A description is given of the essential points of DIN 58185 Optical Transfer Function, in particular of the principles of OTF measurement, the necessary equipment and problems likely to be encountered during measurement. Items of special importance are incoherent illumination, linearity especially of the electronic system, and the existence of an isoplanatic region. These have to be checked before measurement. Other points of interest are the lowest spatial frequency for normalization, the selection of azimuth in the test pieces and the effect of auxiliary systems, insofar as these are used. When measuring phase, a distinction must be made between phase and distortion. Calibration should ensure that phase starts with a horizontal tangent. Information is given on the effect of veiling glare.