Aspects of Photographic Vignetting
by Michael Brunet
Vignetting (V) is a common photographic issue. It occurs in most lenses and to be sure in all cheap compact cameras. It is a radial loss of brightness (due to exposure drop), most visible at the corners.Vignetting can be used as an effect to draw attention to the central subject of a picture, but in general it is undesirable. Vignetting actually has two aspects: The less exposed corners and the bright center, known as the hot spot.
There are four causes of vignetting: 1. Mechanical. 2. Optical. 3. Natural. 4. Pixel.
Mechanical vignetting is typically caused by too thick a stack of filters, or a lens hood attached to the lens, or by secondary lenses. This will reduce the light at the corners and darken them. Depending on the lens aperture, the darkening will be abrupt or gradual. Small apertures create a more abrupt vignetting.
Optical vignetting is inseparable from the lens itself and is caused partly by the combination of several lens elements, partly by the fact that the lens itself has a considerable length. It is also called artificial vignetting.When lenses are used with a wide open aperture, the length of the lens becomes a problem: the edges of the lens will occlude the edges of the aperture and thus darken the edges. Zoom lenses and wide angle lenses are particularly prone to this kind of V. The rear elements of the lens are shaded by the elements in front of them reducing the light radially and causing vignetting. One can often cure optical V by reducing the aperture two or three stops. This kind of vignetting can be reduced by using very large front lens elements and this is often done in wide angle lenses. Finally should be mentioned that the contrast of the sensor or film also plays a part: the stronger the contrast, the stronger the vignetting.
Natural vignetting, or natural light falloff as it is properly termed, is not caused by the lenses or mechanical issues, but is a natural light falloff proportional to the angle the light reaches the film or sensor. Technically the falloff is proportional to the fourth power of the cosine of the angle of the in-falling light on the film or sensor. Lenses in compact cameras are particularly prone to such falloff. So are wide angle lenses. Telephoto lenses show the least falloff. Both optical and natural vignetting are present at large apertures. The combined effect is often called illumination falloff or radial density.
Pixel vignetting is of course not relevant for film, but only for digital cameras. It is created because most sensors have an angle dependency of the in-falling light.Light hitting the sensor at a right angle produces a stronger impression than light hitting at an oblique angle, thus the corners become underexposed. Digital cameras often have a built-in compensation for this, used when converting the RAW image data to tiff or jpg. If one works with RAW images, one will have to do post-processing to remove pixel V.
Source: <a href="http://www.squidoo.com/vignetting-in-photos">Vignetting in Photos</a>.
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New Unique Article!
Title: Aspects of Photographic Vignetting
Author: Michael Brunet
Email: jan@janesmann.com
Keywords: vignetting,digital photography, photography,hobbies, arts and entertainment
Word Count: 501
Category: Hobbies
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