The Effective Instantaneous Field of View (EIFOV) is a new specification for defining system performance that takes into account the system MTF and provides a convenient way to specify and compare the performance of imaging and radiometric systems. The use of EIFOV is illustrated by two examples of first-order designs, each using ƒ/3·5 optics having a diffraction-limited MTF over the spatial frequency range of interest. The systems are to detect reflectance differences of 0·01 with an EIFOV of 30 m in a wavelength interval of 600 to 700 nm from an altitude of 1000 km. One system uses a solid-state array having a noise equivalent signal of 1·0 μJ/m2and elements 15 by 15 μm in size; the other uses aerial film with a diffuse RMS granularity of 0·008. These examples serve to point out the basic differences between electro-optical and photographic systems for imaging through the atmosphere. Assuming they are of the same size, an electro-optical system is preferred when radio-metric measurements of the imagery are needed and a photographic system is preferred when the imagery is to be examined visually.