Reciprocity Law
The principle that exposure depends only on the product of light intensity and time, so halving one and doubling the other leaves the result unchanged.
Formula / rule
H = E × t
Measured in
lux-seconds (lx·s)
In depth
Reciprocity is the reason equivalent exposures work at all, and it holds essentially perfectly for digital sensors across their entire usable range. Film departs from it at very long and very short exposures — a phenomenon called reciprocity failure — which is why film photographers carry correction tables that digital photographers never need.
Examples in practice
On a digital sensor, 1 s at f/16 and 1/60 s at f/2 produce identical brightness; on film, the one-second frame would come out visibly thin.