Chromatic Aberration

Chromatic aberration appears in all cameras and even telescopes.  The aberration appears as a color fringe separating a light and dark object.   Lens manufacturers pour money into reducing or eliminating chromatic aberration.  Digital cameras produce another type of fringe which is caused by the method of reconstructing the colors at each photosensor.

All lenses bend the red, green and blue wavelengths slightly differently.  Therefore each color is focused at a different point.  In the example below, a white point is projected onto the film plane (the small point on the right represents the image viewed straight on).  In this example, the green wavelength is focused precisely on the film plane.  The blue is focused in front because it is bent with a sharper angle, and the red is focused behind the film plane.  Where the red and blue merge on the film, a magenta color is produced (red + blue = magenta).  Where all three colors meet, white is produced (red + green + blue = white).  Astronomers were the first to notice this while trying to focus on a white point of light on a black background.  A small purple (magenta) halo appeared around the star (top image).  Lens manufacturers realized that a concave lens could cause all the wavelengths of light to converge back to a single point by using the same aberrant properties of glass but in the opposite direction (bottom image).  Unfortunately lens manufacturers have not completely eliminated this problem, especially with wide angle lenses. Chromatic aberration can be reduced by stopping down the lens, reducing the size of the cone (see depth of field)

Digital camera owners have to deal with another source of aberration which is unrelated to the lens system.  each CCD sensor can distinguish only one color because a red, green or blue filter is placed in front of a sensor, but all trhee colorsmay have reached a sensor.  A program in the camera then reconstructs the three colors of a pixel by analyzing the colors adjacent to each pixel. Imagine a black and a white object contacting each other. Within the black object, all sensors surrounding a pixel will not be stimulated by light and will not affect the average, and within a white object all sensors surrounding a pixel will be maximally stimulated and will not affect the white. A problem, however, occurs at the border between white and black. The internal program will try to guess the color for a pixel by analyzing the surrounding pixels. In this region, however, some of the sensors will show black and some will show white. The averaging of red, green and blue pixels at the border will produce a color fringe, usually magenta (purple). Fortunately, the average photo with mixed lighting will not display this purple fringe.  Now, is it possible to correct this with an image editor?  Not really, but you can make the problem less objectionable. 

The enlargement of the bird sitting on a fence (taken with a long lens) shows a purple fringe around the cotton nest material in the bird's mouth.  Enlarging even further, we can see the extent of the problem (top two images).  I should point out that in the original image, before enlarging, it was barely noticeable.  Since there is no magenta in this photo except that produced by the aberration, we can remove magenta without affecting the color balance of the photo.  Choose Image/Adjust/Hue Saturation in Photoshop or other image editor.  Select magenta from the drop down menu, and move the saturation slider to the left (desaturate).  Desaturating does not get rid of the fringe, but it makes it grey not magenta (bottom two images).  You might also adjust the lightness slider to lighten or darken the shade of grey to match the tonal value of the object behind the fringe.  For this example, the bird is grey so you don't even notice a grey fringe on a grey bird.  The small amount of red to the right of the nest material is not magenta so it was not affected.  The object, however, does not have to be grey to benefit from this procedure.  Replacing magenta with grey (a neutral color) will improve any image having chromatic aberration.  Fortunately, most RGB photos have little magenta in them, however, even if there is a lot of magenta in the photo, you may still benefit from this procedure.  Create a marquee around the area having the chromatic aberration.  Since the mask that you just created will shield the rest of the image, you are free to remove the magenta hue from the exposed area.  In this example, I could have created a circular mask around the cotton material, including the magenta portion, and removed magenta from this circular area only, leaving the rest of the photo untouched.