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Glossary Terms

Diffraction is an optical phenomenon caused by the wavelike nature of light, which sets a theoretical limit on the resolution of any optical system.

Optical designers compute ray-traces through the elements of a lens as if these took idealized geometric paths. However this is a simplification: In fact, light waves spread outwards somewhat as ocean waves do, albeit on a microscopic scale. The effect is especially relevant for light grazing the edges of an obstruction, such as a diaphragm stop or the perimeter of a lens element.

For this reason, no optical system can focus light to a perfect pinpoint. Instead a tiny, blurred bulls-eye pattern known as an Airy disk is formed[1]. Perhaps surprisingly, the diameter of this disk is not affected by the focal length of the lens, or the image's dimensions: it is determined solely by the f-ratio in use.

For a typical lens, diffraction has minimal effect at its widest apertures. Instead, optical aberrations limit resolution, particularly towards the corners of the frame. As a lens is stopped down, aberration control improves; but the effects of diffraction increase. Thus a lens often gives its best image quality when closed down two or three f-stops from its widest setting[2]. When using the smallest available stop, critical sharpness may be noticeably reduced, even though depth of field will be at its greatest.

It should be emphasized that diffraction is not a lens aberration or a manufacturing defect: It is an unavoidable consequence of the physics of light. Diffraction is also a primary design consideration for pinhole cameras, and determines the pinhole diameter which will yield optimum sharpness.

The independence of Airy disk diameter from the dimensions of the image has several photographic consequences. For users of large-format view cameras, the blur disk is small relative to the image size; thus the "f/64 Group" photographers[3] could maintain acceptable sharpness even stopping down to such small f-ratios. Conversely, the sensor of a compact digital camera may be only 4.5×6 mm. In this case, the Airy disk diameter is a significant fraction of the image height—it may cover quite a number of individual pixels. Thus degraded sharpness may be observed even at moderate apertures such as f/4.5 or f/5.6. Some digital compacts employ a neutral density filter as a supplement to their regular aperture control, permitting an equivalent to smaller f/stops but without the loss of resolution.


  1. See the article Airy disk at Wikipedia
  2. Only testing of a particular lens design can determine the exact f-number where this occurs.
  3. "Group f/64" article at Wikipedia