Here are all the photo dictionary entries in which either the term or the definition contain the text “diffraction.”
Airy disc/disk.
Also Airy’s disc. An image artefact caused by circular aperture diffraction.
When light from a small point is shone through a tiny aperture (hole) the result is not a perfect point on the image plane but rather a blurry dot surrounded by soft concentric rings. This is a side-effect of diffraction (the effect worsens - discs become larger - as the aperture decreases) and is a particular problem for telescope designers, since stars are essentially point sources of light from the point of view of an optical system on Earth. It also affects the sharpness of a photographic image when the lens is stopped down. (eg: stopping down past f/16 or so on a 35mm camera usually results in gradually worsening image sharpness owing to diffraction)
The phenomenon is named after British Astronomer Royal Sir George Bidwell Airy (1801-1892). Airy is of particular note for inventing the first spectacles to correct astigmatism and his work in building the Airy Transit Circle observatory in Greenwich, England, which has marked the prime meridian (0 degrees longitude and the line upon which Greenwich Mean Time or GMT is based) since 1884. Less auspiciously he is also remembered for an alleged lapse in attention which lead to German astronomers discovering Neptune (a huge political blow for Victorian England) and also for a long-running feud with Charles Babbage (1791-1871), the inventor of early computing machinery, which unfortunately was partially responsible for the British government withdrawing funding for Babbage’s work.
cf. aperture, astigmatism, diffraction.
Entry last updated 2002-05-09.
Diffraction.
Scattering of light which occurs when light passes through a very small opening.
When a lens is stopped down to a very small aperture, therefore, diffraction increases and the image quality is reduced. Diffraction can also be used in a controlled fashion for lens construction. Canon have built very expensive but relatively lightweight lenses which use diffractive optic principles - their DO lenses.
Diffraction is a side-effect of the wave aspect of light. Really tiny apertures result in light interference - a kind of beat effect which results in light dark and areas.
cf. Airy disc, aperture, DO, f stop, light interference.
Entry last updated 2002-05-09.
DO.
(Canon) Diffractive optics. A lens technology developed by Canon which uses a lens elements with extremely fine grooves - diffraction gratings - engraved into them. These elements use the principle of diffractive optics to bend light.
The advantage of DO lenses is that they can be made smaller and lighter than regular lenses. The disadvantage is that they are, at present at any rate, incredibly expensive. DO lenses are identifiable by the light green ring painted around the end of the lens barrel.
cf. diffraction.
Entry last updated 2002-04-03.
Pinhole camera.
A very simple form of camera which does not use a lens. A pinhole camera is simply a container of any size with a tiny pinhole at one end. Light enters this hole and exposes some photosensitive material mounted on the far wall of the camera.
Pinhole photography is often a slow process, since the tiny size of the hole means very little light enters. Several second or minute exposures are usual. Pinhole photographs are notable for many reasons - they have almost infinite depth of field and tend to be very soft towards the edges because of diffraction.
Though not a practical form of photography for snapshots, many artists and hobbyists enjoy the creative possibilities of pinhole work. A big part of the pleasure is building unusual homemade cameras - cameras from oatmeal containers, beautifully handcrafted wood, fabric-lined cars or rooms, film canisters put inside the photographer’s mouth to get a mouth-eye view of the world, and so on.
cf. camera, depth of field, diffraction.
Entry last updated 2002-04-04.
Scattering.
Light (and other forms of radiation, particularly UV and infrared) which is reflected off small suspended particles in the air or which passes through a small hole. The effect is an overall one of light diffusion.
It’s light scattering which causes the blue of skylight - Rayleigh scattering. Light scattered by dust and fog is Mie scattering. Light scattered by small apertures is the cause of diffraction.
Entry last updated 2002-04-03.
Sweet spot.
A vague colloquialism describing a setting or area of a lens which offers the best optical performance.
1. The aperture which offers the best optical performance for a given lens.
Generally lenses do not perform at their best when shooting wide open. If the lens is stopped down to a very small aperture, however, some diffraction can occur. Most lenses tend to work best when set to an aperture of 2-3 stops down from wide open, though this is of course just a rule of thumb and not a guarantee.
cf. aperture, diffraction, wide open.
2. The area towards the centre of an image area. Most lenses are slightly softer towards the edge of the field, though of course high-quality professional lenses tend to be relatively sharp across the field.
Entry last updated 2002-04-03.