Wednesday, March 17, 2010
All About Fringe
I’ve heard lots of great photographers talk about color fringing in their digital image files, but alas I’ve never noticed it myself. Until now. I was recently wandering through the city park one winter day when a woman pointed me to a tall tree just a few hundred yards away, in the top of which were perched two beautiful bald eagles. I was armed only with a APS-C sensor and a 200mm-equivalent zoom—not ideal for wildlife, especially bald eagles that would have preferred a 600mm image stabilized extreme telephoto. When I got the pictures into the computer, I quickly zoomed in and cropped to create a composition in which the eagles practically filled the frame. Horror of horrors what did I see? Bright purple fringes at the high contrast edges of the objects in the frame. I did a bit of research and found out there’s some misinformation out there along with the real facts about fringe, and it can be difficult to discern the difference. So here I’ll try to briefly set the record straight.
There are two primary causes of color fringe. There’s chromatic aberration, which comes from lower-quality lenses or extreme zooms used at their extremes. Different colors of light are focused at slightly different distances, and if a lens isn’t optically optimized to compensate, fringe can appear.
The other cause is bichrominance. It’s the “purple fringe” that people still debate the exact cause of, but conventional wisdom seems to be it happens more on small sensors with densely packed pixels. It would appear that microlenses don’t focus all light evenly, and sometimes stray magenta (i.e. purple) light can be misallocated into adjacent pixels. The problem is most evident in edges of extreme contrast—say a dark tree branch backlit by bright sky. That is, in fact, where I saw it. (In fairness, the fringe I found was fairly minimal. In some cases it’s extreme and hugely distracting.)
To prevent color fringe, you can use lenses coated to minimize aberrations. You can also limit yourself to prime lenses or smaller zoom ranges, as they’re less prone to chromatic aberration since they’re not so “extreme” in their capabilities. Of course, that’s a sacrifice too.
Another helpful prevention is to avoid shooting wide open (like f/2) or stopped down (like f/32). Somewhere in the middle is bound to be the sharpest aperture anyway, and you’re going to lessen the possibility of color fringe there too. Some photographers even suggest using a UV filter on the lens to minimize ultraviolet (magenta/purple) light—thereby minimizing the opportunity to have excess purple light to create fringe.
When color fringe occurs—and it will—you’ve got a lot of options to repair it. Perhaps the easiest approach is to adjust noise reduction sliders in the RAW conversion. Lightroom, Aperture and Camera RAW all offer intuitive tools to minimize fringe. But even if you don’t shoot RAW, you can always process images in third-party programs like Noise Ninja which has a lens correction tool designed for precisely this purpose.
Photoshop also has a Lens Correction filter which works in much the same way. Or you can use simple hue/saturation sliders to reduce the appropriate channel’s saturation and lightness—in the case of purple fringe, work on the magenta channel. If other portions of the image contain purple tones you’d like to retain, though, you’ll have to selectively adjust the fringe alone by selecting a large area, the sky for instance, expanding that selection to include the fringe, and then removing the original selection to retain only the edges.
The takeaway from all of this is that fringe is bound to happen. When it does it may be barely noticeable or it could ruin the shot. But now you know what causes it, how you can work to prevent it and some simple approaches for eliminating it in post production.
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