Reciprocity Failure, Stock by Stock

What reciprocity failure is

The reciprocity law says that exposure equals intensity times time. Halve the intensity, double the time, and the film gets the same amount of light. The law holds for most everyday exposures and breaks down at long ones.

When exposures stretch past about one second, film stops responding linearly to light. The silver halide grains require a critical number of photons within a short time window to register, and at very low light intensity those photons arrive too slowly. Some grains never accumulate enough to expose. The film acts as if it is less sensitive than its ISO rating, so you need to add exposure time to compensate.

Different stocks fail at different rates. Some lose a fraction of a stop at one second. Others lose two or three stops. The correction is not optional for anything past a few seconds.

How it scales

The correction follows a power law: actual exposure equals metered exposure raised to a stock-specific exponent. The exponent for most modern black and white stocks is around 1.3. For older or simpler formulations like Fomapan it can reach 1.5. For Kodak T-Max and modern Ilford Delta films it stays close to 1.1.

That means a metered exposure of 1 second is roughly 1 second on T-Max, 1.3 seconds on Tri-X, and 1.5 seconds on Fomapan 100. A metered exposure of 10 seconds becomes 13 on T-Max, 26 on Tri-X, and 56 on Fomapan 100. At 60 seconds metered, Fomapan 100 wants about eight minutes.

Correction tables for common stocks

Approximate exposures for common stocks. Use these as starting points; your own testing will refine them.

These are starting points. Manufacturers publish technical sheets; for any stock you shoot regularly, your own bracketed tests will give you exact numbers for your developer combination.

Colour shifts on slide film

Reciprocity failure on colour slide film is not just about exposure: each of the three colour layers fails at a different rate, which shifts colour balance during long exposures. Velvia 50 in particular develops a strong magenta cast past 30 seconds, which manufacturers recommend correcting with a 5CC to 10CC green filter for exposures up to a minute, and stronger filtration past that.

For long-exposure colour work, slide film is harder than negative film for this reason. Portra and Ektar shift more gracefully and the lab scan can usually neutralise minor casts.

How to meter for reciprocity

Meter the scene normally to get the base exposure. If it lands anywhere past about one second, apply the stock-specific correction. The simplest way is to look up the table or use a meter that has the table built in.

For Zone System work, place your tone first, then read the corrected exposure off the meter. Bellows extension, IR filter, expired film, and reciprocity all stack: a 1-second base exposure on Fomapan 100 with a 1:1 macro bellows can become four minutes once every correction is in. Each stop matters because they all multiply.

Where Zone Light Meter helps

The app ships per-stock reciprocity curves for 693 stocks across 70+ brands and lets you override or add custom curves. Select your film stock and meter normally. If the metered exposure is past about one second, the app applies the curve automatically and displays the corrected exposure alongside the raw meter reading, so you can see what changed.

For long-exposure work the bulb timer takes the corrected exposure and counts down with haptic feedback so you can keep your eye on the camera instead of the phone.