Grating acuity, the ability to resolve high-contrast square-wave gratings, was measured in a falcon and in humans under comparable conditions. This behavioral test of falcon acuity supports the common belief that Falconiformes have superb vision—the falcon's threshold was 160 cycles per degree, while the human thresholds were 60 cycles per degree. Falcon acuity, however, was much more dependent on luminance, declining sharply with decreases in luminance.