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As digital SLR cameras grow in popularity, most manufacturers are working to develop additional lenses. An increasing number of the new products--primarily zooms--are designed exclusively for use with digital SLRs with the "APS-C" size sensors employed in most cameras. Such lenses (discussed in more detail later) are usually more compact because they need to cover only the smaller image circle of the smaller sensor. If used with a 35mm SLR, or a camera with a full-frame sensor (such as the Canon EOS-1Ds or Kodak Pro SLR) such lenses will produce vignetting, darkening at the edges, at some or all focal lengths. Naturally, some of the new products shown at photokina were multi-platform lenses, optimized for use with digital SLRs, but equally suitable for use with 35mm SLRs.

Digital-Optimization Technology
Virtually all new multi-platform lenses (and all digital-only lenses) are "digitally optimized," but that expression is somewhat generic. The specific features and technology used to achieve this goal varies from manufacturer to manufacturer. Even within a single brand, the strategy can vary from extensive with a pro lens, to minimal with an entry-level $199 zoom. Note, too, that the manufacturers are somewhat secretive about proprietary technology as well as the specific features applied to each of their digitally-optimized lenses.

But why do we need different lenses for digital SLRs than for 35mm cameras in the first place? Well, we don't need them per se, because virtually all conventional lenses will work for digital capture. On the other hand, they may not provide the best possible results, because lenses designed for 35mm SLR cameras pose a few problems in digital photography. These can be minimized or eliminated with digitally-optimized lenses.

The first problem is increased internal flare that can degrade contrast. In a worst-case scenario, "ghosting" can occur: reflections of the lens diaphragm that can mar images. Those effects can be caused by the very highly reflective nature of CMOS and CCD sensors and the protective glass cover over the chips. (The surface of film is not nearly as reflective.) In order to prevent these problems, multilayered antireflective coatings must be applied to the rear lens elements as an absolute minimum. Superior coatings on all elements are preferable and several optical manufacturers have developed new, more effective chemicals for this purpose.

The second issue is more complicated but worth understanding. Digital cameras produce the best results with light that strikes all the areas of an imaging sensor at a 90Þ angle. With conventional wide angle lenses, light can strike the edges of the sensor at increasingly more acute angles, reducing sharpness at the edges of a photo. This effect does not occur with long lenses and may be insignificant when any lens is used at a small aperture, such as f/11-f/22.

Finally, peripheral light falloff--darkening at the corners of the image--can be quite serious with some conventional lens designs when used on a digital SLR camera. Less light reaches pixels at the edges of the image area causing vignetting. Very little information is available as to the optical technology used to achieve "increased corner luminosity" by each manufacturer and it probably varies, depending on the lens type and the steps required for optimal results.

Note: Some digital SLR manufacturers employ in camera technology to minimize vignetting and loss of sharpness at the edges of the frame. Nonetheless, even those companies make digitally-optimized lenses, as well as digital-only lenses, that should provide even better results with their APS-C size sensors.

None of the digital-optimization strategies produce an adverse effect in 35mm photography. In fact, they should produce better silver-halide images: with less contrast-degrading flare as well as high edge sharpness and brightness, particularly when using short zooms at wide apertures. Does that mean that SLR camera owners (digital or 35mm) should sell off all their conventional lenses to buy digitally-optimized or digital-only models? Of course not. Still, the benefits are worth considering if you're shopping for a new lens and want superior image quality.

Multi-Platform Lenses
Sigma (www.sigma-photo.com) already markets six DG (Digitally Optimized) lenses for 35mm and digital SLR cameras, including three new models with wide apertures great for low-light shooting at lower ISO settings. The 24-70mm F2.8 EX DG MACRO incorporates premium-grade optics: two Special Low Dispersion (SLD) elements and three aspherical elements, to control all types of aberrations. (Street price, $429.) Substantially smaller and lighter (67mm filter; 18 oz), the new Sigma 28-70mm F2.8 EX DG zoom incorporates similar high-grade optics plus Super Multi Layer (SML) coating to control flare caused by highly reflective digital sensors. (Price not yet set.) A pro-grade lens, the new Sigma APO MACRO 150mm F2.8 EX DG HSM allows for true life-size or 1x magnification. Features include ultrasonic HSM focus motor in some mounts with full-time manual focus override, two SLD elements plus a floating focus system to correct for chromatic, spherical, and astigmatic aberration. (List price, $749.)