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Robert D FeinmanCorrecting Wide Angle Vignetting in Images |
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| One of the
problems with ultra wide angle lenses is light fall off in the
corners. This is caused by a variety of factors, especially the extreme oblique angle of the rays reaching the corners. Here is an example of the original image shot with the Voightlander 12mm lens on 35mm film. The horizontal angle of view is about 112º. Notice the darkening of the snow in the lower corners. Other optics with a similar problem include the X-Pan camera and the Super Angulon lenses. |
 Original
Image
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The traditional way to
offset this is by means of a center darkening filter. This is a
filter which is darker in the center and clear at the edges. It
compensates for the light falloff when the picture is taken. The
downside to this is the expense of the filter and the loss of one
to two stops to compensate for the filter density. We wish to correct this digitally. The first step is to use a negative film rather than transparency film. Negative films have enough latitude to accommodate the exposure variations. Set the film speed to 2/3 to one stop lower than the rated speed. So, for example, shoot 200 ISO film at 100 or 125. This way the corners will get full exposure while the center will fall within the overexposure latitude of the film. |
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Here is a way to achieve similar results using Photoshop and the versatile Panorama Tools set of filters. Using a standard 8 bit RGB
image we open the correct filter. Choose the radial luminance
option and then press the options
button. Set the Red, Green and Blue to the same value. For this lens, film and scanning adjustment 30 seems to work well. Half this value will be subtracted from the center of the image and half added to the corners. |
 Panorama Tools Correct Filter  Luminance Values |
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Here are the values after the filter. Since the sample points were not at the center or the extreme corners the changes are not quite 15. |
 Adjusted Image Brightness Readings |
| This method is quick, but
it suffers from a couple of shortcomings. First, if larger values are used the contrast of the image is noticeably lowered in the corners. Second, the rate at which the change occurs from center to edge cannot be set. Third, this only works on 8 bit color images. Fourth, the filter doesn't work on 16 bit images. The next technique will solve the first three limitations. See the following tip for a way to fix 16 bit images. |
 Image After
Luminance Filter
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| First we add a gradient
fill layer above the image. Set to radial, black to white and
150% as shown. You can adjust the rate from center to corner by changing the scale or by modifying the gradient itself. Click on the gradient to open the edit gradient dialog. Set the midpoint as needed if the defaults are not suitable. |
 Gradient Fill
Dialog
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| We set the blend mode to
overlay and the opacity to 16%. If you set a couple of samples in the center and corner of the image you can watch the values change as you adjust the opacity. |
  Layers Palette & Info Palette - Overlay Mode |
| Here is the result. The
contrast between the center and edges has been
reduced. |
 After Overlay |
| Here is another variation
using the Soft Light blend mode and a 50%
opacity. |
  Layers Palette & Info Palette - Soft Light Mode |
| Once again the image
after the adjustment. The differences are slight, but that is
exactly what we want since we can fine tune the results to give a
pleasing result with different lenses, films, scanners or
aesthetic intent. |
 After Soft Light |
| Here is the final image
after some overall brightness, color balance and perspective
adjustments. It is important to capture all the values in the negative and not permit the scanner software to clip the bright or dark values or they may block up when then the brightness curves are applied. As always the aim is to make subtle adjustments which compensate for deficiencies in the photographic process and not to make the adjustments obvious. See the next tip on how to do this with 16 bit images. |
 Final Image |
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© 2003 Robert D Feinman
