The measurement of the photographic films in their level of sensitivity to light is defined by term as ‘film speed’. Among the different numerical scales the ISO system is the most recent to measure the sensitivity of digital imaging system. Exposure to light is given as per sensitivity of the particular film. Comparably films that are less sensitive and have lower speed index need greater exposure to light to give the density of image same as that of more sensitive films. Thus they are the ‘slow films’ whereas those with higher sensitivity level are termed as ‘fast films’