Specific image quality issues
As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests. The Sigma 18-250mm F3.5-6.3 OS Macro turned out to be a reliable performer when used on the Canon EOS 650D, consistently delivering decent image quality.
The 18-250mm Macro, like many complex zooms, can be quite susceptible to flare when shooting into the light. Place the sun in the corner of the frame and you'll get a diagonal pattern from internal reflections that gets progressively more intense and better-defined on stopping down, along with a 14-ray star pattern from the 7-bladed aperture. (In the example below you can also see some radial red-green-blue patterning around the sun, but this tends to be rather camera-dependent.)
|18mm, F22, sun in corner of frame||73mm, F8, sun just outside frame|
In the second example you can also see flare from pointing the camera into the light, such the sun is just outside the frame. The Sigma's not done terribly here (no other superzoom is likely to do much better), and in any case this kind of flare can often be mitigated by watching out for it in the viewfinder, and shielding the front element from the light with your hand if necessary.
Background blur ('bokeh')
One genuinely desirable, but difficult to measure aspect of a lens's performance is the ability to deliver smoothly blurred out-of-focus regions when trying to isolate a subject from the background, generally when using a long focal length and large aperture. This lens can allow you to achieve quite substantially blurred backgrounds, especially at longer focal lengths and large apertures.
The 18-250mm isn't doesn't provide the prettiest-ever rendition of out-of-focus areas, but isn't massively worse than other lenses in its class either. For close-ups the bokeh is quite acceptable, with reasonably smooth blurring of the out-of-focus regions. More distant backgrounds, however, can look quite 'busy', with doughnut-like, hard-edged point highlights.
|250mm F6.3||155mm F6.3|
|Background detail, upper centre||Background detail, lower left|
The 18-250mm, like pretty well all superzooms, shows significant lateral chromatic aberration at each end of the zoom range. This is visible as strong red/cyan fringing towards the edge of the frame at wideangle, and green/magenta fringing at telephoto. In the middle of the zoom range there's scarcely any CA visible at all. The two extremes are shown in the examples below; the fringing is is accentuated in the telephoto shot as it's out-of-focus, making it more diffuse.
|F8, Canon EOS 650D||F6.3, Canon EOS 650D, ISO 100|
|100% crop, top right (JPEG)||100% crop, upper left (JPEG)|
Lateral CA can be removed pretty effectively in software if necessary. Most current and recent Nikon DSLRs will automatically correct it in their JPEG output, too, but unfortunately Canon SLRs won't (their lens correction options only work with Canon's own lenses).
Like most superzooms, the Sigma 18-250mm Macro suffers from more than its fair share of distortion - barrel-type at wideangle, and pincushion through most of the rest of the zoom range. Note that because it's a third-party lens, the distortion correction functions built into modern SLRs won't work with it, as the required profiles are only available for the manufacturers' own lenses.
The Sigma's distortion is relatively complex in character, and this means that it's relatively difficult to correct completely in software, unless you use a program like DxO Optics Pro, or Photoshop's Lens Correction Module, that uses specific profiles for each individual lens. This is illustrated in the examples below - the corrected versions use Photoshop's generic distortion correction routines, and lines along the edges of the frame aren't perfectly straight (although they're a lot better than the originals).
18mm corrected in PS
73mm corrected in PS
The 18-250mm's 'Macro' tag hints at its class-leading close-up performance, which provides a bit more flexibility compared to other superzooms. It can focus very close indeed, to the extent that if you shoot with the lens hood attached it will end up just 4cm away from your subject, and can easily block out the light.
There's another catch for macro work, too. Like almost all image stabilization systems, Sigma's OS becomes progressively less effective the closer you focus. This means you still need to keep shutter speeds up quite high to get properly-sharp images, and therefore will often need to use high ISOs (at least if you're not using a tripod). With the high ISO capabilities of modern DSLRs you can still get pretty decent results this way.
|1/250sec F8 ISO 3200 (EOS 650D)||100% crop|