Because the Nikon D40 and D40X do not include an AF drive motor, they can autofocus only with lenses equipped with an AF motor, such as the Sigma HSM and Nikon Silent Wave (AF-S) series. Although the Nikon mount version of the 18-200mm OS zoom was not available at our press time, Sigma confirmed that the model for Nikon will include an AF motor so it will be fully compatible with all Nikon D-SLRs. It's not likely to be the ultrasonic HSM type since that's not being used in the Canon mount version. However, it will allow for autofocus with the D40 and D40X and any future Nikon D-SLRs that omit the AF drive motor.

Particularly at f/11, at any focal length, image quality is impressive in terms of edge-to-edge sharpness/brightness and resolution of intricate detail. (Raw capture, made at f/11 at 1/320 sec with a 42mm focal length at ISO 400 and converted to TIFF with Photoshop CS3.)

Stabilizer Evaluation
Because blur from camera shake is the primary cause of unsatisfactory pictures, the Optical Stabilizer is a valuable amenity. When shake is detected, a signal is sent to a special motor with instructions to shift a group of lens elements in the appropriate direction to counteract the effect of lens shake. This causes incoming light rays to be refracted so the image is returned to the center of the frame; consequently, the projected image is stable, allowing for sharper pictures.

Some of the recent Canon IS and Nikon VR lenses can be used with the stabilizer when the camera/lens is mounted on a tripod. However, Sigma recommends disengaging their OS system when using a tripod unless the camera is being moved for panning. That's not a drawback in my estimation. First, there's no need for a stabilizer when the lens is rock steady and this all-purpose 18-200mm zoom is not the type of lens that's typically used on a tripod.

The Sigma OS technology was certainly effective, as I discovered while shooting from the vibrating deck of a ship or inside dark museums and churches. Without the OS system, I generally needed to use a 1/320 sec shutter speed (on solid land) for razor-sharp images at the longest focal length. After activating the stabilizer, I was able to make equally sharp images at 1/80 sec, and sometimes, even at 1/50 sec. Bracing my elbows on the roof of a car or a concrete wall produced even more impressive results: professionally sharp images at 1¼30 sec at 200mm, a 320mm equivalent with the XTi camera.

While this is not a true macro lens, the close-focusing ability is certainly useful for nature close-ups, particularly at the longest focal length. For the very best results, activate the Optical Stabilizer to minimize the risk of blurring from camera shake, select f/11 or f/16, and shoot when the subject is perfectly still. (Image made at f/11 at 1¼60 sec, using a 200mm focal length and electronic flash, at ISO 400.)

Optical Evaluation
This rugged zoom with SLD and aspherical elements proved to be a fine performer during extensive travel photography. While examining my hundreds of JPEG and raw format images of people, places, and events, I made the following notes:
· Shading (darkening at the corners) is visible--though not obvious--in images made at wide apertures, especially at very short and very long focal lengths. Shading is typical with all affordable zoom lenses, but this Sigma model is better than average. Edge brightness is quite acceptable by f/5.6 at wide angle and by f/8 at telephoto.

· Barrel distortion (bowing outward of lines near the edges) was certainly visible at wide angle focal lengths as it was with every all-purpose zoom that I have ever tested. But since lenses of this type are very rarely used for serious architectural photography, I consider this issue to be irrelevant.

· The multilayered coating plus the lens hood combine for very effective flare control; that was certainly useful while shooting on very bright, sunny days in Italy.

· Image quality is most impressive at focal lengths from about 18mm to 135mm, particularly in the f/8 to f/16 range.

Center sharpness is high even at f/5.6, but it's worth stopping down for maximum edge sharpness.

· At longer telephoto focal lengths, it's wise to avoid using f/5.6 or f/6.3 unless you plan to make only 4x6" prints, which will look just fine. By f/8, center sharpness is high but there's some softness at the edges. That's not a problem in people pictures where the subject is usually close to the center, but for photos of buildings and landscapes, it's worth stopping down to f/11 or f/16. My best long telephoto shots made for very good 8.5x11" enlargements, the largest size that most families usually make or order.

· This lens can focus as close as 17.7", useful for filling the frame with a blossom, especially at longer focal lengths. I was most satisfied with images made at f/11 or f/16; they look great when printed letter-size. Aside from the extra depth of field, this aperture range produced the best edge-to-edge sharpness and resolution of fine detail. (While the OS system can minimize the need for a tripod, no Image Stabilizer can prevent blur caused by wind-induced motion; that still requires fast shutter speeds.)

This all-purpose Sigma lens was ideal for most types of travel photography since the zoom range includes my favorite focal lengths. Far more versatile than the typical kit lens, this 28-300mm equivalent zoom would also be a fine choice for families who want to carry only a single lens on all of their outings. (JPEG capture at f/8 and 1/250 sec, at 18mm and at 200mm, using a Hoya S-HMC polarizer.)

Final Assessment
Like most zooms in the affordable or all-purpose category, this one provided the best edge-to-edge sharpness and brightness at mid-size apertures. By comparison, Sigma's large/expensive/fast pro lenses are optimized for high performance at wide apertures. That's a benefit because it allows for using faster shutter speeds at low ISO levels where any camera provides the "cleanest" images. On the other hand, this zoom's Optical Image Stabilizer reduces the need for fast shutter speeds to prevent blur from camera shake, making ISO 100 suitable for most daylight photography. Switch to ISO 400 when using a polarizer or on very dark, overcast days when using mid-size or smaller apertures. Most of today's D-SLR cameras produce excellent image quality at ISO 400, certainly adequate for very nice 11x15" prints.

The Optical Stabilizer makes this versatile Sigma zoom a particularly fine choice for any occasion when you simply don't want to carry a bagful of lenses. Especially on a compact D-SLR, the lens would be a great companion when hiking, cycling, touring, or during long outings with your family. After all, this 28-300mm (approximate) equivalent lens covers 80 percent of the most popular focal lengths in a single barrel. Sure, there are smaller/lighter/more affordable all-purpose zooms without a stabilizer including some from Sigma. But if you agree that the OS system adds genuine value, you'll gladly pay a bit more and accept the slightly greater size/weight.

No OS Lenses For Sony Or Pentax Mounts
Sigma has no plans to make OS lenses in Sony Alpha/Maxxum or Pentax mount, because those D-SLR bodies (except for older Pentax *istD models) already incorporate a CCD-shift stabilizer.

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

A long-time "Shutterbug" contributor, stock photographer Peter K. Burian (www.peterkburian.com) is the author of "Mastering Digital Photography and Imaging" and the co-author of several "Magic Lantern Guide" books about 35mm and D-SLRs. He also teaches two online digital photography courses at BetterPhoto.com.