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Monday, November 26, 2007

The Digital Lens Revisited

In a world of marketing buzzwords like 'optimized' and 'designed for digital,' what's really going on behind all the hype?

Chromatic Aberration
While the retro-focus design solved one problem, it created another problem typically associated with telephoto lenses—chromatic aberration or “color fringing.” Because lenses disperse the different colors of light, it causes them to focus at different points (Fig. 3). Telephoto lenses and wide-angles with an inverted-telephoto design are especially prone to this phenomenon.

Extra-low-dispersion (ED) and ultra-low-dispersion (UD) glass are the common solutions to this problem. ED and UD lens elements help keep each of the colors focused as close as possible to the same point in the image plane.

Many of Tamron's Di and Di II lenses have unique aspherical glass elements that substantially improve optical quality while eliminating chromatic aberrations that can degrade image contrast and sharpness. Stacie Errera of Tamron USA says, “These elements consist of LD or AD glass that's perfectly bonded to an aspherical layer of optical resin. Because of the fragile physical nature of the glass during the polishing and coating process, and the specific temperature required for molding the resin, engineers had to develop a brand-new method of production to make these special glass elements a reality.”

Richard Pelkowski, product manager of D-SLRs for Olympus, points out that dealing with color fringing also is more critical with digital capture than with film because of a sensor's micro lenses. “Chromatic aberration can be more of a critical issue with digital because you have micro lenses on your sensor that can be prone to producing chromatic aberrations,” says Pelkowski. “So you not only have to get all three colors to line up coming out of your primary optic, but then when it goes through the micro lenses on the image sensor, you've got to deal with that as well. So you want to keep problems with chromatic aberration to an absolute minimum with the taking lens, so when the light gets to the micro lenses, it doesn't get magnified there.”

In the case of Hasselblad, technical specialist Paul Claesson says they deal with chromatic aberrations in a different way. “We, or I should say Zeiss, never had to redesign the lenses to meet the needs for digital capture. We do, however, have our HC line of lenses for the H cameras, which are optimized for the parameters of digital capture, and they're more than sufficient for film capture as well. Any lens anomalies like color fringing or vignetting at the edges can be handled via software in post-capture.

“Dealing with aberrations this way was a conscious choice by the company,” adds Claesson. “We could have gone one of two different directions. We could have made a lot of these corrections physically inside the lens, which would have increased the size and weight of the lens, and also the cost of the lens to the consumer. Or we could use a good-quality lens and make improvements from a software application. That's the route that we chose to go.”