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One look at the specs of this new Tamron zoom and you'll understand why it's immediately attractive to anyone carrying around a digital SLR in their day bag. Weighing in at around 15 oz and measuring just under 3x3.2", the lens is quite the marvel of size for what it delivers in focal length and aperture options. Indeed, if someone told me that a constant aperture, 17mm wide lens would be this size a few years back I'd have thought they had lost their optical marbles. To be fair, however, that 17mm is not really a 17mm in 35mm equivalent, thus practical terms, and I wonder why lenses like this are still labeled that way. This lens is only for APS-C sensors, which means it has the "35mm equivalent" of a 27mm wide angle view and 80mm tele.

Tamron's new 17-50mm zoom, available in Canon, Nikon, Konica Minolta (read Sony), and Pentax (read Samsung) mounts is highly portable and compact.

But terminology aside, any constant aperture lens is always my first choice for a zoom, as exposure will not shift in the middle of a shot. With some variable aperture lenses a handheld image can be prone to camera shake when you zoom in. This is not so bad in the f/2.8-3.5 slide that is fairly common, but becomes a true pain in the less expensive f/3.5-5.6 types, where you might as well permanently attach a tripod to get a steady shot with the lens.

One of the real benefits of this new lens is the fact that you can maintain the same close-focusing distance throughout the range, so you can get in tight with the wide and then try out different framings without having to move back. This advantage allows you to set a tripod and then decide about composition, using the IF (Internal Focusing) mechanism to get within 10.6" at all times. It isn't a 1:1 reproduction ratio, but 1:4.5, which, in digital (APS-C) terms is really like 1:3 equivalent, close enough in my book for all but those who would require a fixed focal length 1:1 macro-dedicated lens.

Tamron was one of the first companies to bring forth "digitally-optimized" lenses, and for a while none of us could figure just what that meant. There are a few items that do seem to make a difference--how sensors gather light, the greater resolution capability of digital, and the way ghosting and flare plagues image crispness. This lens addresses all three matters.

Along with the benefits of a constant aperture throughout the zoom range, this close-focusing lens allows for a constant minimum focusing distance as well. This rosebud was photographed using the 50mm setting and exposed at f/5.6 at 1/160 sec using a Canon EOS 30D. All Photos © 2006, George Schaub, All Rights Reserved

First off, the lens has been designed to constrict the angle of incidence of light striking the sensor, which plays to the fact that photo sites are more efficient as light striking them becomes more perpendicular. Too acute an angle can cause problems, something that plagued early wides in the digital realm, and why we didn't have many wide angle lenses when digital SLRs first came out the gate. (In truth, though, the micro lenses placed by manufacturers over their sensors to direct light in a more efficient manor can have as much influence over optical quality as how the lens does the same, but I guess having a double backup can't hurt.)

The lens has been optimized with Extra Refractive Index glass (the XR designation) and two hybrid aspherical elements, plus a low dispersion glass element to compensate for chromatic aberrations. And multi-coating of a number of internal elements cuts down ghosting and flare as well, chief enemies of snappy images. Plus, included with the lens is a flower-shaped hood, a good item to keep attached to the lens to cut down on light striking across the surface of the lens.

To show sharpness and detail, a severe crop was done on the stem, which shows all the texture and edges in fine fashion. Note the very nice character of background unsharpness in both images.

I worked with the Tamron SP AF17-50mm F/2.8 XR Di LD Aspherical (IF) lens (that's Tamron's official name for it) on a Canon EOS 30D in close focusing, tele, and wide setups, and found it a good match for just about any lighting and subject scenarios. The only nit to pick is the lack of a manual focus override when in AF mode; indeed, trying to tweak AF manually might cause some damage. With an MSRP of $763 the lens is considerably less expensive than others in its class, and worth the investment as that one moderate wide to tele constant aperture zoom that's always a handy item in any photo day kit bag.

For more information, contact Tamron USA, Inc., 10 Austin Blvd., Commack, NY 11725; (631) 858-8400; www.tamron.com.