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What is Vignetting? and How to Avoid It!

By Didi Swoboda

Vignetting (V) is a common photographic problem. Most lenses and certainly all compact cameras show it. It is most visible at the corners as a radial loss of brightness due to lowered exposure. Vignetting can be used as an effect to draw attention to the central subject of a picture, but in general it is undesirable. There are in fact two aspect to vignetting: The common underexposed corners and the bright centre, called the hot spot.

There are four kinds 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. All three may reduce the light at the corners and darken them. The darkening will be abrupt or gradual depending on the aperture. The smaller the aperture, the more abrupt the vignetting.

Optical vignetting is inseparable from the lens itself; it is caused partly by the combination of several lens elements, partly by the fact that the lens itself has a considerable length. Another name for optical vignetting is artificial vignetting. The length of the lens is a problem because when the lenses are used with a wide open aperture, the edges of the lens will occlude part of the aperture from oblique incoming light, thus shading the edges of the image. This kind of V is most pronounced in zoom lenses and wide angle lenses. The light in the lens is also reduced radially because the rear lens elements are shaded by the front lens elements, thus causing vignetting. One can often cure optical V by reducing the aperture two or three stops. Very large front lens elements tend to reduce this kind of vignetting and are typically used in wide angle lenses. Lastly, the contrast of the film or sensor plays a part: the stronger the contrast, the more pronounced the vignetting.

Natural vignetting (also called natural light falloff) is a, well, natural light falloff proportional to the angle the light reaches the sensor or film; it is not caused by the lens. Technically the falloff is proportional to the fourth power of the cosine of the angle that the light reaches 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. At large apertures both optical and natural vignetting are present. 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 reaching the sensor at an oblique angle generates a weaker sensor reaction than light reaching it at a right 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 jpeg. If one works with RAW images, one will have to do post-processing to remove pixel V.

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