Chromatic Aberration

Foiling the Fringe in the Camera and in the Digital Darkroom

The Fringe

You know the old saying, "There is nothing that is certain except death, taxes and Chromatic Aberration". Chromatic Aberration (CA) is a physical property of light when focused by a lens and the result is varying degrees of color fringing which you may have noticed, especially on the edges of your images. 





From Wikipedia

The Physics
Ever since Newton passed sunlight through a prism, we have known that different wave lengths of light are bent to different angles as they pass through a lens. This is called dispersion and because a simple lens cannot focus all colors of light on the same plane (your camera's sensor), colored halos or fringing is created. This effect is seen most prominently on the edges of images and in areas of high contrast. Red and green bands are most common, but blue, yellow and purple may also be seen. This is all called Chromatic Aberration and although saying "Chromatic Aberration" may make you sound intelligent at the photo club meeting, the color banding can contaminate your images. Fortunately there is much that can be done to minimize or eliminate the fringe, both in the camera and in post processing.


In the Camera
Throw Money

As expected, more expensive lens typically do better with CA. 

From Wikipedia

special coatings and combinations of multiple lens elements can reduce and compensate for light dispersion. Achromatic and apochromatic lens also use special materials, such as fluorite, that have properties of low dispersion. Think of this when you are pondering how a lens could cost several thousand dollars. 


 

Avoid High Contrast Situations.
Creatively it is not always possible or desirable to limit high contrast in your images, but, especially on the edges, contrast control can make a big difference.

Stop Down
CA is more apparent at the widest apertures. Stopping down just a step or two from wide open may limit your beautiful bokeh, but it can reduce the CA contamination dramatically .

Avoid the Extremes of Zoom Lens
It is hard to engineer zoom lens that can offset dispersion across the full range of the zoom. You will usually get the best results somewhere in the middle of the range. Extreme wide angle lens may be especially susceptible to this effect.

Get Ready for the Digital Darkroom
There are a couple of things that can be done to get the image in the camera ready for the work that can be done in the digital darkroom. First, this another situation where shooting in RAW can make a big difference. The great flexibility of RAW images makes it easier to correct for CA in post processing programs such as Camera RAW. Also, one simple approach is to frame images with extra room around the edges which will then allow cropping of the worst areas of CA without sacrificing the composition.

In the Digital Darkroom 
 

Fading Crimson

You've done what you could in the field, but you still notice ugly fringing at the edges of your images. Fortunately, with every new version of Photoshop, the tools to control CA have become more effective and easier to use. Here I will be speaking specifically of Photoshop and the newest version of Camera RAW, although Lightroom approaches the problem in a similar manner. Other image editing software, such as Paintshop, have their own solutions to this problem. 

Lens Correction in Camera Raw

Fringed

In newer versions of Photoshop's Camera RAW correction of CA is largely automatic. Within the Lens Correction menu, the "Color" tab features a simple radio button for Chromatic Aberration control. It is almost magic. The program can reads the image meta data to determine the camera and lens used and in most circumstances applies the correct adjustment without further input. This is much easier than in earlier version in

Aberration Control

which individual red/green and blue/yellow sliders required careful manual adjustment. After the automatic adjustment, some fringing may still remain, especially the more difficult axial CA in the purple/green range. Below the automatic button, purple and green sliders allow fine tuning of the adjustment. In addition to controlling the amount of adjustment, the Hue slider can precisely target the color affected. 

Facing the Dawn, Rockport, Maine

Fringed Rungs

De-Fringed

Dealing with Chromatic Aberration can be frustrating, but, with awareness of the problem, adjustments in the camera and in the digital darkroom can be effective in foiling the fringe.
 

Solstice, Spofford, NH