Monday, June 21, 2010
All About Color Fringing—06/21/10
The causes and fixes for crippling chromatic aberration
COLOR FRINGE CAUSES
There are two main causes of color fringe—chromatic aberration and bichrominance. Chromatic aberration is seen most often in lesser-quality lenses or in zoom lenses at their widest or most telephoto extremes. Because different wavelengths (i.e. colors) of light focus at slightly different distances, lenses that don’t deal with this subtle distinction can amplify the problem and produce images with dramatic color aberrations. A lens with strong chromatic aberration characteristics will produce images with multiple color fringes and blurry, out-of-focus details.
Bichrominance is more commonly called “purple fringe” and it’s a somewhat more controversial topic. For years photographers have debated whether it’s simply a variation on chromatic aberration, but the conventional wisdom these days seems to be that purple fringe is caused by the interaction between microlenses (the tiny array of lenses positioned immediately above a camera’s sensor) and small pixels tightly packed together on small sensors. Magenta wavelengths of light spill from one microlens to an adjacent photosite that causes the errant purple halo. Microlenses primarily are designed to focus green wavelengths of light because green is given the most weight in the sharpness of an image made on a DSLR or point-and-shoot camera that utilizes the Bayer Pattern filter for CCD and CMOS sensors
Chromatic aberration fringe can be seen in any area on the sensor, but it’s most common—as is purple fringing—at the edges of high-contrast areas, most often when a dark edge element is strongly backlit. This is very commonly manifested, as it was in my experience, on tree branches silhouetted against a bright sky.
If you’re concerned with keeping color fringe at bay, use of a lens designed to minimize aberration is a good place to start. You know all those coatings you read about in lens specifications? Many of them are designed expressly for the purpose of minimizing chromatic aberration. Beyond simply selecting high-quality optics and coated lenses, consider lenses that don’t get too extreme with their zoom range since chromatic aberration most often occurs at the ultrawide and ultratelephoto ends of a lens’ focal range.
Purple fringing and chromatic aberration can each be minimized by avoiding shooting at the extremes of the aperture range too. Wide open (ƒ/2, for example) and stopped down (ƒ32, let’s say) are more likely to produce aberrations and color fringes than the lens’ sharpest aperture—typically two or three stops from wide open, most often somewhere around ƒ/5.6 or ƒ/8. You can test your lenses for sharpness by photographing a resolution chart or even fine newsprint to determine not only the sharpest aperture but also the one least likely to produce color fringing.
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