Vignetting is a visual effect of an image or photos with darkened corners used to help frame an image or soften the frame outline.
or it could be defect of an optical system in which light at the edges of images is cut off or reduced. It is caused by an obstruction in its original construction; for example, when the elements used in a
lens are too small.
A vignette is often added to an image to draw interest to the center and, in effect, frame the center portion of the photo.
In photography vignetting is a common feature of
photographic images produced by
toy cameras such as this shot taken with a Holga.In photography and optics, vignetting (vin-ye-ting) is a reduction of an image's brightness or saturation at the periphery compared to the image center. A similar effect occurs when filming projected images or movies off a projection screen, the so-called hotspot, defining a cheap home-movie look where no proper telecine is used.
Although vignetting is normally unintended and undesired, it is sometimes purposely introduced for creative effect, such as to draw attention to the center of the frame. A
photographer may deliberately choose a lens which is known to produce vignetting. It can also be produced with the use of special filters or post-processing procedures.
There are several causes of vignetting. Sidney F. Ray distinguishes the following types:
A fourth cause is unique to digital imaging:
Mechanical vignetting occurs when light beams emanating from object points located off-axis are partially blocked by external objects such as thick or stacked filters, secondary lenses, and improper lens hoods. The corner darkening can be gradual or abrupt, depending on the lens aperture. Complete blackening is possible with mechanical vignetting.
This type of vignetting is caused by the physical dimensions of a multiple element lens. Rear elements are shaded by elements in front of them, which reduces the effective lens opening for off-axis incident light. The result is a gradual decrease of the light intensity towards the image periphery. Optical vignetting is sensitive to the aperture and can be completely cured by increasing the aperture of the lens by 2-3 stops.
Unlike the previous types, natural vignetting (also known as natural illumination falloff) is not due to the blocking of light rays. The falloff is approximated by the cos4 or "cosine fourth" law of illumination falloff. Here, the light falloff is proportional to the fourth power of the cosine of the angle at which the light impinges on the film or sensor array. Wide-angle rangefinder designs and the lens designs used in compact cameras are particularly prone to natural vignetting. Telephoto lenses, retrofocus
wide angle lenses used on SLR cameras, and telecentric designs in general are less troubled by natural vignetting. A gradual grey filter or post processing techniques may be used to compensate for natural vignetting, as it cannot be cured by stopping down the lens.
Pixel vignetting only affects
digital cameras and is caused by angle-dependence of the digital sensors. Light incident on the sensor at a right angle produces a stronger signal than light hitting it at an oblique angle. Most digital cameras use built-in image processing to compensate for optical vignetting and pixel vignetting when converting raw sensor data to standard image formats such as JPEG or TIFF. The use of microlenses over the image sensor can also reduce the effect of pixel vignetting.
Vignetting can be used to artistic effect, as demonstrated in this panorama.
Vignetting can be applied in the post-shoot phase with digital imaging software.For artistic effect, vignetting is sometimes applied to an otherwise un-vignetted photograph and can be achieved by burning the outer edges of the photograph (with film stock) or using digital imaging techniques, such as masking darkened edges.
In digital imaging, this technique is used to create a more film-like appearance in the picture.
A simple tool like the Vignetting Reducer will reduce vignetting to a minimum by performing some computer magic. Interested?
Download the free program below.
Current version is 1.3, built 5-Mar-2009. This program is distributed as Freeware. This means that it is completely free for anybody to use without any limitations. The program may be freely distributed as long as it is distributed in the form of the original installer.