Vignetting is the reduction in image brightness in the image periphery compared to the image center.
Vignetting is sometimes used for creative effect (e.g. to draw attention to the center of the frame), and 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:
- Mechanical (or physical) vignetting
- Optical vignetting
- Pixel vignetting
Mechanical or physical 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 (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(θ) or "cosine fourth" law, where light falloff is roughly proportional to the fourth power of the cosine of θ (where θ 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 result 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.
Pixel vignetting only affects digital cameras and is caused by the physical depth of the photon wells that capture light in today's digital cameras. 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 will have greater impact 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.
References
- This article contains information originally taken from the Wikipedia article "Vignetting". You can see the authorship and revision history of that article here.
- Van Walree's webpage on vignetting uses some unorthodox terminology but illustrates very well the physics and optics of mechanical and optical 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).