For the purposes of this article, I will use the Adobe RAW converter to establish the correction values, but it can almost as easily be done with the Lens Correction filter or a plugin/standalone application like ptlens.
Although the procedure is described for a prime lens, it can just as easily be applied to a zoom; just repeat the procedure a variety of focal lengths. You don't have to cover every mm- a few focal length samples will suffice.
Overview of Process
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Find suitable diffuser
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Take sample shots over range of
wide apertures
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[Adobe RAW settings]
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Determine best
correction values
Diffuser
In order to determine the best correction values, you will need to take sample shots of a uniform surface or through a uniform diffuser of some sort. For this article, the sample shots were taken of the northern sky (away from the sun) through an ExpoDisc. One or two coffee filters held over the lens will also work fine, too. Whether you shoot a uniform surface such as a smooth wall, or through a diffuser, you will want all of the shots to be exposed in the middle of the camera's dynamic range for best results.
Taking the Sample Shots
Since vignetting is most pronounced at wide-open apertures, and usually disappears within a couple stops of wide open, start at maximum aperture. Using Av mode (shutter speed doesn't matter here) and at the lowest ISO setting (to minimize noise,) take a shot through the diffuser into a uniform light source such as the sky. Use this image to set the custom white balance- uniformity across the color spectrum isn't strictly necessary, but it does make determining correction values easier.
Once CWB is set, take successive shots of the sky through your diffuser, incrementing the aperture setting by 1/3 or ½ stops. How many sample shots you take depends on how significantly your lens vignettes, but usually within two stops of wide open the vignetting will disappear. Try to hold the lens axis parallel to the direction of the light, so that the sensor is evenly illuminated. This will make your job easier later.
Transfer these images to your computer, and open Adobe Bridge.
Adobe RAW Settings
Assuming that your images are in RAW format, open the first white balance-corrected image in the Adobe RAW converter.
Figure 1. The RAW Image, Default Settings
Set the Exposure, Shadows, Brightness, and Contrast settings to zero. Set Saturation to its lowest setting. You'll notice that color channel information in the histogram suddenly disappears, and you'll just be looking at the luminosity. This step isn't absolutely necessary, but it makes the process visually simpler (see below.)
Figure 2. Adjust Settings
You will need to monitor the luminosity at various points around the image, so use the Color Sampler tool to set points in the center of the image, the four corners, and in a line extending from the center point to one of the corners. (You may be surprised at the lack of uniformity among the corners, and from one side of the image to the other.) With Saturation set to -100, each color sampler should also show R=G=B, or very close to it.
Figure 3. Color Sampler Placement
Determining Correction Values
Select the Lens tab in the RAW converter. While monitoring the color samplers in the corners, adjust the Amount setting until the corner values are as close to the center value as possible.
Figure 4. Vignetting Amount Adjustment
Then adjust the Midpoint setting (to a lower value) until the samplers extending from the center to the corner are as uniform in value as possible. There will still be some variation, but with a real image it will be unnoticeable. If you prefer a visual method instead of watching the numbers, you will notice that the histogram narrows as you adjust the settings- the goal is to get the histogram to look as much like a narrow spike as possible.
Figure 5. Vignetting Midpoint Adjustment
Record the Amount and Midpoint settings for each aperture. Once the Amount goes below 10, for all practical purposes the vignetting gone, and you can stop.
Figure 6. Spreadsheet
Using the Correction Values
Much of the time you won't notice vignetting in your images. But when you do, go to the table of values you created using this method, and plug those values into the RAW converter.