What is reciprocity failure?

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.

Does digital suffer from reciprocity failure?

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).

How is the corrected time calculated?

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.

When does reciprocity failure become significant?

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.

Why are the colour-film figures marked "approximate"?

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.

PhotoCalcs

Reciprocity Calculator — Film Long Exposure Correction

Calculate corrected exposure times for reciprocity failure in long-exposure film photography.

Metered Exposure Time

The shutter time your light meter indicates, in seconds (e.g. 10 for a 10s reading)

Film StockEach film has its own Schwarzschild exponent. B&W values are from published data; colour stocks are approximate

How We Calculate This

Reciprocity failure correction uses the Schwarzschild equation:

Corrected time = Metered time ^ p

Where p is the reciprocity exponent specific to each film stock. A higher p value means more correction is needed. The exponent is determined experimentally and published by the film manufacturer.

For example, with Ilford HP5 Plus (p=1.31) and a metered time of 10 seconds: corrected = 10^1.31 = 20.4 seconds. The same 10s reading on Kodak Tri-X 400 (p=1.54) needs 10^1.54 = 34.7 seconds — Tri-X falls off much faster.

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.