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Sometimes optical problems in a lens can create visible artifacts in your image. Lens aberration correction attacks two different kinds of optical problems. Vignetting is a darkening that can occur in the corners of your image. Vignettes most often occur with wide-angle lenses, and sometimes a vignette can add a nice effect to an image; it can bring attention to the center of the frame. Most of the time, though, you don't want vignettes in your image, and your camera has a feature to remove these when you're shooting JPEG images.
Chromatic aberration occurs when the lens doesn't focus all wavelengths of light to precisely the same point. The practical upshot is that you'll see colored fringes around some areas of your image. As with vignetting, your camera can address these issues when you're shooting in JPEG mode. Lens aberration correction, which is in the first page of the shooting menu, is where you will find controls for both correcting vignettes and chromatic aberration. This big thing up here is showing me the lens that I currently have attached; I have the Canon 24 - 105 f/4; that's the kit lens that comes with the Mark III, and it says Correction data available.
Mark III has a database of 25 Canon lenses in it that are profiled, so that it knows exactly what it's vignetting and chromatic aberration characteristics are. So here I have got Peripheral illumination. That's basically vignette correction. I can turn it on or off. So if you're finding a vignetting problem -- this defaults to Enable -- if you're finding you still have a vignetting problem, you might want to consider looking at a different copy of your lens, because the camera should be able to correct a good amount of it away.
If you're still getting it, there might be something wrong with your lens. If you like having the vignette in your image, you can simply disable this, or if you find that it's a little too aggressive, and it's brightening the corners, you could disable this. Chromatic aberration is going to occur when you are shooting high contrast edges. So, say, telephone lines up against a blue sky, or the edge of roofs up against a blue sky. You might see purple, or reddish, or bluish fringes along those edges, and you are going to find them worse at wide angles, and at certain apertures.
Chromatic aberration reduction will simply try to eliminate those for you. It does a very good job, but if you, for some reason, find that it's degrading your image somehow, you might want to Disable it. Again, as with any in camera processing operation, except for noise reduction, these features only work on JPEG images. If you are a RAW shooter, you don't need to worry about these. You are going to have to figure out a way to fix these problems in post-production. If you are JPEG shooter, this is a very simple, easy way to deal with these two kind of pesky image editing hassles.
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