The frequency response of an optical system may be derived from photoelectric image scanning data by a comparatively simple technique. The scanning movement is used to drive a sine-cosine potentiometer, fed by the photoelectric scanning signal, and integration of the product so formed is carried out with a low-inertia motor. Sine and cosine terms are available, and the successive Fourier coefficients are obtained by an alteration of potentiometer gearing. Values obtained with the apparatus are compared with those resulting from an equivalent digital electronic computation.