Fighting Chromatic Aberration

I use good glass. Still, sometimes chromatic aberration rears its ugly head. Even with quality lenses, sometimes I see color fringing at the edges of my subjects. And sometimes it’s obnoxiously bright! So how can I keep it from appearing, and can I fix it when it does?

– First, what exactly is chromatic aberration? Also known as color fringing or purple fringe, technically it’s when light of different wavelengths (red, blue and green) is focused at different points on the sensor, which translates to the colors effectively being “out of register.” At the ultraviolet end of the spectrum (i.e. near purple) lenses are most vulnerable to this chromatic aberration, and it shows up around the edges of darker image elements against lighter backgrounds—areas of high contrast. Fringe can actually happen with almost any color, although purple is the most—for lack of a better word—popular. When you’ve got it, you’ll know it.

After Fixing

– The first way to minimize chromatic aberration is to use a high-quality lens designed to minimize chromatic aberration. Special optical coatings are designed to minimize reflections from glass to glass and the sensor to the glass, which in turn helps focus all the color wavelengths to focus at the exact same point. Optical improvements, too, help minimize the aberrations that occur when light passes through glass.

– Another way to minimize chromatic aberration is to use a UV filter on the lens. Since it’s ultraviolet light that’s bending in a funny way and outlining image elements, eliminating the ultraviolet light largely eliminates the problem. Some say even polarizer filters will cut the appearance of fringing, which makes sense if they’re able to cut UV light too. If you see fringe appearing on the LCD preview, consider adding a UV filter to your lens.

– The dreaded purple fringe is most pronounced when shooting with a wide-open aperture, or at least large apertures such as f/1.4, f/2, f/2.8, etc. If you see signs of purple fringing, consider switching the aperture to f/8, f/11, or f/16 and adjusting the shutter speed and/or ISO accordingly. This goes a long way to minimizing purple fringe.

– Of course, you could always avoid shooting subjects against a bright background, and avoid overexposing those images. That might not eliminate chromatic aberration but it should hide it fairly effectively. If you’d like to keep shooting the subject you want in the way you want, purple fringe might be inevitable. To remedy that, you’ll have to turn to the computer.

– In Lightroom, RAW files can have their chromatic aberration reduced with a tool found in the Develop Module’s Lens Corrections pane. By simply checking the box, Lightroom will automatically work to identify and correct color fringing. Further adjustments can be made simply by increasing or decreasing the sliders for amount and color. Make sure you’re zoomed in close enough to see the purple fringe as you make these edits. 200% is a good place to start.

– In Photoshop, the controls are similar, but they’re found under the Filter menu’s Lens Corrections heading. They offer auto correction options, and they also provide a bit more control with custom fixes to various colors of aberration that can occur. As with any automatic approach, the more custom controls you have, the better the results will usually be.

– Another approach in Photoshop is to go all manual and use the clone stamp to paint away particularly aggressive purple fringe. Match the brush shape to the edge you’re working on (a hard edge requires a hard brush, and vice versa) and alt-click to set a starting point and then click away to hide the fringe. On particularly tricky fringes, consider using the Clone Stamp’s Color mode, or even Lighten or Darken settings, in order to target the fringe specifically. Further, you can use the color channels to isolate the fringe color and eliminate it with the same tools but limited to the channel on which the fringe appears. For instance, for a bluish fringe, you can select the blue channel and then simply paint on it with a brush the way you might paint on a layer mask to hide or reveal more of the blue channel—so you can watch as right before your eyes the fringe disappears.

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