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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

This photo of Mono Lake, California, shows a fair amount of chromatic aberration.

The Lens Corrections palette in Adobe Lightroom 5. Figure 1: You can see the fringing in this enlargement. Figure 2: In Photoshop, go to Filter > Lens Correction to pull up the chromatic aberration tools.
We've all seen it—that glowing violet fringe that sometimes lurks like a dirty halo in out-of-focus areas and along high-contrast boundaries of dark objects photographed against a bright sky. It's called chromatic aberration, and although it's most often visible in its purple robes, it can appear as other colors. Lateral chromatic aberration is seen as red-green or blue-yellow fringe. Axial chromatic aberration is purple or green. This unpleasant phenomenon occurs because it's very difficult to coax more than two colors of light to focus on the same point. We call it an "aberration," but in reality, it's natural for light to be uncooperative.

Think back to grade school when your science teacher showed you what a prism could do to a shaft of light. Remember how the seemingly white light broke down into a rainbow of seven brilliant colors? Red was on one side, and violet on the other. You learned the Roy G. Biv mnemonic to better recall the order of the colors. That's the natural sequence in the visible portion of the electromagnetic radiation spectrum. I went home after school that day thinking that prisms were special objects that had the power of leprechauns to produce rainbows, otherwise how could they conjure up color where none existed? I had completely missed the point—or my teacher had. What I actually had witnessed was an enormously amplified demonstration of what optical engineers call chromatic aberration.

Prisms can't change the nature or composition of light. You might say that a prism allows us to watch light doing what light is supposed to do. When light encounters a transparent object, it either bounces off (reflects), gets absorbed or passes through, usually a combination of all three. Snell's Law essentially tells us that when light passes through the boundary between two different types of media, it bends or refracts in a predictable way. Light changes speed as it moves from one medium to another, which is what makes eyeglasses, telescopes and camera lenses work.

Figure 3: The Photoshop Auto Correction does a decent job of reducing the fringing. Figure 4: You can see the reduced green and red edges.
Chromatic aberration occurs when a lens is unable to focus red, blue and green wavelengths on the same point. The color fringe is produced by light waves that spill over and don't align with the others.

There are other possible reasons for the presence of extraneous colors. Chromatic aberration always gets blamed for color fringing, and deservedly so. But it's not always the culprit. Color fringe can be caused by CCD charge leakage or by reflections that occur within the microlenses on the sensor itself. And, occasionally, it can be traced to the rear element of the lens reflecting off the CCD.

There are a number of ways color fringing can be minimized or eliminated entirely. The first line of defense is lens coating. Color bleed caused by lens-to-lens or lens-to-sensor reflections can be dramatically reduced by application of optical coating to the air-glass surfaces.


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