A résumé of the treatment of image formation from the standpoint of the theory of passive linear systems is given, it being shown that image formation for an incoherent object satisfies the basic postulates of superposition and stationarity. It then follows that the spatial frequency response of an optical system will be given by the Fourier transform of its impulse response, this latter being simply the distribution of intensity in the image of a narrow self-luminous line. There follows an account of work done in the author's image-assessment group at Imperial College. This includes the diffraction theory of optical frequency response, aberration tolerance theory and the numerical evaluation of frequency response and diffraction integrals together with examples of the response curves for particular cases. A number of methods for the measurement of frequency response are described, and the theory of these methods and results showing the comparison of theoretical and measured response curves are discussed. The final section describes methods for the measurement of the Fourier spectra of photographic images, and their application to the study of the influence of the detector properties on recorded images.