The Lens Corrections tab is designed to give photographers a tool to fix common problems with photographic hardware, such as lens vignetting and chromatic aberration. However, this tab also provides creative photographic controls that go well beyond the simple functionality of fixing problems.
Fixing lens vignetting
Vignetting is a lens defect that occurs with certain lenses (especially wide angle lenses), causing the edges and especially the corners of an image to be darker. In many instances, especially in architectural photography, the result of lens vignetting is undesirable. You can use the Lens Vignetting sliders to compensate or remove the vignetting (Figure 36a).
Figure 36a Use the controls on the Lens Corrections tab for creative results or to fix lens vignetting or chromatic aberration.
Drag the Amount slider to the right to lighten the corners or drag to the left to darken the corners.
Use the Midpoint slider to expand or contract the vignetting effect. For example, you might drag the Amount slider to the left to darken the corners. Then you might decrease the Midpoint value to increase the “reach” of the vignetting so that the darkening extends from the corners toward the center of the image. Conversely, you could increase the Midpoint value to restrict the darkening adjustment to an area closer to the corners.
Adding creative lens vignetting
Many photographers are now using the Lens Vignetting and Post Crop Vignetting controls to darken the corners of images (Figure 36b). In previous versions of Camera Raw you could only add a vignette if the image had not been cropped. New to Camera Raw CS4 are the Post Crop Vignetting controls, which allow you to brighten/darken the edges of your photo. In addition, you can control the shape and edge of the effect with the Roundness and Feather controls. Either way, whatever controls you use, modifying the edge of your photo adds a unique look and feel. Adding a lens vignette will affect the overall tone of an image. Thus, once you have applied the effect, be sure to go back to the Basic and Tone Curve tabs to make final tone adjustments.
Figure 36b This photograph was taken on a recent surf trip to Baja, Mexico. The lens vignetting was added in Camera Raw by decreasing the Amount and Midpoint values.
Correcting chromatic aberration
Chromatic aberration is a lens defect that results in images with color fringing, in which the details near the corners of an image have a red or cyan fringe. To identify chromatic aberration, zoom in to an area near the corner of an image and look for color fringing. While color fringing isn’t all that common, except when using wide angle lenses, it’s worth noting how to fix it in case you encounter it.
In the Camera Raw Lens Corrections tab, adjust either of the Fix Red/Cyan Fringe or Fix Blue/Yellow Fringe sliders. As you adjust these controls to the left and right, watch the fringe in the image increase or decrease until you find the correct setting. The goal is to remove as much of the color fringing as possible.