You can't expect the same performance from a superzoom lens like the Sigma 18-300mm F3.5-6.3 DC Macro OS HSM Contemporary ($579) as you can from a prime lens or a shorter high-quality zoom like Sigma's own 17-70mm F2.8-4 DC Macro OS HSM. But the convenience of an all-in-one zoom is not to be discounted, and the 18-300mm is a solid choice for photographers who desire wide angle coverage, telephoto reach, and close focus capability without having to swap lenses. But it's on the pricey side for a lens that isn't going to deliver top-quality images, and for that reason we rate the Sigma's less expensive, less ambitious 18-200mm F3.5-6.3 DC Macro OS HSM Contemporary slightly higher.

Design
Despite its long zoom range, the 18-300mm is fairly compact. It measures 4 by 3.1 inches (HD), weighs 1.3 pounds, and supports 72mm front filters. Because the lens is designed to match the image sensor found in consumer SLRs, it's much smaller than similar superzooms for full-frame bodies, including the AF-S Nikkor 28-300mm f/3.5-5.6G ED VR ($846.95 at Amazon) (4.5 by 3.3 inches; 1.8 pounds). To give you an idea of the zoom range in real-world terms, look at the next two images. The moon is a small dot when the lens is set to its widest, and in clearer view at 300mm.

Sigma sells the 18-300mm for Canon, Nikon, Pentax, Sigma, and Sony SLRs. The Pentax and Sony versions omit image stabilization, as that is handled in-body in those camera systems, but the other versions of the lens include it. All versions include a reversible lens hood. Like most SLR lenses, the 18-300mm is finished in black. Its barrel is hard composite, similar to other lenses in Sigma's line of Global Vision lenses, which are easily identified by their Art, Contemporary, or Sports designations.

The barrel does extend when the zoom is set beyond 18mm. That's typical for a consumer-grade zoom. What's slightly atypical is the design of the focus ring. It's narrow and sits at the very front of the portion of the barrel that doesn't extend. Most focus rings don't move when the camera is set to autofocus, but this one does. On the plus side, the focus distance scale (listed in feet and meters) is able to be printed directly on the focus ring. But I had to stop myself from resting my fingers on the narrow ring when shooting with the lens—on more than one occasion I found myself holding on to it, preventing the autofocus system from properly doing its job.

The zoom ring is much wider than the focus ring, and where the focus has a molded texture to it, the zoom is covered in rubber with a similar knurled finish. It's comfortable to turn, and several focal lengths (18, 28, 35, 50, 80, 135, 200, and 300mm) are marked. A lock switch is included to keep the lens set at 18mm, preventing it from creeping out when your camera hangs at your side or around your neck.

In addition to the lock, there are two switches on the barrel—one to change between autofocus and manual focus, and the other to turn the image stabilization system on and off. You'll want to leave it enabled when shooting handheld, and turn it off when using a tripod.

The lens can focus as close as 15.3 inches. At wide angles that doesn't give it much magnification capability—as you can see from the markings on the telescoping portion of the lens barrel. But at 300mm, the zoom projects objects onto your camera's image sensor at one-third life size (1:3) when working at the minimum distance. You can add the Sigma AML72-01 Close-Up Lens filter ($45) and improve that to 1:2, but you will lose the ability to focus on distance objects with the filter attached. Sigma occasionally bundles the filter with the lens at no added cost.

Image Quality
I tested the sharpness of the lens paired with the 24-megapixel Canon EOS 80D. We want to see images that hit 1,800 lines per picture height on Imatest's center-weighted scoring metric, a figure that the 18-300mm sometimes failed to manage. At 18mm, f/3.5 results are solid, with a 1,871-line center-weighted score. As you move further away from the center, however, results are not as good. The middle third of the frame shows about 1,650 lines and the edges just 1,135 lines. Results are nearly identical at f/4. At f/5.6 the overall score ticks up to 1,947 lines, and the middle third is crisper (1,719 lines), but edges are still muddy (1,379 lines). Edges are better at f/8 (1,502 lines) and f/11 (1,590 lines), but the average score hovers around 1,950 lines in both cases. Moving beyond f/11, regardless of focal length, does more to hurt image quality than to help it.

