The Tamron SP 15-30mm f/2.8 Di VC USD ($1,199) couldn't have been an easy lens to design. It covers an extremely wide angle of view, has a maximum f/2.8 aperture, zooms, and is optically stabilized to boot. That said, there are a couple of trade-offs—it's a big lens, and it shows significant distortion throughout its range—neither of which is uncharacteristic of a such a wide lens for a full-frame SLR camera. The 15-30mm is a strong performer if you're in love with wide angles, but it has a few too many compromises to earn Editors' Choice honors. 

Design
The 15-30mm ($999.00 at Amazon)  is big. It measures 5.6 by 3.9 inches (HD) and adds a full 2.4 pounds of weight to your camera. The front element is bulbous, so there's no filter thread (and no support for rear drop-in filters), and the included lens cap slides over the integrated hood. The hood should help you from accidentally fingerprinting the front element, but if you do the outer fluorine coating will make it easier to clean than other lenses. The 15-30mm is available for Canon, Nikon, and Sony SLRs.

There are two switches on the barrel, one to toggle the focus mode and one to enable or disable the Vibration Control (VC) stabilization system. The barrel itself is made from a hard composite plastic material, with a rubberized manual focus ring near the base and a larger zoom ring just behind the front element. It has markings for the 15mm, 18mm, 20mm, 24mm, and 30mm positions. The focus distance is shown in a window on the top of the barrel. It's displayed in both feet and meters, but there's no printed depth of field scale. Zoom lenses typically omit this, as depth of field varies with focal length, but it is possible to incorporate one with an ultra-wide zoom. The Leica Tri-Elmar-M 16-18-21mm f/4 ASPH. includes one, for example.

Close focus is a strength. The lens focuses on objects as near as 11 inches from the image sensor, so you can get up close and personal with subjects. Focusing close with an ultra-wide lens is an interesting visual effect, simply because of the vastness of the background behind your subject.

Optics
I used Imatest to check the performance of the lens when paired with the 36-megapixel Nikon D810 ($1,996.95 at Amazon) . At 15mm f/2.8 it scores 2,895 lines per picture height on a center-weighted sharpness test. That's better than the 1,800 lines we look for in an image. There is a drop in image fidelity as you move away from the center of the frame, but even the very edges show 1,900 lines. There's a modest improvement at f/4 (3,021 lines), and a more significant jump at f/5.6—an average score of 3,397 lines with edges that approach 2,800 lines. Peak resolution is at f/8; the lens is just shy of 3,500 lines there, and edges show about 3,000 lines. Diffraction is a minor issue at f/11 (3,390 lines) and more so at f/16 (2,930 lines). Flare can be an issue at 15mm, even with the integrated hood.

At 20mm f/2.8 the lens is a strong performer, showing 3,015 lines, but again with some drop in fidelity as you look at the outer third of the frame (2,301 lines). The average score improves at f/4 (3,209 lines) and f/5.6 (3,334 liens), but edge performance stays steady. The lens is at its best at f/8—the average score is 3,464 lines and edges are just shy of 3,000 lines. There's a slight drop in resolution at f/11 (3,404 lines) and a more significant one at f/16 (2,991 lines). Numbers are very similar at 24mm.

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At the maximum zoom, 30mm, the lens shows 2,737 lines at f/2.8. Edge performs lags behind a bit, but the outer third of the frame still shows 2,317 lines. Image quality is stronger at f/4 (2,990 lines) and f/5.6 (2,974 lines) with similar improvements at the edges of the frame. Resolution improves at f/8 (3,191 lines) and the outer third of the frame shows a nominal drop, recording about 2,800 lines. Peak resolution actually hits at f/11—the lens shows 3,264 lines when stopped down that far. Diffraction is an issue at f/16; the lens dips to 2,975 lines, but that's still a vey respectable number.

While there are no issues with clarity, the 15-30mm shows strong distortion. That's not a surprise, as most ultra-wide zooms distort images—it's just their nature. At 15mm barrel distortion is extreme, 4.5 percent, giving images a mild fisheye look. Distortion is nonexistent at 20mm, but by the time you get to 24mm you end up with 2 percent pincushion distortion, which has the opposite effect of the barrel variety. That increases to 2.4 percent at 30mm. Fall-off, which darkens corners of even illuminated subjects, is noticeable at f/2.8 and f/4, but it's not a problem in field conditions by the time you've narrowed the aperture to f/5.6.

Thankfully, both distortion and fall-off can be compensated for using software tools like Lightroom ($9.99/Month at Adobe) . The software includes a lens profile for the 15-30mm, which corrects issues with a single click. Please keep in mind that the field of view will be slightly narrowed when correcting the barrel distortion due to its extreme nature.

Conclusion
The Tamron SP 15-30mm f/2.8 Di VC USD is a strong contender for your dollar if you're in want of an ultra-wide zoom lens for a Canon, Nikon, or Sony SLR. It sets itself apart from the competition with an image stabilized design and a maximum f/2.8 aperture; there are other f/2.8 zooms out there that cover an extremely wide angle of view, but none that offer optical stabilization. It's very sharp throughout its range, but it does make the optical compromises that you'd expect from such a wide zoom: distortion is strong, there is some loss of light at the periphery at wider apertures, and flare is definitely a concern, especially when shooting at 15mm. Still, if you shoot with a full-frame camera and really dig an ultra-wide perspective, the 15-30mm is definitely worth considering, even if you've never picked up a Tamron lens before. APS-C shooters are better served with a lens that's designed for a smaller image sensor, as they offer significant savings in weight, size, and cost.

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