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Tuesday, October 22, 2013

Defeat Chromatic Aberration

How to banish color fringes from your photos forever

This Article Features Photo Zoom

Figure 5: The Custom tab gives you more controls. Figure 6: You can see the dramatic reduction in fringing from the custom settings.
For more than 500 years, engineers and scientists have been arranging concave and convex lens elements into different configurations to improve image quality. The first breakthrough improvement, the achromatic lens, employs lenses made of different types of glass and focuses two colors of light (generally, red and blue) on the same point. This lens design significantly reduces chromatic and spherical aberration, but doesn't entirely eliminate either. Apochromatic lenses, which we usually call "APOs," take correction one step further and bring three colors of light into nearly perfect focus on the same point.

All transparent material has an index of refraction that correlates to the angle light is bent as it passes through. A perfect vacuum has an index of 1.00 and fluorite about 1.40. Scientifically speaking, light travels 1.4 times slower through fluorite than through a vacuum. The tendency for different wavelengths to bend at different angles when passing through the same medium is called dispersion. When you hear lens manufacturers like Sigma or Tamron, for example, talking about Low Dispersion or Special Low Dispersion glass, they're referring to optical material that minimizes this effect. Lens designers reduce aberrations by selecting glass material of specific refractive indices.

The shape of the glass elements plays as important a role as the composition. We're all familiar with concave and convex lenses that bend light in or out. Concave lenses cause light waves to spread apart, or diverge. A convex lens—for example, a magnifying glass—causes light waves to converge. Both of these are classified as spherical because of how they're made.

In this diagram, you can see that chromatic aberration is caused by varying wavelengths of light (colors) coming to sharp focus at different points.
Most glass lenses are spherical and are ground and polished by rotating a glass blank on a central axis against a fixed abrasive surface. The process is similar in some ways to placing a finger upon a lump of soft clay as it spins on a potter's wheel—the resultant shape, unavoidably, is spherical. That's physics. Aspherical lenses, on the other hand, aren't uniformly shaped. While more expensive to make, aspherical lens elements allow designers to correct aberrations more effectively.

Many of the latest generation of point-and-shoot cameras, principally those with 24mm wide-angle zoom lenses, rely on complex algorithms and their internal image-processing engines to subdue chromatic aberration and other unpleasant aberrations. One of the reasons why cameras of this type often deliver outstanding results is their ability to automatically compensate for the lens' shortcomings—before you see the picture.

Subject selection and composition play a role, too. You can minimize the impact of chromatic aberration and other lens defects, but conscientious work habits alone can't prevent them entirely. Specifically, chromatic aberration is most visible when a dark edge is photographed against a bright background—a tree limb against the sky is the common example. Avoiding backlit subjects can improve lens performance. But from a practical standpoint, it's more efficient to learn how to use your image editor to remove the chromatic aberration artifacts during postprocessing.


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