Difference between revisions of "Diffraction"
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Lens 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. | Lens 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 | + | 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<REF>See the [http://en.wikipedia.org/wiki/Airy_disk article Airy disk] at [http://en.wikipedia.org/wiki/Main_Page WIkipedia]</REF>. 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 soley by the f-ratio of the lens. |
At wide apertures diffraction has minimal effect, but at small apertures it significantly softens sharpness and reduces contrast. For a good quality lens, best sharpness and contrast are normally achieved between one and two stops less than maximum aperture. For example, a 50mm f2.0 lens will normally have best sharpness and contrast at f2.8 or f4.0; at f2.0 optical aberrations usually reduce sharpness and at f5.6 or smaller diffraction will reduce both sharpness and contrast. | At wide apertures diffraction has minimal effect, but at small apertures it significantly softens sharpness and reduces contrast. For a good quality lens, best sharpness and contrast are normally achieved between one and two stops less than maximum aperture. For example, a 50mm f2.0 lens will normally have best sharpness and contrast at f2.8 or f4.0; at f2.0 optical aberrations usually reduce sharpness and at f5.6 or smaller diffraction will reduce both sharpness and contrast. | ||
Revision as of 17:41, 30 January 2012
Diffraction is an optical phenomenon caused by the wavelike nature of light, which sets a theoretical limit on the resolution of any optical system.
Lens 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 soley by the f-ratio of the lens.
At wide apertures diffraction has minimal effect, but at small apertures it significantly softens sharpness and reduces contrast. For a good quality lens, best sharpness and contrast are normally achieved between one and two stops less than maximum aperture. For example, a 50mm f2.0 lens will normally have best sharpness and contrast at f2.8 or f4.0; at f2.0 optical aberrations usually reduce sharpness and at f5.6 or smaller diffraction will reduce both sharpness and contrast.
It should be emphasized that diffraction is not a lens aberration: 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 disc 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 could maintain acceptable sharpness even stopping down to such small f-ratios. Conversely, the sensor of a compact digital camera may be only 6×8 mm. In this case, the Airy disc 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.
Notes
- ↑ See the article Airy disk at WIkipedia
Links
- "Diffraction" at Wikipedia.
- Sample photos illustrating sharpness at different f/stops, from Voxphoto on Flickr
