If you've been taking photographs for long enough, you've probably heard of chromatic aberration. It actually goes by a couple of different names. The first one, "chromatic aberration" is the technical term. It seems a bit overly-technical, really, as if it’s just meant to make beginners scratch their heads. The second name for chromatic aberration is quite the opposite. "Purple fringing" sounds like something you might find on the sleeves of a jacket from the 1960s, not like something that has anything to do with photography. So if you are confused about chromatic aberration or purple fringing, keep reading. A little explanation will go along way.
Definitions ”Chromatic” means “of, related to, or produced by color.” “Aberration” means “a departure from what is normal, usual, or expected.” In short, chromatic aberration is an area of color that doesn’t belong. Technically speaking, chromatic aberration happens when different wavelengths of electromagnetic radiation are refracted through slightly different angles.
There are actually two types of chromatic aberration — the first is “axial” chromatic aberration, which causes blurred colors in front of and behind the focus point. The other type is called “lateral” chromatic aberration, and it is only seen at the edges of an image, usually in areas of high contrast. This is the type of chromatic aberration that you’ll also hear called “fringing.” It is typically the most noticeable variety and the one I’m going to discuss in this article.
Famous Purple Fringing by Flickr user Flop Eared Mule
It’s not really important for you to understand the hows of chromatic aberration, but you do need to know something about the whys. Chromatic aberration is a symptom of your lens—as a general rule, it tends to appear more often in less expensive lenses, which I’m sure is not a hugely surprising revelation. Manufacturers use a low-dispersion glass in higher end lenses, which helps reduce chromatic aberration, though it can never really be completely eliminated in all situations.
But less expensive lenses aren’t made with low-dispersion glass, which means that they may do a poor job of focusing all of the colors in a scene to the same point of convergence. When you’ve got an area of dark tones against an area of lighter tones (you commonly see chromatic aberration at the division between the sky and a darker structure such as a tree branch), you may get a sort of smearing of color right along that border. That’s where the term “purple fringing” comes from, although the color isn’t always purple.
Chromatic aberration is actually really common, though it’s possible you’ve never actually noticed it. A lot of hobby photographers never look at their images at 100 percent zoom, which means that details like chromatic aberration often go completely unnoticed.
Even though it’s not the sort of thing you’re likely to spot immediately when looking at your digital photos, it’s still really important to learn what to look for and how to correct it. Remember that flaws are sometimes more apparent in print images than they are in digital images, and you don’t want to wait until you’ve spent a few bucks on an 8x10 before you notice the rampant purple fringing. It’s good practice to examine every one of your best shots at 100 percent, paying special attention to those areas of high contrast. When you spot purple fringing (or blue fringing, green fringing or any other sort of color fringing), you need to take steps to remove it.
[ Top image lighthouse by Flickr user jovike]
What You Can Do
The first and most obvious way to address this problem is to buy an expensive lens. But I’m not going to tell you to do that, because expensive lenses aren’t in everyone’s budget.
Modern DSLRs sometimes have chromatic aberration correction built-in. Newer Nikon DSLRs, for example, will automatically correct chromatic aberration without any input from you whatsoever. So if you’re in the market for a new DSLR it may be one of those features you want to put on your must-have list.
Fortunately, there's another way to fix chromatic aberration without upgrading your camera.
Fix it in post
Unlike focusing problems, burned-out highlights and severe underexposure, this is one of those problems that really can only be corrected in post-processing. Fortunately it is generally a pretty easy fix—most good post-processing packages have a function that corrects specifically for this error. In Photoshop, go to Filter > Lens Correction and select the “Custom” tab. Look for the chromatic aberration sliders—they appear as follows: Fix Red/Cyan Fringe, Fix Green/Magenta Fringe, Fix Blue/Yellow Fringe. To use them, determine what color the fringe is and then move the associated slider until you see the fringe start to fade. Note that sometimes you may have more than one color in the fringing—for example, chromatic aberration that appears blue may actually contain both blue and cyan tones, so to completely remove it you will need to move both sliders. Make sure you’re viewing your image at 100 percent or larger when you make these adjustments—often there are only a few pixels involved so you’ll need to be pretty close to make a judgment call about how well your adjustments are working.
dfx V2 by Flickr user ken--
If you’re not getting good results using the dedicated filter, you can also fix chromatic aberration using the hue/saturation tool. Go to Image > Adjustments > Hue/Saturation and select the color of the fringe from the drop-down menu (for example, “magenta.”) Now use the eye dropper tool to click on the part of the image where the fringing occurs, and then move the saturation slider until the fringing starts to fade. You may need to select additional colors to get complete results, and you do need to be very careful, because if your photo has a lot of that color channel naturally in other areas, you may end up desaturating parts of the image you don’t want to desaturate.
[ Top image Hand In Hand by Flickr user bobafred]
You may not be able to avoid chromatic aberration, but the good news is that fixing it really isn’t much of a learning curve. In most cases a little bit of tweaking will greatly reduce or even eliminate the fringing, and you can do it without making it at all obvious to the viewer that there was ever a problem with the image. Not may photographic problems are that easy to fix in post—so although it does mean some extra time in front of your computer, you can rest assured that you won’t need to throw any money at the acquisition of a new lens in order to banish a little chromatic aberration from your life.
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