Menu: Adjust -> Chromatic Aberration
A simple lens will not focus different colors in the exactly same place because the refraction depends on the wavelength. Short (blue) wavelengths are refracted more than long (red) wavelengths. Camera lenses are made to correct or minimize the impact of aberration. Lens may be corrected for two primary colors (achromatic lens) or even for three primary colors (apochromatic). Camera lenses are very complex “masterpieces.” A very good lens will show just very little if any aberration defects. However, poorly a designed lens will suffer from aberration a lot.
The lens is more affected by aberration usually in tele mode with the aperture wide open. Especially large zoom lenses will suffer from this because it is virtually impossible to make corrected lenses for all zoom values.
Digital cameras suffer more from aberration than film cameras because of the large density of pixels in CCD - the uncorrected light can propagate to dark neighbors making the aberration more visible in high contrast images.
So the best chance to catch chromatic aberration with your digital camera lens is to look at images with high luminance contrast such as looking through tree on front of a bright sky etc.
The color of the aberration varies from blue to purple.
The Chromatic Aberration Auto-Removal tries to fix the aberration problems. The process may also change the appearance of some colors on your image (In many cases it is usually toward better looking colors).
If the image suffers just a little from the aberration, it is probably safer to leave it as it is unless you like the color effect of the result.
The only thing you need to do is to choose the color which best matches a typical aberration of your lens. In many cases it will be blue. Once you are satisfied with the result, you don't need to change it for other images from that camera - the lens always performs the same way.
It is true that chromatic aberration will tell you a lot about the quality of your lens (and also the camera). In a very good camera you will probably have a hard time finding an image which shows aberration problems. If you look at the sample on the top of this page, and if you have very good lens, this is probably the worst case which will happen. (And you will have to go through many photos to find one which exhibits it). Lenses on cameras such as Sony 505v, Canon S300, S10, S20 etc. are proven to be the highest quality with very few aberration problems.
Tips:
The removing of aberration is one task, but you may experiment with this function also on any other image just to get a bit different color look.
Note:
The human eye is also a simple lens and as such it theoretically must suffer from chromatic aberration as well. However, we don’t see such problems partially because our brain seems to filter any unwanted “data” and also a yellow pigment in the fovea helps to correct this problem by absorbing blue light.