This paper deals with the question of the evaluation of the optical transfer function from the image of a sector disc. It is interesting to note that the optical transfer function evaluated by this method differs from the actual OTF, depending on the number of sectors in the object structure used. If the number of sectors is large the values obtained by the present method tally with those of the actual OTF. It is seen that for such measurement, an object target with at least 30 sectors, i.e. thirty bright and thirty dark lines, should be used to obtain reliable results. The image intensity pattern of an edge has also been computed considering the edge object as a limiting case of the sector target. © 1986 Taylor & Francis Group, LLC.