At longer focal lengths, images made at f/5.6 exhibit high central sharpness but slight softness in the periphery. However, with the OS system, I rarely needed to shoot at such a wide aperture, except in the low light of a heavily overcast day. And since the most significant subject elements are usually near the center of the frame in super telephoto images, most photographers should be happy with many of their 400mm photos made at f/5.6.
For the ultimate in image quality at very long focal lengths, I stopped down to f/8 or f/11 for ultrahigh resolution of fine detail plus excellent edge to edge sharpness. This tactic produced another benefit in air show images that included the sky: it eliminated the darkening of the corners that was obvious in images made at wider apertures. Frankly, vignetting is a problem with most zooms but it's not readily noticeable in images with more typical backgrounds such as foliage.

Optical Stabilizer Evaluation
Because blur from camera shake is the primary cause of unsatisfactory pictures, the OS system is a valuable amenity. And the Sigma technology is certainly successful, as I discovered while shooting at low ISO (film and digital) to record animal portraits at a wildlife safari park, the action at a junior soccer game, and the parked aircraft at a couple of air shows. Without the OS system, I generally needed to work at around 1/500 sec for razor-sharp images at the longest focal length. After activating the stabilizer, I was able to make equally sharp images of static subjects at 1/90 sec. Bracing my elbows on a solid object produced even more impressive results: many professionally sharp images at 1/30 sec and occasionally at 1/15 sec at 400mm.

Because I rarely needed to worry about long shutter speeds when using the OS system, I could shoot with confidence at ISO 50 even when using a polarizing filter and small apertures for an extensive range of apparent sharpness. On sunny days when using an aperture of f/16 and a polarizer, the shutter speed was around 1/45 sec, fast enough for sharp images at many focal lengths. Naturally, the exposures were substantially longer on dark, overcast days, so I switched to wider apertures or higher ISO settings in order to maintain a suitable shutter speed.

In order to clearly illustrate the effectiveness of the OS system, I made these photos at a very long, 1/15 sec exposure at 400mm, with and without the stabilizer. For critical sharpness at this focal length however, I would recommend using a 1/90 sec shutter speed. (Canon EOS-1Ds; 1/15 sec at f/5.6; ISO 1250.)

Note: My tests of the effectiveness of the OS system were conducted with a 35mm camera and a digital SLR with full-frame sensor at 400mm. On an SLR with a smaller sensor, the effective focal length at the long end is 600mm, 640mm, or 680mm, depending on the camera's exact sensor size. Although this effect is actually produced by field of view crop, not by greater magnification, it's worth using slightly higher shutter speeds with such cameras. That's because the smaller image will require greater magnification for making prints of any desired size, so any blur (from camera shake) will be more obvious, especially in oversized prints.
Most 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 or unless there is a great deal of camera shake, as on a very windy day when using a flimsy tripod. I don't see this as a disadvantage because there's no need for a stabilizer when the lens is rock steady. Working without OS also helps to conserve battery power, a significant benefit because power drain is increased by about 50 percent with full-time stabilizer use.

(SIDEBAR) Sigma's OS system consists of two types of sensors inside the lens to detect the angle and speed of vertical and horizontal camera movement. 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.

Note: Owners of Nikon-mount cameras should be aware that OS will not operate with all SLR models. (The same applies to Nikon's own VR system.) At the time of this writing, the Sigma OS system is compatible with the following: F100, N80, N65, N75, D2H, D1 series, D100, D70, and the Fujifilm FinePix S2 Pro. (It should also work with the FinePix S3 Pro.) When used with the Nikon F5, autofocus will also cease to function.

Two OS modes are available. In Mode 1, the system detects and compensates for both vertical and horizontal movement of the camera, ideal for most types of photography. Switch to Mode 2 if you plan to pan with an action subject moving across your line of vision. Now, the system will compensate only for vertical (up/down) camera movement, and not for the intentional horizontal movement used when following a moving subject. This feature is useful especially for sports and racing photography, where you want to use a long shutter speed. For the best results, pan the camera/lens at exactly the same speed as the subject's motion. In the resulting images, the subject should be fairly sharply rendered, while the background will be smoothly streaked--without blurring from up/down camera shake--for a convincing effect of motion in a still image.

Final Assessment
It's unfortunate that this lens is not available in Minolta and Pentax mounts, because it would finally give the owners of such systems access to a lens with an image stabilizer. Canon, Nikon, and Sigma SLR owners will find a great deal to like about this lens in terms of optics, build quality, and features. While an ultrasonic focus motor would make the Sigma 80-400mm f/4.5-5.6 EX OS APO even more effective for sports and bird photography, it should certainly satisfy the vast majority of photo enthusiasts. Some photographers prefer to use tripods for all their work, but others will consider the OS to be a blessing. In any event, the addition of the OS system makes this Sigma tele-zoom highly versatile and it will surely help many photographers to increase their success ratio of technically excellent images.

For more information, visit Sigma's website at: www.sigma-photo.com.

Technical Specifications
Minimum Aperture: f/22-f/32
Optical Formula: 20 elements in 14 groups
Minimum Focus Distance: 71"
Maximum Reproduction Ratio: 1:5
Filter Size: 77mm
Controls: Autofocus/Manual Focus, OS, and Zoom Lock switches
Tripod Mount: Detachable (included)
Vibration Reduction: Two modes; for static subjects and for panning
Accessories: Case and hood (included); compatible with Sigma EX APO tele-converters
Dimensions And Weight: 3.7x7.5"; 61.7 oz
Street Price: $1199

A long-time contributor to "Shutterbug" and "eDigitalPhoto," stock photographer Peter K. Burian is the author of a new book, "Mastering Digital Photography and Imaging" (Sybex). Covering the technology, equipment, and techniques, this book provides 270 pages of practical advice.