from Wikipedia
« »  
This breakdown in the usual tradeoff between aperture and shutter speed is known as reciprocity failure.
Each different film type has a different response at low light levels.
Some films are very susceptible to reciprocity failure, and others much less so.
Some films that are very light sensitive at normal illumination levels and normal exposure times lose much of their sensitivity at low light levels, becoming effectively " slow " films for long exposures.
Conversely some films that are " slow " under normal exposure duration retain their light sensitivity better at low light levels.

1.816 seconds.