A Trio Of Tamron Zooms; SP AF17-50mm F/2.8 XR Di II LD Aspherical [IF], AF28-200mm F/3.8-5.6 XR Di Aspherical [IF] Macro, And AF70-300mm F/4-5.6 Di LD Macro 1:2 Page 2

Evaluation: For a versatile lens that's surprisingly affordable, this one turned in a respectable performance. Barrel distortion is certainly noticeable in test images made around the short end but that's common with most zooms of this type. At normal focusing distances, peak performance was provided in the 28-150mm range at f/8 or f/11. Image quality was certainly adequate for a very good 5x7" print or a decent 8x10" print after some tweaking with Unsharp Mask or Smart Sharpen/Lens Blur in Adobe's Photoshop Elements 5.0.

Images made at long focal lengths exhibited some softness, particularly near the edges of the frame. Stopping down to f/11 was useful for the best possible results. Again, some sharpening was required in imaging software in order to make very good 5x7" prints, the largest that most families usually want. The same findings apply to performance in close focusing; the best results were available at the mid range of focal lengths and apertures.

Especially on a compact SLR camera, this highly portable lens would be ideal for hiking or for long days of touring during vacation trips. Since it covers many of the most popular focal lengths, the 28-200mm Di model can take the place of several other lenses. Attractively priced, it will satisfy many families with nice photos for their scrapbooks while providing good value for the money.

Tamron's 70-300mm Di Zoom
Downright inexpensive, this telephoto zoom feels solid and incorporates a metal lens mount like the other two Tamron models and is covered by the same Tamron six-year U.S.A. warranty. It's longer than the 28-200mm Di lens and the internal barrel extends by 2.5" when zoomed to 300mm. The larger size allows for wider zoom and focusing rings but this 70-300mm zoom does not include internal focusing. Hence, the front element rotates during AF operation; that calls for setting a polarizer after focusing has been completed, a slight annoyance. More importantly, autofocus is a bit slow at 200mm and longer focal lengths, particularly when a polarizer is used. That's understandable because the filter's neutral density (gray) glass reduces the amount of light that reaches the focus detection sensor. For any action photography, it's definitely worth shooting without a polarizer.

This 70-300mm Di zoom bears a 1:2 designation, indicating extremely close-focusing ability (to 37.4") for magnification up to 1/2 life size, or 0.5x at the 300mm end. The "Macro" mode, selectable only in the 180-300mm range, allows for frame-filling shots of flowers, jewelry, and other small (though not tiny) objects. When used with a digital SLR camera with an APS-size sensor, the apparent maximum magnification is even greater, roughly 0.75x. That's because the sensor is smaller than a 35mm film frame so it records only a portion of the large image circle that's projected by this multi-platform lens. Consequently, a small subject appears larger--in the viewfinder and in the final digital photo--because less of the scene is included in the image area.

(Top) In normal use, the 70-300mm Di focuses fairly close for moderately tight close-ups. (Above) Switching to Macro mode (available in the 180-300mm range only) allows for much closer focusing for greater magnification. (Canon EOS Digital Rebel XT; Bogen Manfrotto tripod; 300mm at f/13; a +1 in camera sharpening level.)

Evaluation: This lens incorporates only one piece of Low Dispersion glass, omitting the other high-tech elements, but image quality is better than you would expect. That's because a telephoto zoom without wide maximum apertures requires less correction for optical aberrations than many other types of zooms. In my tests, optimal performance was provided in the 70-200mm range at f/8 with high edge-to-edge sharpness and brightness; the images were suitable for very good 8.5x11" prints. Continuous tracking focus was also most reliable in this range of focal lengths. But even images made at 300mm were highly acceptable at f/5.6 and surprisingly sharp and finely detailed by f/11.

True macro lenses (such as Tamron's SP AF90mm F/2.8 Di) provide the best image quality in extremely close focusing because they are optimized for that application. Still, this inexpensive 70-300mm Di zoom is fine for occasional high-magnification use, especially at focal lengths under 250mm, at f/11 or f/13, apertures that also provide the necessary depth of field. However, I found that extremely close focusing at longer focal lengths produced images that are soft, without the same definition of intricate detail. As well, they were not always accurately focused when I used the Digital Rebel XT's AF system; manual focus was definitely preferable. Avoid the extremes of focal length and focusing distance in "macro" photography, select the "best" f/stops and use a rigid tripod for images that should be highly acceptable, particularly for 8x10" prints or for images needed for eBay.

As with any brand, the premium-grade telephotos always receive the highest ratings. However, such lenses are hefty, large, and expensive, making the 70-300mm Di zoom a far more attractive alternative for families. Considering the modest price, this lens offers very good value with competitive performance for its category and much closer than average focusing ability.

Tamron's "Digital Integration" Technology
Until about 2004, the vast majority of autofocus lenses were designed to produce optimum results when used with 35mm SLR cameras. Although they can also be used with digital SLR bodies, some lenses do not produce the same level of quality on the new, high-tech cameras. Particularly at wide apertures, internal reflections can cause flare, "ghosting" (reflections in the shape of the lens diaphragm), low contrast, as well as light falloff or darkening at the edges of the frame.

Most of these problems occur because CMOS and CCD sensors are highly reflective. When light reflects from the sensor (or from the protective glass cover), it bounces to the rear element of the lens creating flare that can degrade image contrast and apparent sharpness. In most lenses for 35mm systems, the rear element is not coated with multiple layers of anti-reflective compounds because the matte film surface produces only diffused reflections. The stronger reflections in a digital camera can degrade image quality to a greater extent, calling for additional anti-reflection countermeasures.

Newer lenses of most brands address this issue. For example, the Tamron designers specified "Internal Surface Coatings" for their Di and Di II zooms: multiple layers of new chemicals on more elements to minimize flare caused by both internal and external reflections. As well, "upgraded advanced quality control raises the level of resolution performance standards and prevents flare due to aberrations," according to a company rep. The new strategies should also provide a benefit in 35mm film photography (with Di lenses), producing more snappy contrast in strong sidelighting, for example.

While extensive scientific comparison testing would be required to prove the true value of the "digitally integrated" technology, I can offer the following assessment about one of the three tested lenses: The SP AF17-50mm f/2.8 Di II zoom is superior in most respects to the Tamron SP AF20-40mm f/2.7-3.5 model that I used frequently for stock photography for several years. In digital capture at a 20mm focal length, the newer lens produced higher contrast, less flare, and superior edge sharpness/brightness by f/4. And as a bonus, the images exhibit less purple fringing (chromatic aberration) around subject edges while barrel distortion is more effectively controlled.

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

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.