At 50mm, the maximum aperture is narrowed to f/5. Images are a bit crisper—2,012 lines at f/5, 2,175 lines at f/8, and 2,199 lines at f/11—and the midparts of the frame hold up well to scrutiny. Edges are still soft, though—1,175 lines at f/5 and 1,502 lines at f/8—until you get down to f/11, where they improve to 1,846 lines.

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There's a noticeable downturn in clarity at 100mm. The maximum aperture is now f/5.6, and the lens manages just 1,691 lines when shot wide open. It improves to 1,919 lines at f/8 and 2,078 lines at f/11. Edges are very soft—just 900 lines at f/5.6, 1,248 lines at f/8, and a more acceptable 1,698 lines at f/11.

And the lens is worse at 150mm. At f/6.3 it shows just 1,386 lines across the frame, with edges that are very blurry (757 lines). There's modest improvement at f/8—a 1,574 line average with muddy edges that fall a bit shy of the 1,000-line mark. The story is better at f/11, but even then the lens hits just 1,830 lines and edges fall shy of 1,500 lines.

There's noticeable improvement at 200mm. At f/6.3, the lens scores 1,873 lines. If you're paying attention it won't be a surprise that the edges are not good, though, showing just 1,127 lines. Stopping down to f/8 (2,021 lines) and f/11 (2,050 lines) goes a long way to sharpen the center and midparts of the frame, but you'll still be left with 1,222 lines and 1,482 lines (respectively) at the periphery.

There's another dip in performance at 300mm—that's not surprising as long zoom designs often suffer at their extremes. At f/6.3, the zoom scores 1,623 lines on the sharpness test, with a crisp center that gives way to a disappointing middle third (1,350 lines) and blurry edges (823 lines). Stopping down to f/8 does nothing to help the periphery, but it does boost the average score to 1,741 lines and nets better results in the middle third of the frame (1,625 lines). You'll want to stop down to f/11 for the best results—1,879 on average, with midparts that show 1,850 lines and edges that fall just shy of 1,300 lines.

Sharpness isn't the only compromise you make to achieve this level of zoom in a lens. Distortion is also prevalent—there's 3.2 percent barrel distortion at 18mm, which causes straight lines to be captured with an outward curve. It gives way to pincushion distortion, which causes those same lines to bow inward, as you zoom—there's 2.2 percent at 50mm, 1.6 percent at 100mm, about 1 percent at 150mm and 200mm, and 1.4 percent at 300mm. If you work with an image editing application like Adobe Lightroom ($9.99/Month at Adobe) this distortion is easy to remove—there's a lens profile that will straighten those lines with a single click.

Lightroom can also correct for dim corners and edges. When shooting with the 18-300mm at its maximum aperture you'll notice that the illumination is not even across the frame. I used an ExpoDisc and Imatest's Uniformity tool to evaluate the severity of this effect. It's at its worst at 18mm—at f/3.5 the corners are about 3.5 stops dimmer (-3.5EV) than the center, and sides lag behind by 1.5 stop. Narrowing the aperture to f/5.6 brings the sides within acceptable tolerances, but corners still show -2.5EV when compared with the center—narrowing the aperture further doesn't do anything to alleviate this at 18mm.

The lens is a better performer in this regard when zoomed in. At 50mm f/5, 100mm f/5.6, 150mm f/6.3, 200mm f/6.3, and 300mm f/6.3 the corners are about 2 stops dimmer than the center, but the sides are within a stop. Narrowing the aperture to f/8 brings the corners within a stop at all of those focal lengths.

Conclusions
Long zoom ratios come with compromises in optical quality, and that's very true of the Sigma 18-300mm F3.5-6.3 DC Macro OS HSM Contemporary. The lens is compact and light, covers a very wide range, and offers a solid 1:3 macro magnification ratio at its longest focal length. But it's not as sharp as shorter zooms or prime lenses, has a narrow maximum aperture that limits light gathering and depth of field control, and is on the pricey side at $579. If you're in the market for this type of lens, Sigma's 18-200mm comes with many of the same caveats, and doesn't quite have the same telephoto reach, but does cost just about $400. If you're not willing to make as many compromises consider pairing Sigma's excellent 17-70mm Contemporary zoom with the Canon EF-S 55-250mm f/4-5.6 IS STM ($299.00 at Amazon) or a similar lens for your SLR system of choice. You won't have the convenience of an all-in-one lens, but you will enjoy crisper images and more consistent performance from a two-lens kit.

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