In the brown falcon the relationship between acuity, as measured using a task designed to test minimum-separable acuity, and luminance was investigated as well as the possible determinants of that relationship. The maximum acuity was 73 c/deg and was found to be resistant to decreases in luminance. Maximum behavioural acuity corresponds closely with the maximum anatomical resolving power of the eye, though it is postulated that, in the first instance, acuity is optically constrained. The flat acuity-luminance function of the falcon results from adaptations which increase the optical sensitivity of the eye compared with the globose eye of strongly diurnal falconiformes.