Correcting long-exposure reciprocity failure on film
Last updated 2 June 2026
On long film exposures, reciprocity failure means the metered time is no longer enough, so the shadows come out thin and the frame looks underexposed. EditMe includes a reciprocity calculator in its darkroom companion: you pick your film, enter the metered seconds, and it returns the corrected exposure. It uses a per-film power law, corrected = metered raised to the film's p-factor, and only kicks in past one second, since shorter exposures need no correction. The built-in film table ships starting p-factors like HP5 Plus at 1.31, so you can correct a reading without a paper chart.
How the correction works
Below or at one second, the calculator leaves the time untouched, because reciprocity failure only matters on long exposures. Past one second it raises the metered seconds to the film's p-factor, a single exponent that captures how that emulsion falls off. A higher p-factor means a steeper correction.
So a film with a p-factor of 1.31 takes a 10-second meter reading to about 20 seconds, while one second stays one second. You read the scene as usual, then let the calculator stretch the time for you.
- Pick your film from the list.
- Enter the metered exposure in seconds.
- Read the corrected time and use that on the camera.
The built-in film table
EditMe ships starting p-factors for a set of common black-and-white films, plus a generic fallback for anything not listed. These are sensible starting values, not lab-calibrated for your exact development, so treat them as a base and adjust to taste.
- HP5 Plus 1.31, FP4 Plus 1.26, Pan F Plus 1.33.
- Delta 100 1.26, Delta 400 1.41, Delta 3200 1.33.
- Tri-X 400 1.54.
- T-Max 100 and T-Max 400 at 1.0, and color Portra 400 (C-41) at 1.0, since those need little or no correction at these times.
- A generic Other entry at 1.3 for films that are not in the list.
The reciprocity calculator sits alongside the rest of the darkroom companion: f-stop print timing, temperature compensation, push and pull, and a dilution mixer.
Free editor, Pro darkroom
The photo editor and film-negative inversion in EditMe are free. The analog darkroom companion, including this reciprocity calculator, is part of the optional EditMe Pro subscription. Everything runs on the device, with no cloud and no account, and works on iPhone and iPad.
How it compares
| Factor | EditMe | By hand |
|---|---|---|
| Per-film p-factor | Built-in table | Paper chart |
| Correction past 1 s | Automatic | Manual math |
| HP5 Plus starting value | 1.31 | Look it up |
| Color and T-Max (low/no need) | Set to 1.0 | Remember to skip |
| Runs on the device | Yes | n/a |
Frequently asked questions
What is reciprocity failure?
On long exposures, film stops responding to light in a straight line, so the metered time underexposes the frame. You have to extend the exposure to compensate, which is what the calculator does.
How does EditMe correct it?
It raises your metered seconds to the film's p-factor, a single per-film exponent. The correction only applies past one second, since shorter exposures need none.
Which films are included?
A table of common black-and-white films with starting p-factors, including HP5 Plus at 1.31, plus T-Max and a color stock set to 1.0 where little correction is needed, and a generic fallback for anything not listed.
Are the p-factors exact for my development?
They are sensible starting values, not lab-calibrated for your exact film and chemistry. Use them as a base and fine-tune from your own results.
Is the reciprocity calculator free?
The EditMe photo editor and film inversion are free. The darkroom companion, including this calculator, is part of the optional EditMe Pro subscription.
Does it work offline?
Yes. EditMe runs entirely on the device, with no cloud and no account, on iPhone and iPad.