Science Fair Project Encyclopedia
Vignetting is sometimes used for creative effect (e.g. to draw attention to the center of the frame), and are then deliberately introduced by the photographer by the use of special filters or post-processing procedures.
Vignetting is also used to describe unwanted darkening of the corner of a photograph. There are three different types of unwanted vignetting, which I will refer to as follows:
- Mechanical (or physical) vignetting
- Optical vignetting
- Pixel vignetting
Mechanical or physical vignetting is caused by light beams emanating from object points located off-axis are partially blocked by external objects such as thick or stacked filters, secondary lenses, and (misaligned) lens hoods, as well as various elements positioned inside the lens to limit aberrations, causing brightness to drop in the image periphery. Mechanical vignetting is sensitive to aperture. Stopping down a lens prone to mechanical vignetting one or two stops will remove the vignetting effect.
Optical vignetting is light falloff that is inherent in the lens design and is approximated by the cos4(theta) law where light falloff is roughly proportional to the fourth power of the cosine of theta (where theta is the angle off axis). Wide-angle designs and the lens designs used in compact cameras and rangefinders are more prone to optical vignetting than longer lenses and retrofocus lenses used in SLR cameras because these designs results in a larger theta. Optical vignetting is not reduced by stopping down. The visual effect of optical vignetting can be remedied by using a gradual grey filter or by image processing.
Van Walree's webpage  uses some unorthodox terminology, but it illustrates very well the physics and optics of mechanical and optical vignetting.
Pixel vignetting only affects digital cameras and is caused by the photon wells that capture light in today's digital cameras has physical depth. Just like more light reaches the bottom of a well when the sun is in zenith, light hitting a photon well at a right angle have greater inpact 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.
For a more detailed description of pixel vignetting, see the paper by by Catrysse et al. 
- Van Walree: Vignetting
- Peter B. Catrysse, Xinqiao Liu, and Abbas El Gamal: QE Reduction due to Pixel Vignetting in CMOS Image Sensors; in Morley M. Blouke, Nitin Sampat, George M. Williams, Jr., Thomas Yeh (ed.): Sensors and Camera Systems for Scientific, Industrial, and Digital Photography Applications, Proceedings of SPIE, vol. 3965 (2000).
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