Reciprocity failure is a substantial problem for pinhole photographers. Most B&W films begin to experience serious reciprocity effects at exposure times of one second or more, which is quite common in non-lens photography. The only way to compensate for reciprocity is to increase the already lengthy exposure time.

In general, exposure can be described by this equation:

E = I x T

Where E is exposure, I is intensity and T is time. This is the law of reciprocity. This is a reciprocal equation
(eg as intensity increases, the time must be decreased proportionally). If the intensity of light is doubled and the time is halved, the exposure will stay the same. If the light level falls to a quarter, then the exposure time would have to be four times as long.

Within wide limits, the reciprocity law is true. However, reciprocity fails when light intensity is very weak and the exposure long (generally, when the exposure is longer than a second). Reciprocity also fails when light intensity is great and the indicated exposure time is very short (less than 1/1000th of a second).

Reciprocity failure means the exposure necessary to produce adequate minimum density will be larger than calculation from a light meter reading indicates because of reciprocity. Additional exposure is needed to compensate for reciprocity failure. The correction is surprisingly large, and it varies with the kind of film used.