Reciprocity Failure Calculator｜Corrected Shutter Time for Long Exposures
Enter your metered seconds and your film's reciprocity exponent p to get the corrected shutter time and the extra stops you need for long exposures. Anything under one second is treated as needing no correction. Built for film shooters working nights, pinhole and astro.
💡 About this tool
Once an exposure runs past about one second, film no longer behaves the way your light meter assumes. The emulsion loses effective sensitivity in dim light over long durations, so a metered reading leaves the frame underexposed. This is reciprocity failure, also called the Schwarzschild effect. Between roughly 1/1000 s and 1 s the meter reading is fine, but outside that window you have to stretch the exposure beyond what the meter says.
How much extra depends on the film. Manufacturers like Ilford and Kodak publish a shutter-time exponent p for each stock, and this tool applies the standard relation corrected = metered^p to turn your metered time and p into a corrected time. Choose a preset and p fills in automatically (Ilford's published figures are FP4+ p = 1.26 and HP5+ p = 1.31); if your film isn't listed, switch to Custom and type p yourself. It saves you digging through a data sheet on a dark tripod-bound shoot.
🧐 Frequently Asked Questions
Does reciprocity failure happen on digital cameras? No. It is specific to chemical film emulsions. Digital sensors do not suffer reciprocity failure, so this correction only matters for film.
When does correction kick in? Roughly at exposures of one second or longer. This tool treats anything under one second as needing no correction and returns your metered time unchanged.
What are the "extra stops"?
They tell you how many stops of extra exposure the correction adds, computed as log2(corrected / metered). It is a handy cross-check if you would rather open up the aperture than lengthen the shutter.
My film isn't in the presets — what do I use? Pick Custom and enter p by hand. The most reliable value is the shutter-time exponent from the manufacturer's data sheet. When no figure is published, Generic (p = 1.3) is a reasonable starting point for typical black-and-white film.
Why is Fomapan 100's p so high? Fomapan is famous for breaking down fast under reciprocity failure — Foma's own datasheet asks for roughly 8x correction (about three stops) by a metered 10 seconds. A single exponent can't reproduce that whole curve, but p = 1.62 is a published community fit that lines up well in the long-exposure range, which is why the preset uses it.
📚 How different films behave
The spread between films is huge. Fuji Acros is the long-exposure favourite precisely because it shows almost no reciprocity failure, holding up to a metered two minutes without correction, with Kodak T-Max films close behind. At the other end, films like Fomapan need several times the metered exposure within seconds. A practical habit among long-exposure shooters: trust the manufacturer figure as a starting point, then run a test roll, because development and temperature shift the real numbers. The single-exponent model is a clean approximation, not a guarantee — useful for the field, worth confirming for work you cannot reshoot.