Sigma 18-125mm 1:3.8-5.6 DC OS HSM Lens Review

The "kit" 18-55mm zoom lens that comes with many Canon APS-C DSLRs is an "OK" starter lens, but has some issues with sharpness, chromatic aberration, and focusing -- and has a limited zoom range. Looking for a better quality replacement, I decided to give the Sigma 18-125mm DC OS lens a try. Is this a good upgrade from the standard "kit" lens? Read on to find out...

There are a number of "super zoom" lenses out now for APS-C DSLR cameras, with zoom ranges starting about 18mm and going up to 125, 200, 250, even 300mm at the long end. While having impressive zoom ranges, often these "jack of all trade" lenses are masters of none, having to make so many compromises to get that big zoom range that they're not especially good at any focal length. For most of my shooting, a shorter long end would be sufficient, so I decided to try the Sigma 18-125mm lens out, thinking its shorter zoom range would mean less compromises, and better performance.

The Sigma has a number of advantages over the standard 18-55mm OS "kit" lens: Obviously the greater focal length range, topping out at 125mm instead of 55mm. It also has internal focusing, so the front of the lens doesn't rotate when focusing -- a major pain with the "kit" lens when trying to use a polarizing filter. It also has Sigma's "HSM" (equivalent to Canon's "USM") fast and quiet autofocus motor.
The build quality of the Sigma lens is very good. While it sill uses plastic for some parts, it's very solid and well-built without being overly heavy. The lens mount is all-metal instead of the plastic mount of the kit lens. And it comes with a petal-shaped bayonet lens hood, which the kit lens lacks (and even the recommended Canon EW-60C hood for it lacks the petal shape, limiting its effectiveness).
Both lenses have optical image stabilization ("IS" on the Canon, "OS" on the Sigma), which my tests showed to be roughly equivalent; I averaged 3 stops better than 1/focal length shots on both lenses, impressive for the Sigma even at 125mm focal length.

(Canon 18-55 @18mm on top, Sigma 18-125 @18mm on bottom)

Sharpness

Comparing the two lenses at 18, 24, 35, and 50mm gave me a good range of test shots to judge the sharpness of the two lenses. All of the test shots were done on a tripod with a remote release, using aperture-priority autoexposure on a Canon T2i DSLR. The RAW files were processed in Adobe Camera RAW using the same settings, with all sharpening and noise reduction turned off. The two shots above are full-frame images from the two lenses at f/8.

At 18mm, the results are a mixed bag; the Canon kit lens is basically soft over the entire image, not especially sharp anywhere. However, the image is about the same level of sharpness over the entire frame, with little additional softness at the image edges. The Sigma was considerably sharper at the center at 18mm, but drastically softer at the corners of the frame than the kit lens.

(Canon 18-55mm lens @18mm f/8, center)

(Sigma 18-125mm lens @18mm, f/8, center)

(comparison of extreme frame edges @18mm f/8)

You can see from the comparison image above the poor performance of the Sigma lens at the corners of the frame at 18mm. Things improved at f/11 and f/16, but the Sigma's frame edges were softer than the Canon's at every aperture.

Things improved quickly on the Sigma as I zoomed from 18mm to 24mm, however; at 24mm, the Sigma lens is still sharper than the Canon at the center of the frame, and the extreme edges now matched the kit lens performance. By 35mm, the Sigma beat the Canon in sharpness both at the center and edges of the frame, and the same was true at 50mm. In fact, the only place the kit lens was sharper than the Sigma was at 18mm at the edges of the frame -- in every other test, the Sigma was at least as sharp as the kit lens, and better nearly everywhere. For such a big zoom range, the lens did quite well.

Here are a few of the test shots, 100% crops, for comparison:

(comparison @35mm, f/8, center image)

(comparison @50mm, f/8, center image)

(comparison @50mm, f/8, frame edge)

Here's a chart of how I *subjectively* rated the sharpness of the two lenses at a variety of focal lengths and apertures:

False Color (Chromatic aberration)

In this category, the Sigma wins hands-down. At all focal lengths and apertures, the Sigma had very little to no chromatic aberration visible at high-contrast edges, while the kit lens had visible color fringing at most focal lengths and apertures. The comparison image below (at 4X magnification to make things more visible) was shot at 50mm @ f/8 -- you can clearly see the blue-purple fringing on the left image from the Canon kit lens, while the Sigma's image has virtually none:

Vignetting

Again, the Sigma is the clear winner here. The lens exhibits a bit of vignetting at 18mm wide-open, but it's minor and quite a bit less noticeable than the kit lens at the same focal length and wide open. No vignetting was visible at any focal length on the Sigma at f/8 and beyond, while even at f/11 the kit lens still showed visible vignetting between 18 and 35mm focal lengths.

Conclusion

Overall, the Sigma is a fine replacement for the standard kit lens, offering a number of advantages and only one real downside (very soft frame edges at 18mm). Comparing the Sigma 18-125 with some of the longer "super zooms," it appears most of the "super zooms" get very soft and lack contrast beyond 125mm anyway, and many of them don't do as well at the shorter focal lengths as this lens does.

I'm pleased with the lens, but will pay attention to the corners of the frame at its widest focal length. While this Sigma lens doesn't match most Canon "L" lenses in performance, it doesn't cost nearly as much as they do either -- about $339 US most places. The price-performance ratio makes it a very good value. Try one out if you are looking for a good kit lens replacement with a bit better performance and a longer zoom range.