Fig. 62. Comparison of various gratings. The upper right is a high-frequency, high-contrast sine wave grating. The upper left is a sine wave grating with the same contrast as the upper right but with a lower frequency. The lower left is a sine wave grating with the same frequency as the upper left but with a lower contrast. The lower right is not a sine wave grating but a Ronchi ruling. Unlike the sine wave grating, which gradually changes from dark to light, the Ronchi ruling changes abruptly. In a sense, Snellen optotypes resemble Ronchi rulings more than sine wave gratings.