A gradient is a difference in brightness from one side of the picture to the other; it generally occurs when you're photographing close to the horizon or the Moon is in the sky. Vignetting is underexposure at the edges of the picture, usually caused by the limited diameter of the light path. Both of these can be corrected by software; in fact, MaxDSLR and similar packages can usually correct them automatically (Figure 14.8). Of course, if the vignetting originates in the camera, flat-fielding is a potentially more accurate way to correct it.
You can also correct vignetting manually in Photoshop. Briefly, the technique is:
• Learn to use the gradient tool in Photoshop to draw gradients, both linear and circular.
• On top of your image, draw a gradient of the same shape and position as the one you want to correct, using Color Burn rather than Normal as the blending mode.
• Then, on the main menu, choose Edit, Fade Gradient, and reduce the opacity of your gradient until it makes the desired correction.
When in doubt, undercorrect.
An impressive Photoshop plug-in for semi-automatic correction of gradients and vignetting is Russell Croman's GradientXTerminator (www.rc-astro.com). It corrects color shifts as well as differences in brightness.
There is an important mathematical difference between the usual gradient (from moonlight or skyglow) and the usual vignetting (from tube obstruction). Vignetting is multiplicative; that is, vignetting reduces the light intensity to a fraction of what it would otherwise have been. Gradients are additive; that is, they consist of extra light added to the original image. The optimal algorithms for correcting them are different, although, in practice, you can usually get away with using the same techniques on both.
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