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Wednesday, May 11, 2011

Lens Vignetting

Lens Vignetting

Vignetting is the effect caused by more light reaching the center of an image rather than reaching the edges. For a visual example see the photo above. There are several types of vignetting caused by mechanical, optical and post production.

Mechanical Vignetting is caused by a physical obstruction that prevents light within the lens' field of view from reaching the camera's image sensor. The obstruction can be caused by the lens barrel, a filter, lens hood(improperly misaligned or designed) or anything else in the way. To try this technique, look through your viewfinder as you use your hand to block out the light from reaching the edge of your lens. You will typically see a strong, dark circular darkening most apparent in the corners of the image.
The Mechanical vignetting will go away as the lens is stopped down (narrower aperture).
Example from Toothwalker:
Figure 7. A typical case of mechanical vignetting. (Distagon 28/2 @ f/11 + Contax metal hood #3)
Figure 7 illustrates a typical case of mechanical vignetting. The image was taken with the Distagon 28/2 equipped with Contax metal hood #3, which is simply too long for this lens. A graphical explanation is given in Fig. 8. With the Distagon used at f/11 and infinity, an image corner relies for its illumination on the orange ray pencil, which comes from infinity heading towards the entrance pupil (in red). The angle with the optical axis is the semifield angle, which amounts to 37 degrees. In the absence of a hood the oblique ray pencil has full access to the entrance pupil, but in the presence of the hood the entrance pupil is invisible to this pencil. The pupil is eclipsed by the hood and the image corner receives no light at all.
Figure 8. The Distagon 28/2 without and with Contax metal hood #3. The oblique ray pencil is blocked when the hood is attached.
Optical Vignetting is caused by light hitting the lens aperture at a strong angle - an internal physical obstruction. This effect is usually noticed in images taken with wide angle and wide aperture lenses used with wide open apertures. This will occur even with the best lenses because the light hitting the lens is coming from a strong angle which is partially blocked by the aperture. Light hitting the lens from the front is allowed to pass through the aperture unobstructed. This again will disappear when your lens is stopped down (narrow aperture)
Example from Toothwalker:
Figure 1. Optical vignetting with a 50/1.4 lens. Left: f/1.4. Right: f/5.6.
Figure 1 illustrates optical vignetting for a Carl Zeiss Planar 50/1.4 with an ever exciting subject like a brick wall. At full aperture the image reveals a 'hot spot': a brighter center and a darkening towards the corners (left photograph). When the lens is closed down to f/5.6, the light falloff has disappeared and an evenly illuminated wall remains (right photograph).
Is Lens Vignetting Bad? Not necessarily. Some videographers and photographers purposefully incorporate vignetting to improve other attributes of a lens such as contrast and sharpness. Many videographers want vignetting in their images for effect and they may even add it during post production in Photoshop.

Can they design lenses with no vignetting? Yes, the could create lenses with very minor vignetting. However, they would be much larger and heavier and no outdoor videographer would want to carry them along.

If you have a full understanding of your lenses, you will be better positioned to use them most effectively. Understand when you are trading depth of field (narrow aperture) for shutter speed, background blur - and vignetting. 
- The Digital Picture


Lens Vignetting
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