Vignetting

In photography and optics, vignetting (/vɪnˈjɛtɪŋ/; French: "vignette") is a reduction of an image's brightness or saturation at the periphery compared to the image center. The word vignette, from the same root as vine, originally referred to a decorative border in a book. Later, the word came to be used for a photographic portrait which is clear in the center, and fades off at the edges. A similar effect occurs when filming projected images or movies off a projection screen, resulting in a so-called "hotspot" effect.

Vignetting is often an unintended and undesired effect caused by camera settings or lens 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 which is known to produce vignetting to obtain the effect, or it may be introduced with the use of special filters or post-processing procedures.

When using superzoom lenses, vignetting may occur all along the zoom range, depending on the aperture and the focal length. However, it may not always be visible, except at the widest end (the shortest focal length). In these cases, vignetting may cause an exposure difference of up to 0.75EV. An example of vignetting characteristics, dependent on focal length, can be found here: http://slrgear.com/reviews/zproducts/pentax18-135f35-56wr/1vignet.gif