law of reciprocity

Now sit back and watch the law of reciprocity kick in.

(If you haven't heard about the law of reciprocity, read Cialdini.)

There is one law; the law of reciprocity, which can be observed by a selfless conscience, together with a sense of awakened justice.

The law of reciprocity describes how light intensity and duration trade off to make an exposure-it defines the relationship between shutter speed and aperture, for a given total exposure.

An example of the use of the law of reciprocity is the Sunny 16 rule which gives a rough estimate for the settings needed to estimate the proper exposure in daylight.

Essentially, reciprocity failure (that is, the failure of the law of reciprocity) means that at very long and very short shutter speeds, film emulsions do not behave in the theoretically correct manner.