This second edition introduces the theory and applications of the modulation transfer function (MTF), used to specify the image quality achieved by an imaging system. It starts with the relationship between impulse response and transfer function, and the implications of a convolutional representation of the imaging process. Optical systems are considered first, including effects of diffraction and aberrations on the image, with attention to aperture and field dependences. Then electro-optical systems with focal-plane arrays are considered, with an expanded discussion of image-quality aspects unique to these systems, including finite sensor size, shift invariance, sampling MTF, aliasing artifacts, crosstalk and electronics noise. Various test configurations are then compared in detail, considering the advantages and disadvantages of point-response, line-response and edge-response measurements.