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Robert D Feinman

Correcting Wide Angle Vignetting in Images

 

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
Original Image

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.

original info panel
Original Image Brightness Readings
Center Snow #1
Lower Right Corner Snow #2

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.

options dialog
Panorama Tools Correct Filter

options values
Luminance
 Values

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.


info after filter
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.

after filter
Image After Luminance Filter
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
Gradient Fill Dialog

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.

overlay layer overlay info
Layers Palette & Info Palette - Overlay Mode
Here is the result. The contrast between the center and edges has been reduced.
overlay mode image
After Overlay
Here is another variation using the Soft Light blend mode and a 50% opacity.
softlight palette softlight info
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.
softlight image
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
Final Image
 

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© 2003 Robert D Feinman