 In 1913, Kron formulated an equation to describe the effect in terms of curves of constant density, which J. Halm adopted and modified, leading to the "Kron–Halm catenary equation" 
or "Kron–Halm–Webb formula" 
to describe departures from reciprocity.In photography, "reciprocity" refers to the relationship whereby the total light energy – proportional to the total exposure, the product of the light intensity and exposure time, controlled by aperture and shutter speed, respectively – determines the effect of the light on the film.That is, an increase of brightness by a certain factor is exactly compensated by a decrease of exposure time by the same factor, and vice versa.