You’ve probably seen it. You’ve probably heard about it. You may not yet understand it, but you know that everybody hates it: it’s “chromatic aberration” (or, abbreviated as “CA”) and it’s a hot-button topic now that low-cost high-definition camcorders are everywhere. And CA is more prevalent on lower-cost equipment, so as more and more low-cost HD camcorders hit the market, we hear more and more complaints about “purple and green fringes.”

So what is CA, and why do we have to put up with it?

Okay, enough of the technoblabber, what does it look like and why is it bad?  Here’s an example of chromatic aberration, a pixel-for-pixel extraction taken from the lower right corner of the frame from a $3,000 HD camcorder.  You can see that there’s an extreme purple fringe on the left edge of the image, and a green fringe on the right.  This is classic chromatic aberration.

Here's another example of chromatic aberration, this time taken from a $6,000 HDV camcorder.  On the right you’ll find a pixel-for-pixel shot of a goose’s neck.  Again, notice the purple fringing on the left, and green fringing on the right. -->

And here’s an example of chromatic aberration from a $25,000 HD camcorder. This time, the fringing is orange rather than purple, but it still shows as a telltale green color fringe on the right side of the left eraser, and a huge orange fringe on the left side of the right eraser.

As you can see, chromatic aberration can manifest itself in a variety of circumstances, at a variety of price levels. The key factor is that all these shots originate from lenses that are at the bottom of their respective price categories. Expensive HD lenses (i.e., $60,000 and up) don’t exhibit these chromatic aberrations nearly as frequently, or nearly as visibly, as these examples above (indeed, a large part of the price of an expensive lens goes to minimizing or eliminating these very artifacts.)

Expensive, top-end professional lenses use layers and layers of precisely-applied coatings to correct for these aberrations. It’s a slow, painstaking, and intensive process. It is this coating process that is largely responsible for the high price tag of premium high-definition lenses.

So does that mean that those with smaller budgets are doomed to forever suffer the fringing and artifacting as shown in the pictures above? Not anymore – there is usually more than one way to solve any particular problem, and now there’s a new technology that addresses the problem. What used to have to be done at the lens level (painstakingly, expensively, optically correcting the aberrations) can now be largely compensated for digitally.

Chromatic Aberration Compensation (“CAC” Technology)

Look at these two pictures side by side.

Those pictures were taken from an HPX500 camera with a Fujinon lens, using the newly-developed Chromatic Aberration Compensation (“CAC”) technology. This “CAC” function is currently available on certain Canon and Fujinon lenses, and will be available on a variety of camcorders. The HPX500 is the first camera to offer it, but Panasonic has announced that the technology will also be incorporated in their high-end HPX3000 and the technology is available to other manufacturers as well. The CAC function is a way to digitally compensate for the fringing artifacts, rather than optically.

I spent quite a bit of time speaking to a Fujinon representative about the CAC lenses. A common misconception is that the CAC lenses are really just standard-definition glass, but that’s not true. The CAC lenses use the same high-definition glass and have the same resolving power of the more-expensive lenses in the same family, but the CAC lenses are less expensive because they don’t undergo the costly coating process to correct for the aberrations.

Instead, the lens manufacturer makes an extensive map of the properties of these lenses. They mathematically calculate where the aberrations will occur. They factor in the combinations of focal length, aperture, and focus distance, and create a detailed list of the conditions where the aberrations will occur. This detailed map of the lens properties is supplied to the camera via a downloadable data file. For current and future CAC lenses you can download a map file of that particular lens’s unique aberration properties, so the CAC technology can grow to include many future lenses as well.

The CAC lenses are digital lenses and communicate the lens position, focus distance, and iris values to the camera through the lens connector cable. Armed with that information, the camera’s digital signal processor is now able to know exactly what aberrations have been introduced into the image, and the camera’s DSP can now compensate for these artifacts. I haven’t been able to determine whether the DSP compensates for the artifact during the analog sampling phase, or afterwards (as a digital manipulation) but in either case, we can see the results and the results are that it works quite well.

To create those eraser shots above, I used the same identical lens for both. I experimented by plugging in and unplugging the lens cable and comparing the fringing. I’d unplug the lens and pan around and change zoom and focus until there was obvious chromatic aberration in the image. Then I’d plug the lens back in (thus activating the CAC function) and it always cleaned up the image significantly. It’s quite impressive, and given the choice of buying the CAC-enabled version of a specific lens or saving a few bucks to get the non-CAC version, I’d definitely spring for the minor cost premium to get the CAC functionality. It makes a noticeable difference in the overall quality of the images.

The CAC function doesn’t necessarily eliminate all fringing, but it certainly improves it noticeably, in exchange for a little cost premium for the lens (the actual premium is determined by each lens manufacturer of course). I think this would be a fantastic technology to trickle down to the fixed-lens cameras; it seems ideally suited to the concept of a built-in lens, and this innovation could lead to minimizing or perhaps even eliminating the purple and green fringing around objects, which is quickly becoming a telltale sign that the shooter probably shot on a lower-cost camera.

BUT IS IT CHROMATIC ABERRATION? OR FLARE/OVEREXPOSURE?

Take a look at this picture of geese. Look at the massive purple halo around the geese.
Have you ever seen worse chromatic aberration before?

Actually, yes you have. Because this isn’t chromatic aberration at all! It’s purple, yes. And it’s a fringe, yes. But it’s not a chromatic aberration. This is instead an entirely different artifact, caused by overexposure. One way you can tell is because it only affects pure-white objects (or blown-out objects), and also it’s on both sides of the object (chromatic aberration will exhibit a specific color fringe on one side of the object, and the opposite color fringe on the other side of the object). In this example you’re seeing purple fringe above, below, to the right and to the left. That’s not a property of chromatic aberration, so this particular fringe isn’t CA; it’s actually caused by overexposure. You’ll usually see this purple fringing around objects that are “blown out,” so you’ll see it in shots where there are bright white clouds, overexposed windows, or any other overly exposed section in the shot. It manifests itself on the border between properly-exposed and overexposed. And unlike chromatic aberration, this overexposure fringe seems to always be purple.

CAC can’t fix this. Because it’s not a lens property, there’s nothing the CAC function can do to control it. High-end professional cameras have “lens flare” setup menus which are designed to instruct the camera in how to compensate for this particular artifact. Lower-cost HD camcorders typically don’t have those menu settings, so they don’t give you that option. To fight this particular purple fringe on cameras that don’t offer proper flare control menu settings, you really only have one option: don’t let the image blow out. Control your exposure, bring up the light on your subject so you can bring down the exposure on the white/bright objects, and consider setting your knee control more aggressively to limit the amount of image area that overexposes. This kind of artifact is going to show up most frequently on exterior shots with clouds or blown-out skies, or in shots with windows or light fixtures in the shot, and it’ll also show up in jewelry photography or other types of photography where there are pinpoints of overexposed light in the shot. There’s nothing the CAC function can do; eliminating this artifact is really up to your skills in controlling the light to prevent overexposure.

In summary, CAC is a great new technology that will be available from multiple camera manufacturers. It promises to further improve overall image quality while keeping prices headed lower and lower. For those who can afford premium glass, it’s probably still better to have the aberrations corrected for optically – but for those on the lower end of the price scale, CAC is a promising new development that will help us overcome that telltale signature of low-cost video origination: purple & green fringing.