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 1.33 (Tri-X, significant correction needed).

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.

PhotoCalcs

Reciprocity Calculator — Film Long Exposure Correction

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

Metered Exposure Time

The time your light meter indicates

Film Stock

Different films have different reciprocity characteristics

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 by the film manufacturer.

For example, with Ilford HP5 (p=1.31) and a metered time of 10 seconds: corrected = 10^1.31 = 20.4 seconds.

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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.