The Sigma 18-250mm F3.5-6.3 DC OS HSM Macro performs quite creditably for a superzoom; shot at normal working apertures around F5.6-F11, it delivers pretty good sharpness at all focal lengths. In a marked departure from previous Sigma superzooms, though, pincushion distortion is unusually strong in the middle of the range.
|Sharpness||Sharpness results are slightly mixed. The tests show a quite specific weakness, with rather soft edges to the frame when the lens is shot wide open in the 28-50mm range. Stop down to more-usual working apertures of F5.6-F11, though, and things improve dramatically. Like all superzooms the 18-250mm is weakest in the telephoto range, and it's best to shoot at F8 if possible.|
|Chromatic Aberration||Chromatic aberration is kept reasonably low. It's worst at wideangle, where there's moderately strong red/cyan fringing towards the edge and corner of the frame. It diminishes on zooming in, and is practically zero at 80mm, before becoming more visible again at full telephoto. Here the fringing is of the visually intrusive green/magenta type.|
|Vignetting||Vignetting is overall very low. As usual it's worst at wideangle and maximum aperture, but even here it's only a little over a stop, and essentially disappears by F5.6. In practical use this counts as nothing to worry about.|
|Distortion||Distortion is high, even by the inflated standards of superzooms. At 18mm there's quite strong barrel distortion, with substantial re-correction towards the corners. Zoom in and this changes to pincushion, which is most pronounced around 35mm at a startlingly-high -3.75%. This reduces progressively in zooming in further, but never goes away (and is always potentially visible in geometric compositions).|
As the 'Macro' label suggests, the Sigma 18-250mm offers best-in-class close-up performance for an SLR superzoom. The measured closest focus distance (using manual focus) is a fair bit shorter than Sigma specifies, at 28.5cm rather than 35cm. In autofocus mode the camera body won't be able to confirm focus quite so close, though.
Central sharpness at F6.3 isn't bad at all, and gets only fractionally better at F8, before gradually declining at smaller apertures due to diffraction. In our flat-field chart test, corners are soft wide open and improve progressively on stopping down, with optimal sharpness at F11-16. Distortion is minimal, but there's visible blue/yellow fringing due to lateral chromatic aberration.
Full Frame (FX) Coverage
The Canon, Nikon and Sony mount versions of this lens will mount on full-frame DSLRs; on Nikon cameras DX crop mode will be automatically selected (and the camera will therefore shoot at reduced resolution). The lens's image circle doesn't cover the 35mm full frame format properly at any focal length, giving severe vignetting; this effect is very obvious and well-defined at 18mm, but gets increasing diffuse at longer focal lengths. At 250mm the corners are reasonably well illuminated when the lens is focused to infinity, but not with closer subjects.
|18mm, infinity||80mm, infinity||250mm, infinity|
|18mm, 1m||80mm, 1m||250mm, 1m|
The bottom line is that if you shoot using a full frame camera - or plan to do so in the near future - a lens with appropriate coverage really does make far more sense (and for a superzoom, that means something like a 28-300mm).
The 18-250mm features Sigma's own 'Optical Stabilization' system, and the company claims that it allows hand-holding at shutter speeds up to four stop slower than usual without seeing the blurring effects of camera shake. The mechanism is effectively silent when operational, with only the stabilization of the viewfinder image betraying the fact that it's running.
To determine the effectiveness of the OS system we subjected the 18-250mm to our studio image stabilization test, using the wideangle and telephoto settings plus one mid-range focal length (50mm). The subject distance for these tests was approximately 2.5m for 18mm and 50mm focal lengths, and 4m for 250mm; the test camera was the Canon EOS 650D.
We take 10 shots at each shutter speed and visually rate them for sharpness. Shots considered 'sharp' have no visible blur at the pixel level, and are therefore suitable for viewing or printing at the largest sizes, whereas files with 'mild blur' are only slightly soft, and perfectly usable for all but the most critical applications.
|18mm OS OFF||50mm OS OFF||250mm OS OFF|
|18mm OS ON||50mm OS ON||250mm OS ON|
The 18-250mm Macro does pretty well in these tests, although it can't quite match the best in class (the Canon EF-S 18-200mm f/3.5-5.6 IS, which offers almost 4 stops according to our tests). At wideangle we see a bit less than 3 stops of stabilization, with a 70% chance of getting a usable (if not perfectly sharp) image even at 1/4 sec. At full telephoto, it does well too, again providing around 3 stops of stabilization. This is a small but usefull improvement over previous Sigma superzooms we've tested.
Real world examples
The examples below should give you an idea of how well the Sigma's OS system behaves in everyday shooting; in both cases the image would be hopelessly blurred without stabilization. But instead we've been able to take advantage of the ability to hand-hold at slower shutter speeds to use a low ISO and stop down to F8. This should normally give better results than shooting at a larger aperture or higher ISO.
31mm, Canon EOS 650D
212mm, Canon EOS 650D
|1/13 sec, F8, ISO 200||1/30 sec, F8, ISO 100|
|100% crop||100% crop|