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Shoot Expertise: HD Reveals All

Aug 1, 2007 12:00 PM, By Barry Braverman

What is chromatic aberration compensation?


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The $14,000 2/3in. Panasonic AG-HPX500 is the first of a new breed of camcorders with interchangeable lenses and chromatic aberration compensation. CAC can be very effective in improving the performance of less than stellar HD optics.

Along with higher resolution and enhanced picture detail, HD increases the visibility of a lens' shortcomings. The most egregious may be chromatic aberration (CA), an ugly fringing artifact found to some degree in all zoom lenses. For HD shooters, CA has emerged as a significant occupational hazard.

Chromatic aberration, sometimes referred to as a shading artifact, can vary substantially from lens model to lens model. It will commonly show up in overexposed light sources, such as streetlights or along the horizon line at dusk. This issue is critical now for HD shooters because while CA has always been present in SD recordings, sloppy NTSC or PAL edges routinely covered the cursed artifact, rendering this particular concern in most SD lenses moot.

High definition, on the other hand, is a raster of another color, and with the advent of large 16:9 displays, we find a heightened visibility of chromatic aberration defects owing to the increased magnification.

For HD shooters, chromatic aberration (CA) is a serious, potentially lethal artifact as seen on the horizon line above. CA is not art!

Panasonic's unusual approach to chromatic aberration correction is best understood by first looking at one of the most popular low-cost cameras, the AG-HVX200. This camera can capture remarkable HD images despite its relative low price, medium-resolution imager, and modest prosumer-grade optics. This is in large part because the $6,000 camcorder understands the shortcomings of its non-interchangeable optics and can therefore compensate in order to achieve satisfactory lens performance. One doesn't find in the HVX, for example, the typical breathing of focus one might expect when zooming through a low-cost 13X lens. Why? The soft spots identified by the camera manufacturer are mapped out as a function of the camera's image processing.

Chromatic aberration compensation (CAC) may significantly reduce the ugly fringing that would otherwise be apparent in this scene. CAC does not obviate the need, however, for better optics, because other areas of concern—breathing of focus, contrast, and flare—are not addressed.

Now, Panasonic's HPX500 camcorder extends this notion of camera-applied compensation to embrace the challenge of interchangeable lenses. In this $14,000 MSRP 2/3in. camera, Panasonic has uploaded the performance details for four currently available CAC lenses from Canon (two 16X models) and Fujinon (two 17X models). When a CAC lens is mounted on the HPX500 (also the HPX2000), the camera recognizes the make and model, and then applies the required compensation profile from a lookup table (LUT) stored in the camera's memory. In my experience, the performance gains in sub-$10,000 lenses can be substantial, with modest low-cost optics suddenly exhibiting the clean look characteristic of lenses costing twice as much.

You get what you pay for. What's true in life is especially so in lenses. If sub-$10,000 lenses are softer than pricier options, this is due largely to chromatic aberration. It was logical that Panasonic would first approach the major lens manufacturers with respect to CA, then later tackle other serious problems, such as modulation transfer function (MTF). But from a lens manufacturer's perspective, chromatic aberration is extremely tough to control, the fringing being much worse in lower-cost lenses due to the inherent compromises in design and materials. Addressing CA in the camera processing acknowledges the extreme complexity inherent in a modern zoom lens; no other approach would appear to be practical given the constraints in lower-priced optics. Shooters can disable CAC by deleting the profile in the camera or unplugging the lens, but there's not much value in disabling CAC, and it's generally not recommended. Still, shooters are an irksome bunch, and someone will always ask. Otherwise, the CAC profiles cannot be modified.

Currently, full compensation for chromatic aberration, breathing of focus, and other defects can only be achieved in auto-focus, servo-controlled lenses. Fujinon is already applying such broad compensation in some box lenses. The zoom group in these lenses, for example, compensates automatically for shifts in image size while focusing.

The Panasonic HPX500 identifies a particular lens model and then applies the necessary compensation from the appropriate onboard stored look-up table. The camera ships with four profiles to match the four currently available CAC-compatible lenses from Canon and Fujinon. This list can be expanded in the future as more CAC lenses become available.

Price sensitivity in lower-cost camera packages generally precludes the matching of an individual lens to a distinct profile. The technology in the HPX500 limits the amount of camera-applied compensation to lenses of a particular product model or group. In the future, one might expect a unique profile to be stored in camera — one that references an individual lens by serial number, which is something that might be feasible in higher-end products using CAC.

Chromatic aberration is a serious menace to HD shooters. Part and parcel to some degree of all lenses, regardless of price, CAC can't magically transform mediocre lenses into stellar ones — but it can go far to alleviate the most egregious fringing in scenes that would otherwise be unusable.

CAC and other strategies that use the camera's processor to improve lens performance will almost certainly become de rigeur in the future as imagers achieve higher resolution and the need for compensation becomes more critical. Indeed, NHK's Ultra HD with 8K resolution would be unwatchable without some sort of onboard compensation, so great and apparent even the slightest chromatic shift would be.

Clearly, the time has come for HD shooters to wake up and smell the CA. As pricier optics are not always a viable option, the principal of chromatic aberration compensation becomes all the more critical in order to extract the best possible performance from low- and mid-level quality lenses. With CAC and the HPX500, Panasonic has introduced a clever and practical solution to the chromatic aberration menace. Where Panasonic goes in this case, others are sure to follow.


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