The ability of an observer to detect correctly a well-defined pulmonary nodule as a function of viewing distance was experimentally determined at three different nodule brightness levels. Optimal viewing distance was shown to be more critical at low brightness levels. From the data obtained, the visual system transfer function (VSTF) was computed. Both peak frequency and peak amplitude of the VSTF curves were shown to decrease with decreasing nodule brightness. This study indicates that viewing radiographs at a fixed distance creates a considerable risk of failing to detect abnormalities.