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What Causes Chromatic Aberration?
Lens dispersion causes chromatic aberration. The refractive index of the lens elements changes depending on the wavelength of light. In other words, different colors of light pass through a lens at different speeds, similar to how a prism separates white light into a rainbow. The refractive index of most transparent materials, such as the glass in lenses, decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, the refractive index variation affects focusing.
Glass lenses bend light rays, and blue rays bend more than red rays. With a simple lens, red light focuses behind green light, and blue light focuses in front of green light.
Designers have corrected for CA in many lenses, so they focus each wavelength at the same point, giving a high degree of color accuracy and registration. Chromatic aberrations still occur for faster lenses and capturing high contrast areas, such as a dark subject set against a bright background. A colored haze—typically purple, but sometimes red, blue, cyan, and green—appears on a subject’s edges, decreasing clarity and sharpness.
2 Types of Chromatic Aberration
A perfect lens would focus all wavelengths into a single focal point, where the best focus with the “circle of least confusion” is located. In reality, the refractive index for each wavelength is different in lenses. This causes two types of chromatic aberration that have different characteristics but may occur together:
- Longitudinal Chromatic Aberration. “LoCA,” “axial chromatic aberration,” or “bokeh fringing” occurs when each different wavelength of color focuses at a different distance from the lens (focus shift) and they do not converge at the same point after passing through a lens. With LoCA, color fringing is visible around subjects throughout the entire image, in the center as well as on the edges. Longitudinal aberration is typical at long focal lengths. Typically, fast aperture prime lenses—even high-end, expensive ones—are much more prone to LoCA than slower lenses.
- Lateral Chromatic Aberration. Also known as “transverse chromatic aberration” or “TCA,” lateral chromatic aberration occurs when different wavelengths of color focus on the same plane but at different points, due to the angle of light entering the lens and the magnification and/or distortion of the lens varying with wavelength. Unlike with LoCA, lateral chromatic aberrations are visible only at the edges of the frame, not in the center. Lateral aberration is typical at short focal lengths. It is most apparent in nonsymmetrical lenses like telephoto and reversed telephoto lenses.
What Is the Impact of Chromatic Aberration in Photography?
Chromatic aberrations can negatively impact image quality in a number of ways.
- Lens dispersion. Different colors of light traveling at different speeds while passing through a lens can make images appear blurred or create conspicuous red, green, blue, yellow, purple, and magenta edges around objects, especially when shooting in high contrast.
- Longitudinal chromatic aberration leads to color fringing around subjects throughout the entire image, in the center as well as on the edges.
- Lateral chromatic aberration causes purple fringing at the edges of the frame. It is only visible in areas of high contrast, but even when post-process correction makes purple fringing less noticeable, the effect still softens the image in the edges and corners.
11 Ways to Reduce Chromatic Aberration in Photography
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There are several ways you can fix chromatic aberration, either reducing or entirely eliminating it to improve your photographic images. Some fixes are common sense solutions while others are more technical.
- Avoid shooting high-contrast scenes.
- Center your subject in the middle of the frame to make your image TCA-free and crop in post to improve composition.
- When using zoom lenses, avoid using the shortest and longest focal length, which typically causes CA issues you need to address in postproduction.
- Change your color image to black and white.
- Use lenses made of low-dispersion glasses, especially those containing fluorite. They can significantly reduce chromatic aberration.
- To reduce LoCA, simply stop down your lens. Closing down your aperture decreases the amount of light that will reach your sensor, so you’ll have to compensate by decreasing your shutter speed and adjusting the ISO to achieve proper exposure.
- Alternatively, you can also use an achromatic lens, or achromat, which corrects to bring two wavelengths (usually red and blue) into focus on the same plane. The common achromatic doublet features two individual glass lenses with different amounts of dispersion. Typically, one of the doublet’s elements is an ultra-low dispersion glass. Apochromatic lenses, which can correct three different wavelengths of light, provide even better correction for LoCA.
- Fix lateral chromatic aberration using post-processing software like Photoshop or Adobe Lightroom. The latter features both automatic and manual correction for CA, including a “defringe tool” in the “Lens Corrections” module, and can either significantly reduce or completely eliminate this type of fringing. (This is key because stopping down your lens won’t reduce TCA, and, due to the angle rays enter the lens, a standard achromatic doublet won’t either.)
- Use cameras that have in-camera solutions that alleviate transverse aberration. Some cameras, such as the Panasonic Lumix series and newer Nikon and Sony DSLRs, feature a processing step specifically designed to remove purple fringing.
- To avoid lateral chromatic aberration, use a lens designed symmetrically about the stop/aperture ring.
- In general, use high-quality lenses. They will exhibit less CA than cheap lenses, fast lenses when used wide open, old legacy lenses, or cheap teleconverters and wide-angle converters, and cut down on your need for chromatic aberration correction.
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