What is reciprocity failure?

The reciprocity law says halving the light and doubling the exposure give the same result, but at very long exposures film loses sensitivity and this breaks down. The film needs much more time than the meter indicates, an effect called reciprocity failure, which gets stronger the longer the exposure.

How is the corrected exposure calculated?

For Ilford films the correction follows a power law, corrected = metered raised to a film-specific exponent of roughly 1.3, with the time in seconds. Other stocks like Kodak T-Max, Portra, and Fuji Acros use published lookup tables, which this tool interpolates between to match the datasheet.

At what exposure length does reciprocity failure start?

For most films the effect is negligible below about one second and only becomes significant beyond a few seconds. Fuji Acros II is exceptional, showing almost no failure out to around two minutes, which is why it is a favourite for long-exposure work.

Does reciprocity failure affect colour film differently?

Yes. Colour films suffer reciprocity failure in each emulsion layer at a slightly different rate, so very long exposures can produce colour casts as well as underexposure. Bracket important frames and be prepared to correct colour balance, since a single time correction cannot fix a cross-layer shift.

Do I need to change development for long exposures?

Sometimes. Long exposures can raise contrast on black-and-white film, so some photographers reduce development time slightly to compensate alongside the reciprocity time correction. Always test with your own film, developer, and metering before relying on it for important shots.

Film Reciprocity Failure Calculator

When you meter a dim scene and the indicated exposure runs into seconds, film no longer responds in a straight line: it loses sensitivity and needs noticeably more time than the meter suggests. This calculator applies each film’s published reciprocity correction so your long exposures come out properly exposed.

How it works

For Ilford’s films the manufacturer publishes a simple power-law model, with the metered time in seconds:

corrected = metered ^ p

where p is a film-specific exponent (typically around 1.3). So a metered 10 seconds on a stock with p = 1.31 becomes 10^1.31 ≈ 20 seconds.

Some stocks — Kodak T-Max, Portra, and Fuji Acros — are better described by published lookup tables, so this tool interpolates between the datasheet points instead of using a single exponent. Below each film’s threshold (about one second for most, but around two minutes for Acros) no correction is applied.

Worked example

Shooting Ilford HP5 Plus with a metered exposure of 10 seconds:

corrected = 10^1.31 ≈ 20.4 seconds
extra time added ≈ 10.4 seconds
compensation ≈ +1.03 stops

So a meter reading of 10 seconds really needs about 20 seconds on the film.

Tips and notes

Reciprocity failure grows fast: a 1-second error at 10 seconds can become tens of seconds at 100.
Colour films can shift colour balance at long exposures because each layer fails at a different rate.
Long black-and-white exposures may also raise contrast, so consider a small development reduction.
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