Sharpness
The Sigma 18-50mm is decently sharp, less so on the wide end, but improving as the lens is both stopped down and zoomed in towards 50mm.
At the wide end (18mm), the lens shows significant corner softness - 3-4 blur units in the corners, set against a good results in the center - around 1.5 blur units. Stopping down to /4 offers a substantial improvement, reducing corner softness to 2.5 blur units, but that's about it - results for /5.6 through to /11 are essentially the same as those found at /4.
The lens likes the /4 aperture - as the lens is zoomed in towards 35mm, corner softness decreases from what we noted at 18mm, and the center maintains its results of 1.5 blur units in the center. By 35mm the maximum aperture reaches /4, and we note results where the corners are as sharp as the center, at around 1.5 blur units. Stopping down offers marginal improvements to sharpness, achieving optimal results at 35mm and /8, at just over 1 blur unit across the frame.
While diffraction limiting seems to set in at /11, the effects aren't noticeable until /16, and even at that point we still note sharpness results of around 2 blur units (except at 18mm, where it seems to be more like 2-3 blur units, softer in the corners). Fully stopped-down performance, as in most lenses, is best avoided with this one, offering between 4-6 blur units of uneven focus across the frame.
Chromatic Aberration
Chromatic aberration shows up prominently at 18mm, offering a significant red-blue color shift in areas of high contrast in the corners. In the center of the frame it's not bad, and at focal lengths longer than 18mm it's quite well-controlled.
Shading ("Vignetting")
Corner shading is about average for a wide-angle lens: at its worst, the corners are 2/3 EV darker than the center at 18mm and /2.8: between 24-35mm at the widest aperture, it's around a half-stop darker. At /4 at any focal length it's around 1/3 EV darker; at any other setting, it's at a quarter-stop or less.
Distortion
The 18-50mm exhibits the complicated mix of barrel and pincushion distortion that's common in consumer zoom lenses: in the corners, barrel distortion at the wide end, and pincushion distortion at the tele end. Throughout the zoom range there is a generalized barrel distortion, meaning absolutely straight lines are very difficult if not impossible to achieve. At the wide end, we note +0.75% barrel distortion in the corners and +0.4% barrel distortion generally; at the tele end, we note -0.3% pincushion distortion in the corners and just +0.1% barrel distortion generally.
Autofocus Operation
The Sigma 18-50mm /2.8-4.5 OS is designated as an HSM (hypersonic motor) lens, but it doesn't share the full implementation of what we've come to expect from Sigma's HSM lenses. Specifically, it doesn't feature full-time manual override, so if you want to override autofocus results you must first disengage the autofocus. Due to the lens' short focus throw, focusing is quite quick, at around one second to focus from close-focus to infinity, and the lens makes very little noise in the process.
Macro
The lens offers fair macro performance: 0.24x magnification, with a minimum close-focusing distance of 30cm (just under one foot).
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