|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.
|This Article Features Photo Zoom|
|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.
Other Aberrations To Be Aware Of
There are other types of aberrations that every photographer should be aware of. Spherical aberration occurs when light rays traveling parallel to the optical axis, but at different distances from the center, don’t converge on the same point. Special-effects lenses, like the Lensbaby family, exploit this characteristic to produce creative blur.
Curvature of field causes flat objects to project curved images. This is especially problematic when doing document copying or close-up photography. Macro lenses are highly corrected for this aberration. Pincushion and barrel distortion, as their names imply, cause objects to morph as if being squeezed or pinched. Very wide-angle lenses are often unjustly blamed for causing parallel lines to converge. It’s true that the lens itself is sometimes responsible, but as often as not it’s the photographer who fails to keep the camera and lens in correct alignment.
When light meets a small opening—the aperture diaphragm in a lens, for instance—the waves spread out. That phenomenon is called diffraction, and that’s why stopping a lens down to ƒ/22 or even ƒ/16 can produce worse results than shooting at ƒ/4 or ƒ/5.6, depth of field aside. Conversely, when light hits a small object, it appears to bend around it. That’s diffraction, too, but it plays virtually no role in photography except, perhaps, to explain why that speck of dust on the front element doesn’t show up in the pictures.