The Lens Corrections pane contains controls for fixing Chromatic Aberrations and Lens Vignetting. These are the same controls found in Adobe Camera Raw and can also be mimicked using Photoshop's Reduce Noise and Lens Correction filters.
Chromatic aberrations (CA) show up as anomalous color shifts, mostly on the outer perimeter of an image, in areas with distinct edge transitions. The fringing is sometimes there because when light passes through glass, different color wavelengths are separated and shifted in focus ever so slightly. They are mostly evident when wide-angle lenses are used, but can appear with longer lenses as well.
This image of a huge Icelandic power plant was taken near our hotel in Nesbud. Although the chromatic aberrations are not visible at first glance, magnification shows some annoying anomalies on the edges.
This is how to reduce CA:
If neither of these sliders does the trick, try one of the two Defringe settings (Highlight Edges or All Edges) and see if that helps.
Lens Vignetting and Lightroom
Vignetting (darkening at the corners of the frame) can be caused by a mismatched filter/lens hood or a lens (e.g., using a filter on an ultra wide angle lens). It can also be caused by using wide angle lenses not optimized for digital capture (i.e. not optimized for even brightness across the fame). It's one of the easiest things you can fix with Lightroom's Develop module. Conversely, you can also add a vignette to your image which will draw attention to a specific part of any image.
To add or diminish a vignette:
Lens Vignetting is applied to the entire image, border to border, regardless of whether the image has been cropped. A heavily cropped image will therefore appear to have vignetting applied only to a portion of the image.
Understanding How CA Controls Work
It's very useful to understand how the CA controls actually work. They don't simply look for edges and remove or desaturate colors. If you move the control sliders radically one way or another, you will notice a subtle distortion of your image, growing in intensity from the center of the image out to the edges. In fact, the distortion is limited to select colors that are actually expanding or shrinking (i.e., distorting) based on your settings. For example, if you choose to fix Red/Cyan fringing, then either the red or cyan colors will be affected. If you chose to fix Blue/Yellow fringing, only these colors will be affected.
(The Defringe options—Highlight Edges or All Edges—slightly desaturates edges, which can help in some cases, but may also result in thin gray lines.)
The key point is there are important practical implications. First, don't crop your image, apply Chromatic Aberrations controls, and expect good results. The color distortions are based on a lens model; the minute you crop, you've changed that model. Second, don't expect this to work on other aberrations such as a dead pixel or a highlight blooming that appears in the dead center of an image. The effect is more powerful on the edges of your images, and diminished as you move closer to the center. Finally, the results you get will vary depending on the lens you use.
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