The optical image quality is influenced by aberrations, by diffraction, and by other factors, such as flare or stray light originating from the badly polished surfaces of lenses, from the edges of the mount, from striations and bubbles in the glass, and from unwanted double reflections. These effects are harmful, but they cannot always be avoided. Their influence on the measured optical transfer function depends on the measuring method used and on the kind of test object. Experimental results are described.