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Inphotography andoptics,vignetting (/vɪnˈjɛtɪŋ/vin-YET-ing) is a reduction of an image'sbrightness orsaturation toward theperiphery compared to the image center. The wordvignette, from the same root asvine, originally referred to a decorative border in a book. Later, the word came to be used for aphotographic portrait that is clear at the center and fades off toward the edges. A similar effect is visible in photographs ofprojectedimages orvideos off aprojection screen, resulting in a so-called "hotspot" effect.
Vignetting is often an unintended and undesired effect caused bycamera settings orlens limitations. However, it is sometimes deliberately introduced for creative effect, such as to draw attention to the center of the frame. A photographer may deliberately choose a lens that is known to produce vignetting to obtain the effect, or it may be introduced with the use of specialfilters orpost-processing procedures.
When usingzoom lenses, vignetting may occur all along the zoom range, depending on theaperture and thefocal length. However, it may not always be visible, except at the widest end (the shortest focal length). In these cases, vignetting may cause anexposure value (EV) difference of up to 3EV.[1]
There are several causes of vignetting. Sidney F. Ray[2] 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 (laterally or vertically off from the optical axis of an optical system under consideration) are partially blocked by external objects of the optical system such as thick or stacked filters, secondary lenses, and improper lens hoods. This has the effect of changing theentrance pupil shape as a function of angle (resulting in the path of light being partially blocked). Darkening can be gradual or abrupt – the smaller the aperture, the more abrupt the vignetting as a function of angle.
When some points on an image receives no light at all due to mechanical vignetting (the paths of light to these image points is completely blocked), then this results in a restriction of theField of View (FOV) – parts of the image are then completely black.
This type of vignetting is caused by the physical dimensions of a multiple element lens. Rear elements of the lens 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 in light intensity towards the image periphery. Optical vignetting is sensitive to the lensaperture and can often be cured by a reduction in aperture of 2–3 stops. (Anincrease in theF-number.)
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 fall off. Here, the light fall off is proportional to the fourth power of thecosine of the angle at which the light impinges on the film orsensor 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 onSLR cameras, andtelecentric designs in general are less troubled by natural vignetting. A gradual grey filter or postprocessing techniques may be used to compensate for natural vignetting, as it cannot be cured by stopping down the lens. Some modern lenses are specifically designed so that the light strikes the image perpendicular or nearly so, eliminating or greatly reducing vignetting.
In digital cameras, light that strikes a pixel in the camera's image sensor perpendicular to its surface produces a stronger signal than light hitting it at an oblique angle. This creates a fall-off with angle in addition to natural vignetting. 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 asJPEG orTIFF. The use of offsetmicrolenses over the image sensor can also reduce the effect of pixel vignetting.
Vignetting is sometimes applied to an otherwise un-vignetted photograph byburning the outer edges of the photograph (with film stock) or using digital imaging techniques, such as masking darkened edges. The Lens Correction filter inPhotoshop can be used for this.
This can be done for artistic effect, to create alow fidelity or 'retro' appearance in the picture. Post-shoot vignetting can also be used more subtly, to guide the viewer's eyes to an intended center of interest in the photograph. A barely perceptible darkening of bright features near the edges of an image can "fence off" the bright area and keep the viewer's eyes where the photographer wants, without the vignetting being obvious.