What is reciprocity failure?

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.

Does digital have reciprocity failure?

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.

Where do the correction values come from?

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.

Which films have the best reciprocity characteristics?

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.

Do I need to adjust development for reciprocity?

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.

Reciprocity Failure Calculator

Calculate corrected exposure time for film reciprocity failure at long exposures.

Metered Exposure Time

The shutter speed your light meter indicates (in the unit selected). Correction only kicks in above the film's onset time — about 1s for most films, 120s for Acros.

Time UnitUnit for the metered time above — e.g. a 30s meter reading is 30 seconds, a 1/2 minute exposure is 0.5 minutes

Film StockPicks the manufacturer reciprocity data: Ilford uses published Schwarzschild exponents, Kodak B&W uses published stop adjustments, Fuji Acros uses its datasheet step.

How We Calculate This

Reciprocity failure only begins above each film’s onset time — about 1 second for most films, and 120 seconds for Fuji Acros. Below that onset the corrected time equals the metered time: no extra exposure is added, and the calculator never shortens an exposure (reciprocity failure can only ever lengthen it).

For Ilford films the corrected time uses the Schwarzschild law published in Ilford’s “Film Reciprocity Failure Compensation” document: T_corrected = T_metered ^ P, where P is the film’s exponent (HP5+ 1.31, FP4+ and Delta 100 1.26, Delta 3200 and Pan F+ 1.33). For example, 10 s metered on HP5+ becomes 10 ^ 1.31 = 20.4 s.

Kodak does not publish a power exponent — it publishes discrete stop adjustments at 1, 10 and 100 seconds (Tri-X 400: +1 / +2 / +3 stops; T-Max 100: +1/3 / +1/2 / +1; T-Max 400: 0 / +2/3 / +2). This calculator interpolates those published points in log-time space. The colour-negative figures (Portra, Ektar) are approximate fits because Kodak quotes colour reciprocity as filter-pack data rather than a stop curve.

Fuji Acros 100 II needs no correction up to 120 seconds, then a flat +1/2 stop from 120 to 1000 seconds, per the Fujifilm datasheet.

Frequently Asked Questions

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Last updated: March 2026

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