Reciprocity Failure Calculator
Long exposures fool your meter. Pick your film, enter the metered time, and get the time you should actually set on the camera.
Film does not respond to light in a perfectly linear way. For normal shutter speeds it behaves itself, but once an exposure runs past about a second the emulsion starts to lose sensitivity. The meter still reads the light correctly, yet the film needs more time than the meter says to build a proper density. That gap is reciprocity failure, and it grows fast: a metered 2 seconds might really need 3, a metered 30 seconds might need a minute or more, depending on the stock.
The correction follows a power law closely enough to be useful in the field. Corrected time equals the metered time raised to an exponent that is specific to each film. An exponent of 1.0 means no correction (the film holds reciprocity). An exponent of 1.3 means a metered 10 seconds becomes about 20. This tool uses the exponent that Zone Light Meter applies for each of 668 catalogued stocks.
How to use it
Meter the scene as normal and note the shutter time. Choose your film from the list, type the metered time in seconds, and read the corrected time below. If the metered time is one second or shorter, the tool tells you no correction is needed, because the loss at those speeds is too small to matter. For films that are not in the list, choose "Generic / custom exponent" and enter your own value (1.3 is a reasonable middle-ground guess for an unknown black and white film).
Frequently asked
What is reciprocity failure?
Film loses sensitivity during very long exposures, so the metered time is no longer enough to expose the frame correctly. The longer the exposure, the bigger the shortfall. You correct for it by adding time.
When do I need a reciprocity correction?
Generally only when the metered shutter time is longer than about one second. Below that the loss is negligible. Above it the correction grows quickly, so a metered 4 seconds can become 8 or more on some films.
Where does the exponent come from?
Each film responds differently. Manufacturers and testers publish correction data, which fits closely to a power law: corrected time equals metered time raised to an exponent. The exponent in this tool is the value Zone Light Meter uses for that specific stock.
Does reciprocity failure affect digital?
No. This is a property of photographic film and paper. Digital sensors have their own long-exposure noise behaviour, but not reciprocity failure in this sense.
In the field, Zone Light Meter for Android applies this correction to the live reading automatically the moment your metered time crosses one second, so you never have to reach for a calculator.