The methodology employed to calculate radiographic signal to noise ratio (SNR) for the commonly used disk shaped test object is reviewed. Although the “matched” aperture yields the maximum SNR, its shape is difficult to achieve experimentally. On the other hand, a circular aperture having the same size as the object is much simpler to realize. We have performed a numerical analysis of the SNRs obtained with matched and circular apertures for two screen/film systems (a mammographic and a general purpose combination) for a range of test object sizes. We find that for object sizes greater than 0.5 mm in diameter, there is a less than 4% difference between the SNRs obtained with the two apertures. The shapes of some of the matched apertures are also presented. We conclude that the SNR determined with the circular aperture is a simpler and more useful determinant of system performance for the screen/film combinations studied. © 1981, American Institute of Physics for the National Institute of Standards and Technology. All rights reserved.