The animation shows a series of black and white lines gradually getting closer together and, underneath it, the intensity profile that might be recorded on film, a CCD array or the retina. As you can see the clear black and white lines at the start form square waves when their black/white, on/off, is clearly visible. However, as the lines get closer together the eye, or camera lens, is unable to see them clearly because of the blurring caused by the diameter of the Airy disc becoming comparable to and then greater than the line spacing. The profiles then become more sinusoidal in shape and gradually reduce in amplitude.

 The animation pauses at the Rayleigh criteria for resolution. This is how lines with frequency 13 per mm, here we mean 13 black or white lines per mm, would be perceived by somebody with 20/20 vision at a distance of 250mm.

 Alternatively they could be an image with lines at about 60 lines/mm on a 35mm film of a line picture at the limit of the depth of field.

 In relation to depth of focus it is the diameter of the geometrical circle of confusion rather than the diameter of the Airy disc (due to diffraction) that leads to the loss of detail.

© Animation prepared by Dr Ken Raine, formerly of the National Physical Laboratory, Teddington, London.