Chromatic Aberration

The Chromatic Aberration transformation lets you remove color fringes resulting from poorly color-corrected camera lenses. This is especially common when using long focal length lenses which do not incorporate expensive, ED (extra dispersion) glass elements or extreme wide-angle lenses. Chromatic aberration shows up as color fringes most easily seen where there is a sharp edge between light and dark parts of an image. The fringes are absent in the center of the image and become increasingly pronounced the further you move toward the corners of the image. The color gradients of the fringes are oriented radially out from the center. Unlike purple fringing, chromatic aberration has colors on both sides of the edges, one the complement of the other. For example, fringes will be red/cyan, green/magenta or yellow/blue.

Newer digital cameras correct for chromatic aberration when writing JPEGs, and the better RAW converters have camera and lens databases that let them correct for chromatic aberration during the conversion process. If you are using an unsupported camera and lens combination, however, then these options may not be available and the Chromatic Aberration transformation can be used instead.

Chromatic aberration arises from the fact that different colors of light are bent at different angles by most glass lenses. This leads to very slight differences in the effective focal lengths of the lens for red, green, and blue light. This in turn causes the red, green, and blue channels of a digital color image to have slightly different magnifications. While all three channels line up in the center of the image, as you move toward the corners of the image, the increasing spread between the different color layers in the image gives rise to color fringes. Chromatic aberration can be corrected by slightly altering the magnifications of the blue and red channels so that their size matches that of the green channel (most lenses are optimized for green light because the eye is most sensitive at this wavelength). Typically the red channel requires the most magnification and the blue channel quite a bit less and in the opposite direction.

Red Shift and Blue Shift. These sliders let you control how much the red and blue channels of the color image are magnified (while the green channel remains fixed). A shift of zero leaves the corresponding channel unmodified. The larger the shift setting the more magnification is applied; negative settings shift the fringes in the opposite direction. Since different optical systems can have different characteristics you will have to experiment to get the best settings.

Optical Center. A crosshair marking the optical center of the input image is displayed as an overlay over the input image. Initially, this marker is placed in the center of the image window. For the most accurate possible correction of chromatic aberration, it is important that this marker be positioned at the point in the image in line with the optical axis of the lens. You can adjust the position of the marker, if necessary, by clicking and dragging it in the input image. 

Tips:

Start with an un-cropped image of the entire frame so you know where the optical center is.

Touch up any dust specks, scratches or other image imperfections before attempting to remove chromatic aberration, otherwise you will introduce color fringes around each one.

To adjust the sliders, first zoom the preview image to 1:1. Then scroll the preview to one of the edges or corners of the image where there are clearly visible color fringes. As you adjust the sliders, the preview image will show you right away if the color fringes are getting better or worse.

Start by increasing the Red Shift slowly until the red/cyan fringes disappear; then adjust the Blue Shift until any remaining blue/yellow fringes disappear. To correct green/magenta fringes, you need to adjust both the red and blue shift values at the same time. Repeat as necessary until you get the best possible correction.