Most lenses are brighter in the center than at the edges, especially when wide open. This is especially pronounced with wide-angles and some long telephoto lenses. This phenomenon is known as light fall-off or vignetting.
Why does it matter?
Because light is so central to photography, even a minor fall-off risks affecting the way a photograph is perceived. Not only is the information conveyed less efficiently, but the emotional response, or the way the image is scanned by the eye, is affected. Photographers may even sometimes choose to make use of vignetting in order to keep the viewer’s eye within the frame.
1. Gradual darkening towards the corners
The illumination fall-off can be disturbing when the subjects has large zones with even color or brightness. Similarly, when shooting a group of people, one does not want to see the people in the middle brighter than the ones near the corners. The same applies to an architectural shot. 2. Eye tracking is affected
Vignetting offers photographers a very powerful tool for influencing the interpretation of their images by affecting the way the eye scans the image. However, this should be a conscious creative decision, not merely a side-effect of the lens and aperture / focal length used.
(Nikon D70 AF-S DX Zoom Nikkor 18-70 mm f/3.5-4.5G IF-ED)
Photo: David Lamouller
3. Image cropping is more complex
Since vignetting has a radial symmetry, a non-symmetrical crop can be very noticeable.
How complex is it to correct?
1. Lens dependency
Natural vignetting is inherent to lens design, regardless of aperture. With a zoom lens, it generally increases as the focal length decreases. 2. Aperture dependency
Because a lens assembly has a certain physical length, at wider aperture, on the edge of the field, the entrance pupil can be partially shielded by the lens body. This is why optical vignetting increases with aperture. 3. Tone curve dependency
Vignetting depends on the exposure, a phenomenon caused by the tone curve. The tone curve is a non-linear curve used to convert the charges produced by the sensor into digital values. Because of this non-linearity, a constant difference in exposure does not translate into a constant difference in the digital values. This means that pixels of different gray levels subject to the same optical vignetting will exhibit different attenuation of their gray levels. As a consequence, vignetting cannot be corrected with a generic mask for a given capture setting.
4. Over-exposure risk
Vignetting correction gets even more complicated if the part of the image subject to vignetting consists of a bright area and a dark area. In this case, a different mask is required for each area. Generally speaking, the mask must take local image gray level variations into account, and therefore becomes specific to the image being processed.
Photo: Guillaume Gaiotti
5. Noise & color control
In addition, vignetting correction must avoid the appearance of increased noise levels in dark areas, or color alteration, as illustrated in the paired images below. Vignetting correction is clearly image-dependent.
Photo: Guillaume Gaiotti
DxO Optics Pro Vignetting Correction
Per above, underneath its apparent simplicity, vignetting correction is yet an extremely tough process. Common tools offer solutions that can partially correct vignetting, but they require manual settings for every single image.
(Nikon D70 AF-S DX Zoom Nikkor 18-70 mm f/3.5-4.5G IF-ED)
Photo: Philippe Tarbouriech
DxO Optics Pro' s automatic vignetting orrection ensures optimal quality, while addressing all the factors described above:
adaptation to any capture settings (aperture/focus distance/focal length...)
