Chromatic Aberration

Chromatic aberration is an ugly effect that manifests as colored fringes around high contrast areas of photographs.

Here is an example of chromatic aberration. This shot was taken with a Canon Powershot S2 IS at maximum zoom on a day with direct sunlight.



Normally, chromatic aberration is non-existent in the very center of an image and gets worse as you move towards the edges. This is not the case in this example only because it was cropped from a larger picture.

Lenses work by refracting light. Shorter wavelengths are refracted to a less degree than longer wavelengths. This causes light at the red end of the spectrum to not focus in the same place as light at the blue end of the spectrum. This manifests itself as a red fringe on one side of a high contrast line and a blue or blue-green fringe around the other. The blue-green fringes are often less noticeable.

Various techniques are used in modern lenses to reduce this effect. Special "achromatic" glasses which have refractive indexes that vary with wavelength reduce this effect. Also, lens elements can be arranged such that one element cancels chromatic aberration induced in another.

There is a broad range of lens qualities, the old adage that you get what you pay for applies. Generally speaking, chromatic aberration gets worse with focal length because there is more magnification to magnify any aberration in the first elements. With telephoto lenses, reflector lenses (mirrors) don't have chromatic aberration, refractive (glass) lenses do. Glass lenses have higher contrast than reflector lenses.

Zoom lenses are more problematic than fixed lenses because of the changing relationship of the elements which makes it difficult to use the aberration in one element to cancel that of another. The Canon S2 IS is particularly bad in this regard because it has a 12x optical zoom with a max focal length of 432 mm.

That said; I really wish I had a decent SLR with a set of decent fixed focal length lenses because on the Canon Powershot S2 IS this problem is substantial and ruins a lot of what could have been good photos.

Here is another example of chromatic aberration followed by attempted software fixes.



Paintshop Pro X has a feature called chromatic aberration removal (I'm sure Photoshop and other similar programs also have this but I'm too poor for Photoshop, Paintshop is inexpensive). To use it, you invoke it then select the fringe area. It removes and tries to extrapolate the correct color where the fringe was.

Two bad things often happen. One, the color of the fringe includes colors close to colors in the photo and they get removed as well. Here is an example.



The other thing that can happen is that the software can substitute the wrong color when it replaces the fringe.

I took another shot at it and was able to remove most of the fringes and only do minor damage to the skin.



I was able to do less damage by selecting different points in the fringe which had a slightly different color and the overall results were better. Some images just plain are not repairable and I wouldn't consider this one repaired sufficiently for anything but the most casual purposes.