Reciprocity Failure

Reciprocity failure occurs during exposures exceeding one second. Essentially what this means is that the emulsion, having been designed for high-speed exposures, fails to respond in its usual proportions to the amount of light it is exposed to. Every emulsion responds differently to long exposures so it is recommended that you get specific data from the manufacturer. In general the following times can be used as benchmarks:

Colour Negative
1 sec: + 1 stop
10 secs: + 2 stops
100 secs: + 2.5 stops
Colour Transparency
1 sec: + 0.5 stop
10 secs: + 1.5 stops
100 secs: + 2.5 stops
1 sec: expose for 2 secs, underdevelop by 10%
10 secs: expose for 1 min, underdevelop by 20%
100 secs: expose for 20 mins, underdevelop by 30%

Color emulsions will experience uncorrectible color shifts as reciprocity fails at different rates in the various color layers. With color film it is therefore better to adjust the aperture, rather than increase the exposure time, to minimize the effects of color imbalance. With B/W film densities build up proportionately during development, so the exposure is usually altered by shutter speed with a corresponding cut in dev time.