Displaying of images on monitors actually samples them. When a sinusoidal image is displayed on a monitor, a frequency that is much smaller than the frequency of the original image might be seen, which appears as a moiré fringe. Characteristics of this fringe depend on the geometry of the monitor's pixel lattice and the frequency of the displayed image. By changing the image frequency, one can access a moiré fringe of infinite period. In this situation, there is a simple relation between the image period and the monitor's pixel period. The estimated error in determining the number of the pixels of a monitor is a small fraction of 1 pixel.