Black and White Images
A black and white image is made up of pixels each of which holds a single number corresponding to the gray level of the image at a particular location. These gray lev¬els span the full range from black to white in a series of very fine steps, normally 256 different grays. Since the eye can barely distinguish about 200 different gray levels, this is enough to give the illusion of a stepless tonal scale as illustrated below: