the technical side of photography is based on simple relationships.
Exposure depends upon how sensitive the recorder(emulsion or sensor) is to light, how much light passes through the lens, and for how long that light hits the recorder. With film, it is common to adjust aperture and time somewhat haphazardly, Assuming that changing either of them is pretty much without effect on exposure – Raise one by one stop, while dropping the other by one stop should produce the same amount of exposed silver. And for most exposures this hold true. Reciprocity.
However, when the TIME is changed to very short or very long durations, this fails — reciprocity failure.
Film’s response to light is not linear. The failure of film density to increase directly as exposure time increases is reciprocity failure. The table reports corrections for this effect. (Schwarzschild first noticed this aspect of emulsions ).
In the world of view camera a standard practice is to use aperture for depth, while using shutter speed to control exposure. This method often requires using tables to correct for exposure time.
Have some fun tonight.
Take a few hours and shoot under the moon. Jerry Burchard used to time his exposures by the number of beers it took.