The perceptual image quality of natural scenes as a function of the physical system parameter gamma has a definite optimum. This optimum is subject-independent and greater than 1, but was found to vary from one scene to another. If gamma is varied, brightness contrast is the most obviously changing perceptual attribute. Subjects appear to be able to make consistent, global judgments of brightness contrast in natural scenes, despite the fact that local brightness contrast may vary considerably. If scaled perceptual quality is plotted against scaled (perceived) brightness contrast, all curves coincide, suggesting that under the given conditions brightness contrast is the dominant psychological dimension of the perceptual image quality. Taking into account the grey-level distribution of the scene in combination with the luminance- reproduction function of the imaging chain, an effective gamma value can be defined. If scaled perceptual quality and global brightness contrast are plotted against this effective gamma, the differences between scenes disappear, although there are clear differences in the relative sizes of the light and dark parts of the various test scenes. An analysis by scaling global brightness of Gaussian blobs of randomly distributed sizes, modulation depths, and polarities shows that skewness of this distribution does indeed have only a weak effect over a considerable range. The data suggest that the ratio of maximum and minimum luminance determines global brightness contrast for complex scenes under these conditions.