Most imaging systems include a shutter to control the duration (t) of the total exposure (H) given to the sensor. Image illumination (Ei) is controlled principally by the aperture stop so that
H = Eit (7.1)
Quantity t is often called the shutter speed, irrespective of how it is devised. Both Ei and t are in simple reciprocal relationship in that to keep H constant, as either Ei or t increases, the other must decrease. Most photographic materials exhibit reciprocity law failure (RLF) at extremes of the exposure duration range, necessitating exposure correction (Keeling 1985, 1986).
For exposure control, the shutter speed range is usually greater than the aperture range, e.g in an SLR camera a 14 step range ...