reduction (dark frame subtraction).

The corrected time = metered time ^ p, where p is the Schwarzschild exponent for the specific film stock. Values of p range from about 1.15 (T-Max 100, minimal reciprocity failure) to 1.54 (Tri-X 400, significant correction needed) for black & white films, with some stocks documented as high as 1.8.

For most films, correction becomes necessary for exposures longer than 1 second. At 10 seconds, a film with p=1.3 needs about 20 seconds actual exposure. At 60 seconds metered, it could need 3-5 minutes.

Colour-negative films such as Portra 400 and Ektar 100 do not follow a single, clean power law. Kodak compensates them with extra stops plus colour-correction filtration that changes with exposure length, so a one-number exponent only gives a rough corrected time. The black & white exponents (Ilford and Kodak) are taken from published reciprocity data and are reliable. For critical colour work, bracket your exposures.