Reciprocity Failure: How to Correct Long Exposures on Film

Normally, halving the light and doubling the time gives the same exposure. At long exposures, film stops obeying that rule: a metered 10-second exposure may need 30 seconds or more. This loss of sensitivity is called reciprocity failure, and correcting for it is essential for night photography, long-exposure work and close-ups with long bellows extensions.

What reciprocity means

The reciprocity law says exposure equals intensity times time. Within the normal range, roughly 1/1000 to 1 second for most films, the law holds: 1/60 at f/8 and 1/30 at f/11 really are identical. Outside that range, at very long and very short exposures, the film responds less than the law predicts.

Why it happens

A crystal needs a few silver atoms to gather together before it can be developed. In very dim light, photons arrive so slowly that the first silver atoms formed can break apart again before the next photon arrives. Some light is wasted, so the film behaves as if it were slower. The explanation for the latent image is the same one that explains this failure.

How much extra exposure to give

Each film is different, and manufacturers publish data sheets with correction tables or formulas. A few examples:

Film	Correction from manufacturer data	Metered 10s becomes about
Ilford HP5 Plus, FP4 Plus	Corrected time = metered time to the power 1.48	30s
Kodak Tri-X 400	1s: +1 stop; 10s: +2 stops; 100s: +3 stops	40 to 50s
Kodak T-Max 100	1s: +1/3 stop; 10s: +1/2 stop	15s
Fujifilm Acros II	None needed up to 120s	10s
Kodak Portra 400	None needed to 1s; not published beyond	Test; many photographers add about 1 stop

Treat these as starting points and always check the current data sheet for the film you are using. Tabular-grain films such as T-Max, Delta and Acros generally suffer much less than traditional emulsions, which is why Acros has long been a favorite for night work.

Working out a corrected time

Using Ilford's formula for HP5 Plus, a metered 4 seconds becomes 4^1.48, about 8 seconds; a metered 30 seconds becomes about 153 seconds. The correction grows quickly, which is why a stop of extra aperture can save several minutes. Opening the lens from f/16 to f/11 halves the metered time before correction, and the corrected time drops by more than half.

Contrast and development

Reciprocity failure affects shadows more than highlights, because the shadows receive the dimmest light. Long exposures therefore build contrast. Many manufacturers recommend reducing development by 10 to 30 percent for exposures of a minute or more, which also fits the Zone System idea of contracting development for high-contrast scenes.

Color shifts in long exposures

In color film the three emulsion layers fail at different rates, so long exposures shift color as well as density, often toward green or magenta. Data sheets for slide films list correcting filters for some exposure times; with color negative film, much of the shift can be corrected when scanning or printing.

Short exposures

Exposures shorter than about 1/10,000 second, such as those from electronic flash at close range, also cause reciprocity failure. Most photographers never encounter it, but high-speed flash photographers may need slightly more exposure or development.