The square root of the density of exposure quanta at the image plane is a popular measure of image quality for photon images. This is in fact only an upper limit which is rarely realized in the image. It is the density of noise equivalent quanta (NEQ) deduced from image measurements which is relevant to real imagery. In general NEQ is a non-trivial function of spatial frequency. Examples are taken from screen/ film mammography, electrostatic mammography, and computed tomography. The signal-to-noise ratios computed from these NEQ values characterize the performance of an ideal observer confronted with the image and a detection task. This sets goals for real observer performance, i. e., image processing and display for human acquisition.