Roughly the same size as the compact Tamron 11-18mm zoom, the Sigma 10-20mm zoom seems even more rugged but is 4.1 oz heavier, though still lighter than the Tokina 12-24mm model. The Sigma is also the most affordable of the trio tested, making it particularly attractive to those on a tight budget. But if you're a serious shooter, don't let the low price dissuade you from buying this lens. It would be a fine choice for anyone who appreciates great value for the money, high image quality, and super fast (HSM) autofocus in Canon, Nikon, and Sigma mount.

	Sigma AF 10-20mm f/4-5.6 EX DC HSM	Tamron SP AF11-18mm f/4.5-5.6 Di II LD Aspherical (IF)	Tokina AF 12-24mm f/4 AT-X PRO DX
Build Quality	Middleweight; rugged metal barrel	Lightweight; solid construction with polycarbonate barrel	Bit heavy but feels most sturdy; metal and polycarbonate barrel, all-metal zoom unit
Light Falloff At	Minimal; none by f/5.6; no vignetting with standard polarizer	Minimal; none by f/8; no problem with polarizer	Minimal; none by f/5.6; no problem with polarizer
Barrel Distortion	Very well controlled; barely noticeable even at 10mm	Very well controlled; barely noticeable even at 11mm	Well controlled; noticeable only at 12-14mm
Autofocus	Very fast, nearly silent with HSM model; focus ring does not rotate	Moderately fast; a bit noisy; ring rotates	Fast; fairly quiet; ring does not rotate
Manual Focus	Well damped but could be smoother	Smooth and well damped	Perfect; reminiscent of a fine manual focus lens
Flare Control	Exceptional	Exceptional at most focal lengths; slight flare at 11-14mm in extreme sidelighting	Exceptional but slight contrast reducing flare at long focal lengths in extreme conditions
Chromatic Aberration	Minimal purple fringing around bright subject edges	Minimal, esp. at longer focal lengths	Minimal, esp. at longer focal lengths
Short Focal Length Performance	Excellent image quality from f/5.6-f/11; quite good at f/16	Very high image quality at f/4.5-f/11; acceptable+ at f/16	Excellent image quality from f/4-f/11; slightly soft at f/16
Longer Focal Length Performance	Very good+ at f/4.5-f/11 at 20mm; good at 15mm (slight edge softness); acceptable at f/16	Very good at f/4.5-f/11, even at the edges; acceptable at f/16	Excellent from f/4-f/11; quite good at f/16
Conclusion	Superb performance esp. at short focal lengths	Fine performance	Pro-caliber performance, esp. at f/8, but some barrel distortion

Tamron's SP AF11-18mm f/4.5-5.6 Di II LD Aspherical (IF)
The lightest of the three lenses tested, this one is nicely finished in satin black, with a wide zoom and focus ring; both provide enough friction for convenient operation. The internal barrel extends very slightly when zoomed to 18mm. The focus ring does rotate during autofocus operation but the response is acceptably fast for an ultra-wide lens and not particularly noisy. The barrel includes an AF/MF switch, distance scales, and focal length markings at the 11, 13, 15, and 18mm position.

When designing this zoom, the engineers specified two types of aspherical elements to correct the angle of light plus other measures for even brightness across the frame. The lens also benefits from a low dispersion plus a high index/high dispersion element to minimize optical aberrations and to optimize color rendition and resolution. The high-grade optical formula paid off, because this lens produces images that can make very good to excellent 11x17" prints.

Outdoors on sunny days the corner-cut lens hood--plus the extensive multilayered coating--prevented flare except in extremely bright sidelighting. Light falloff at the edges of the frame was minor and disappeared by f/8. As with the other zooms, my standard Hoya circular polarizer caused no darkening at the corners but I removed it when necessary to minimize the risk of flare. My best images--made at any focal length at f/4.5 to f/11--exhibit remarkable contrast and clarity, plus moderately high sharpness and resolution of intricate detail. This consistency indicates a particularly effective optical design.

Although each of the three lenses offers a different set of focal lengths, much of my testing was conducted at exactly the same zoom settings, 14mm in this case. This approach proved to be useful for making direct comparisons of all aspects of technical image quality. (Canon EOS Digital Rebel; at f/11 in AV mode; B+W multi-coated polarizer.)

Slightly more expensive than the other two contenders, this Tamron ultra-wide zoom is still relatively affordable. At a mere 12.5 oz, it's particularly portable, great for travel photography or when hiking for long distances. It's also the only one of the models tested that's available for the Konica Minolta Maxxum digital SLRs. If you want the latest technology and appreciate the fine reputation of Tamron lenses, this zoom is worthy of your consideration.

Tokina's AF 12-24mm f/4 AT-X PRO DX
Larger than its two competitors because of the wider, constant maximum aperture of f/4, the Tokina zoom feels exceptionally rugged and boasts superb zooming and manual focus mechanisms. Due to internal zooming, the length of the satin black barrel never changes. Focal lengths are marked at five increments and distance scales are available under glass. Instead of an AF/MF switch, modes are changed by shifting the focus ring slightly forward or backward. Tokina calls that feature a One-Touch Focus Clutch Mechanism; it's quick and easy to use.

The optical formula includes Super Low Dispersion glass plus two aspherical elements. This combination paid off because chromatic aberration is barely visible at 12mm and virtually invisible at 24mm. And the aspherical elements are certainly effective at controlling distortion at most focal lengths while providing great edge to edge sharpness and brightness at wide apertures. Light falloff is barely noticeable and it's gone by f/5.6. Flare is also very well controlled particularly in the 12-18mm range.

The Tokina 12-24mm zoom produced remarkably high sharpness and resolution at every zoom setting--particularly at longer focal lengths--at all apertures from f/4 to f/11. Consequently, I never hesitated to shoot at f/4 for fast shutter speeds in low light. Performance at f/16 is not as stellar but still acceptable. After selecting my technically best images, I was able to make absolutely superb 12x18" prints that look sharp and finely detailed even under close examination.

This zoom is competitively priced but it does not include the very shortest focal lengths available with many others on the market. Still, it extends to longer focal lengths than its competitors, so it's certainly versatile. Finally, like many other Tokina lenses, this one is somewhat heavy by today's standards. Frankly, I doubt that will concern serious photo enthusiasts who appreciate great build quality as well as the potential for pro-caliber images. Anyone who has not previously considered a Tokina lens should certainly check out this fine ultra-wide zoom.

Final Assessment
Any digital SLR owner who is frustrated by the inability to make true ultra-wide angle images with existing lenses would definitely appreciate one of these zooms. Useful for dynamic wide angle perspectives with a powerful near/far relationship, they're also optimized for use with a digital camera, an important factor in lenses of this type. When compared to my older 20-40mm zoom (designed for 35mm systems), they produced images with noticeably less purple fringing around subject edges,
more snappy contrast, and greater edge sharpness at wide apertures.

Manufacturers/Distributors
Sigma Corporation of America
15 Fleetwood Ct.
Ronkonkoma, NY 11779
(800) 896-6858
(631) 585-1144
www.sigma-photo.com

Tamron USA, Inc.
10 Austin Blvd.
Commack, NY 11725
(800) 827-8880
(631) 858-8400
www.tamron.com

THK Photo Products, Inc. (Tokina)
2360 Mira Mar Ave.
Long Beach, CA 90815
(800) 421-1141
(562) 494-9575
www.thkphoto.com