Exposure and shooting
Reciprocity failure
The loss of a film’s effective sensitivity in very long (or very short) exposures, so that more exposure than the meter says is needed.
In the middle range of exposure times, halving the light and doubling the time gives the same result. At very long times this stops being true: the film responds less than expected and needs extra exposure, and colour films can shift in colour. How much depends on the film, and the maker’s own data is the authority.