Fig. 3. The minimum contrast threshold required to detect the presence of a 33° black target for a given spatial resolution. The graph is calculated by convolving the image of the target with the modulation transfer function for a given spatial resolution and then determining the maximum contrast of the convolved image (see Johnsen et al., in press for details). The images above the graph show the appearance of the target when viewed by visual systems with spatial resolutions of 1°, 33°, 60° and 120° (left to right).