At the minimum focusing distance for tight close-ups of colorful vegetables and blossoms, sharpness was excellent, particularly in the f/5.6-f/11 range. By f/16 and f/22, center sharpness was still high, but corner sharpness did begin to suffer, as with any lens, due to diffraction: the bending of light rays as they enter a miniscule aperture. However, the greater depth of field produced an impression of higher sharpness, because all parts of the three-dimensional subject were within the range of sharp focus.

Image quality at f/2.8 is close to excellent in the 70-140mm focal length range and still surprisingly good at longer focal lengths, as indicated by this photo (and the small portion of the image) made at 200mm. Naturally, depth of field (the range of acceptably sharp focus) is very shallow at f/2.8 so only the focused plane will be very sharp. That's an optical characteristic with any lens, and most obvious with a telephoto due to the high magnification. (At ISO 100; Canon EOS Digital Rebel XT.)

The combination of HSM plus wide maximum aperture (that transmits a great deal of light to the AF sensor) definitely paid off. Even in low light, both EOS digital cameras focused quickly and reliably without the need for the focus assist feature provided by electronic flash units. With static subjects, the ability to quickly fine-tune focus manually at any time was a real plus, to get focus on the eye instead of the tip of the nose in a portrait, for example.

However, my primary tests of the Hyper Sonic Motor AF system were made with racing horses and cyclists. Performance with the EOS Digital Rebel XT (one of the more affordable digital SLRs on the market) was very good; nearly all frames in a long series were sharply focused. Switching to the EOS-1Ds Mark II, with its more advanced autofocus system, provided an even higher success ratio of sharp focus when tracking action subjects accelerating toward or away from my position.

In close-focus photography, f/16 is a useful aperture for great depth of field to ensure that all areas of a three-dimensional subject will be sharp: within the depth of field. While few lenses produce optimal image quality at such a small aperture, the Sigma zoom performed surprisingly well with high resolution of fine detail across the frame. (Canon EOS Digital Rebel XT; Manfrotto tripod.)

Conclusion
Anyone who still doubts that a zoom can match a "prime" lens should certainly try this one--it's an eye opener. In terms of sharpness, resolution of intricate details, snappy contrast, freedom from flare and from color fringing, it is definitely an outstanding performer. The very wide f/2.8 aperture is often useful for stop-action photography or for shooting at lower ISOs whenever flash or a tripod is not practical. Overall, this zoom's optics, autofocus, and build quality make it a professional instrument intended for those who want a superior "fast" telephoto zoom and are willing to carry a hefty, oversized lens.

Check out this zoom in person at a photo retailer to determine whether you'll be comfortable in carrying and handling a 3 lb lens. (Removing the tripod mount subtracts about 6 oz.) Frankly, you may find that you prefer something smaller and lighter, and, perhaps, more affordable; Sigma makes many such zooms as well. Though not intended as a mass-market lens, the APO 70-200mm f/2.8 EX DG Macro HSM model will satisfy its target market. Considering its level of performance, this zoom offers good value; it's also mechanically solid and likely to provide long-term reliability. Used with the right shooting techniques, it will pay dividends in terms of professional-caliber images.

As this series indicates, the Sigma zoom, with HSM, was often successful in tracking fast moving (and accelerating) action subjects when used with the Canon EOS Digital Rebel XT. However, serious photographers will want to use a camera with an even more advanced autofocus system for more reliable results with subjects moving at erratic speeds. (At f/6.3; ISO 400; multi-point AF sensor.)

For more information, contact Sigma Corporation of America, 15 Fleetwood Ct., Ronkonkoma, NY 11779; (800) 896-6858; www.sigma-photo.com.

Technical Specifications
Format: Multi-format for 35mm and digital SLRs regardless of sensor size
Minimum Aperture: f/22
AF System: Hyper Sonic Motor; manual focus override available in AF mode
Lens Construction: 18 elements (including four low dispersion) in 15 groups
Angle Of View: 34.3-12.3Þ
Minimum Focusing Distance: 38.4" (100cm)
Filter Size: 77mm
Dimensions: 3.4x7.3"
Weight: 48.7 oz (1380 g)
Accessories: Lens hood included; compatible with optional 1.4x and 2x EX DG APO converters
Available Mounts: Canon, Nikon D, and Sigma AF
List Price: $1199 (dealers often sell for less)

This image, made at 100mm at f/8, confirms the exceptional optical potential of the Sigma zoom lens. When viewed at high affrication on a professional LCD monitor, the most intricate detail is clearly defined. Any linear distortion in this photo was caused by shooting technique and not by the lens, which is actually very well controlled for barrel and pincushion distortion. (Canon EOS Digital Rebel XT.)

A long-time "Shutterbug" contributor, stock photographer Peter K. Burian (www.peterkburian.com) is the author of several books, including "Magic Lantern Guides" to the Maxxum and Sony digital SLR cameras (Lark Books) as well as "Mastering Digital Photography and Imaging" (Sybex). He is also a digital photography course instructor with BetterPhoto.com.