To determine the relationship between the screen brightness and the regularly obtainable resolution, the screen brightness, measured by a S.E.I. photometer was plotted against the minimum width of Fresnel fringe which was clearly visible all round a circular hole in a carbon film. It was found that there is an optimum fringe width at which the brightness is used most economically. It is also shown that unless the brightness exceeds a certain value (for a given resolution) it is not possible to be certain that this resolution can be attained.