A few lenses contain t-tops on one side of the aperture ring, and f-stops on the other side. The f-stops are white. A small amount of light can be lost of a lens contains a number of glass elements. The t-stops function in place of f-stops for setting exposure. T-stops are designed to work with the degree of light striking the film (as opposed to finding the amount using math). The t-stop sets the exposure, and the f-stop shows the amount of depth of field one has.