Chromatic aberration is one of the concepts that are important to understand because it relates both to being a better photographer and buying the right lenses. Ah, you see, those physics lessons you tried to avoid do have an effect on how you spend your money. That should grab your attention!
If you search one of the dusty corners of your brain, then you might remember that Isaac Newton theorized that white light is actually a combination of a spectrum of colors. Each color has a different wavelength. When light enters your camera’s lens and is then registered on the sensor, the goal is for those different wavelengths to all converge at a single focus point. This, however, is an ideal goal that is never perfectly achieved. Some of the colors don’t join precisely with the others. Instead of hitting the target focus point accurately, those uncooperative colors’ wavelengths focus either just in front or just behind the sensor. The waves of light (color) that don’t focus correctly are described as being dispersed. This dispersion is what causes chromatic aberration. “Chromatic” refers to color and “aberration,” as an irregularity or a deviation from the norm or the preferred occurrence.
Chromatic aberration, often abbreviated as CA in lens specifications, is visible as a colored fringe along the outline of an object in your photo. You’re likely to see blue, green, red or purple. Typically, lower quality lenses produce the most easily recognizable CA.
Manufacturers, such as Canon, Nikon and others, understand this principle and have designed “solutions” into their lenses to help reduce CA. Fluorite, a mineral, is added to the making of low dispersion glass elements, which retards the spreading of the color wavelengths. Obviously, a lens that produces less CA is an optimum choice for professional photographers and serious amateurs who want the sharpest and cleanest images. When purchasing a lens, therefore, it’s important to check the glass element types and configuration and to compare the results of third-party chromatic aberration tests.
You don’t have to rely totally on how a manufacturer designs and constructs a lens to reduce CA. You can also control its effect with these techniques.
1. If you find evidence of too much CA at a particularly lens aperture, then choose a narrower f/stop: f/3.5 instead of f/2.8.
2. Using a zoom lens at its greatest focal length (300mm on a 70–300mm lens) can contribute to chromatic aberration. Try to frame your photo, or move closer to the scene or subject, so you can capture the image at 200mm or 250mm.
3. Similar to #2 above is the use of super zoom lenses. These typically have a much greater focal length range, from wide angle, even ultra wide angle, to telephoto, than “common” zooms at 70–300mm. An example of a super zoom is an 18–200mm lens. Super zoom lenses are known for producing acute CA at those wide or ultra wide focal lengths.
4. Your photography equipment budget limitations may have made it necessary to buy a lens with a higher level of CA. If that is the case, then try to shoot scenes and subjects with lower contrast.
5. Even if you are shooting with a high quality lens with low occurrences of chromatic aberration, any dirt on the front or rear exposed glass element can cause CA. This is just one of many reasons to check and clean your lens regularly.
6. Although some chromatic aberration can be corrected in photo-editing software, it won’t rescue some of the image detail that is permanently loss due to CA. The best solution is shooting with the right lens or trying the techniques above.
Photograph by photographytalk.com member Tracey Stott
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