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.
Why reciprocity fails at long exposures
At normal exposure durations (roughly 1/1000 s to 1 s) the reciprocity law holds: halving the light and doubling the exposure produces the same density. But film is a collection of individual silver-halide grains, each of which needs a minimum number of photons in a short window to form a latent image. At very low light levels (long exposures), photons arrive so infrequently that some grains begin to revert before receiving enough to form a stable image — the Gurney-Mott intermittency effect. The practical result is that the film loses effective speed and you need a longer-than-expected exposure to reach the same density.
How the corrections are calculated
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 examples across different stocks
Ilford HP5 Plus (p ≈ 1.31), metered 10 s:
- Corrected: 10^1.31 ≈ 20 seconds (+1 stop)
Ilford Delta 100 (p ≈ 1.26), metered 10 s:
- Corrected: 10^1.26 ≈ 18 seconds (slightly less failure than HP5)
Fuji Acros II, metered 10 s:
- Almost no correction needed below about 2 minutes — one of the strongest reciprocity characteristics of any black-and-white film.
Ilford HP5 Plus, metered 60 s:
- Corrected: 60^1.31 ≈ ~160 seconds — the failure compounds dramatically at longer durations.
This compounding is the key trap: a small correction at 10 seconds becomes a very large one at 60 seconds. When exploring new locations at night, it is worth pre-calculating the corrected time for both your expected and worst-case meter reading.
Film-by-film characteristics
| Film | Reciprocity model | Threshold | Notes |
|---|---|---|---|
| Ilford HP5 Plus | Power law, p ≈ 1.31 | ~1 s | Very popular for night work |
| Ilford Delta 100 | Power law, p ≈ 1.26 | ~1 s | Slightly better than HP5 |
| Ilford FP4 Plus | Power law, p ≈ 1.26 | ~1 s | Similar to Delta 100 |
| Fuji Acros II | Table (published) | ~120 s | Outstanding reciprocity |
| Kodak T-Max 100 | Table (published) | ~1 s | Good for architectural night |
| Kodak Tri-X 400 | Table (published) | ~1 s | Classic, correction needed early |
| Kodak Portra 400 | Table (published) | ~1 s | Colour shift possible beyond ~10 s |
Handling colour reciprocity failure
On colour films, each emulsion layer (cyan, magenta, yellow) fails at a slightly different rate, which produces colour casts at long exposures — typically a warm or cool shift depending on the stock. For long-exposure colour work, bracketing exposures and scanning or printing with colour correction is standard practice. Portra handles long exposures better than most colour negative films but is not immune.
Development adjustments for B&W
Long exposures can increase shadow contrast on black-and-white film, an effect sometimes called reciprocity-failure-induced contrast increase. Photographers doing controlled long-exposure work (landscapes, architecture) sometimes reduce development by 10–15% to bring contrast back in line. Test with your specific film, developer, and dilution before committing a final shoot.