The Sigma 18-250mm F3.5-6.3 DC OS HSM behaves quite similarly to the older 18-200mm in our studio tests, and therefore has a quite different balance of characteristics to most other superzooms. Distortion is unusually low, but this comes at the expense of sharpness - especially towards the telephoto end. Overall performance is therefore somewhat less consistent than any of its direct competitors, the Canon EF-S 18-200mm F3.5-5.6 IS, the Nikon AF-S 18-200m F3.5-5.6G DX VR, or the Tamron 18-270mm F3.5-6.3 Di-II VC.
|Sharpness||Sharpness results are extremely mixed. Typically for its class, the 18-250mm is strongest in the mid-range (24-50mm), but then has a fairly catastrophic dip in sharpness at longer focal lengths before rallying a bit at full telephoto. At almost all focal lengths (except 50mm), the corners are distinctly softer than the center of the frame. At its best (24mm F5.6) the lens returns excellent results across most of the frame; at its worst (135mm F5.6) it's very soft indeed. Overall, like most superzooms, optimum sharpness is normally obtained a stop or two down from maximum aperture.|
|Chromatic Aberration||Lateral chromatic aberration is a mixed bag - there's pronounced red/cyan fringing at wideangle, and even worse green/magenta fringing at full tele, but scarcely any in the middle of the zoom range. To be fair, few superzooms do any better, and some are worse.|
|Falloff||We consider falloff to start becoming a potential problem when the corner illumination falls to more than 1 stop below the center. Falloff is generally low and unlikely ever to be photographically problematic; it measures just 1 stop at 18mm F3.5 (essentially disappearing on stopping down to F4.5), and at longer focal lengths it's too low to be of any concern.|
|Distortion||Sigma has managed to keep distortion unusually low for a superzoom. Barrel distortion at wideangle measures 1.9%, however this is a complex 'wave' distortion with substantial corner re-correction, of a type that is relatively difficult to correct in software if desired. At longer focal lengths the pattern changes to pincushion, reaching a maximum of about -1.9% in the 35-50mm range. Overall, in the same fashion as the 18-200mm OS, this is rather less extreme than we've seen on other superzooms (e.g. the Canon 18-200mm's 3.4% barrel distortion at 18mm, or the Nikon 18-200mm's -2.3% pincushion distortion at 50mm).|
|Maximum magnification is 0.38x, achieved at 250mm and a closest focus distance of about 32.5cm, giving a rather tight working distance of just 10cm from the subject to the front of the lens. This can only be attained using manual focus, though; in AF the lens will focus no closer than Sigma's specified 45cm, giving an image area of 76 x 51 mm at 0.30x magnification (click here for test chart shot).
Image quality is reasonable, with central sharpness peaking around F8-F11. The corners of our test chart image are however distinctly soft wide open, and continue to sharpen up on stopping all the way down to F22. There's visible chromatic aberration, but almost no distortion.
|Macro - 59 x 39 mm coverage
Corner softness: Poor
Focal length: 250mm (400mm equiv)
FX (Full Frame) 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 image circle almost covers full frame, but the extreme corners are still completely black; this might be interesting for specific creative purposes, but not really usable for normal everyday shooting.
The bottom line is that if you shoot using a full frame camera, a lens with appropriate coverage really does make far more sense (and for a superzoom, that means something like a 28-300mm).
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 turned out to be a perfectly competent performer in normal use, although the combination of a slow maximum aperture and relatively ineffective image stabilization can make it difficult to get good results at the long end of the zoom.
The 18-250mm seems reasonably resistant to flare (although the general invisibility of the sun over the majority of the testing period mans we've not been able to get as good a feel for this as usual). With the sun in the frame at wideangle there's a degree of veiling flare, resulting in an overall loss of contrast, and if you stop right down a few bright flare spots can become visible.
Towards the longer end of the range, we saw few serious problems with flare either. However you can get some veiling flare if you attempt to shoot strongly backlit subjects with the sun just outside the frame, but still impinging on the front element; luckily you can normally see in the viewfinder when this is likely to be a problem.
|18mm, F22, sun in corner of frame||147mm, F6.3, sun just outside frame|
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 always the best lens in the world when it comes to rendering out-of-focus areas (although to be fair, superzooms rarely are). For closeups the bokeh is attractive enough, with generally smooth blurring of the out-of-focus regions. More distant backgrounds, however, are often quite 'busy', with harsh edges to transitions, and occasional double-line rendition of out-of-focus elements (so-called 'nissen bokeh').
|250mm F6.3||178mm F6.3|
|25% crop, upper right||25% crop, 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 even stronger green/magenta fringing at telephoto. In the middle of the zoom range there's scarcely any CA visible at all. The samples below give an idea of how this looks in practice - note these are 100% crops from the most demanding APS-C DSLR currently available, the 17Mp Canon EOS 7D.
This CA can be removed pretty effectively in post-processing - indeed on all current Nikon DSLRs except the D3000 it will be automatically corrected in the JPEG output anyway. The examples below give an idea of how effective this can be, using Adobe Camera Raw's Lens Correction module; the visibility of the fringing is dramatically reduced.
|F8, Canon EOS 7D||F9, Canon EOS 7D, ISO 200|
|100% crop, top left (JPEG)||100% crop, lower left (JPEG)|
|100% crop, RAW + CA correction||100% crop, RAW + CA correction|
Softness at mid-telephoto (ca 135mm)
One issue impossible to ignore from our studio tests is the significant drop in sharpness across almost the entire frame encountered at the telephoto end (focal lengths of 80mm and longer), and especially at 130mm. As usual we like to demonstrate how this appears in real-life images, and the samples below show the kind of results you'll get at 120mm both wide open at F5.6 and stopped down to F8, again using the EOS 7D.
Wide open, the center of the frame is distinctly low in contrast and lacking in detail, and the corner is very smeared indeed. Stop down to F8 and both areas of the image improve; but while the center is now quite acceptably sharp, the corner is still distinctly soft. Of course for many typical telephoto subjects - sports or wildlife, for example - corner sharpness is often unimportant.
120mm, Canon EOS 7D
100% crop, center
100% crop, top left
The 18-250mm features Sigma's own 'Optical Stabilization' system, and the company claims that it allows handholding 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 8m for 250mm; the test camera was the Canon EOS 7D.
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's identikit performance to the 18-200mm OS extends to the stabilization results. Once again, we see a solid enough performance at wideangle, with about 3 stops of stabilization (the graphs show similar results at 1/4 sec with OS enabled as at 1/30 sec with it turned off). But the effectiveness diminishes progressively towards the telephoto end, to just 2 stops at 250mm (which unfortunately is where you need it most). Many of the images at the long end are also slightly soft, for exactly the same reason as we observed on the 18-200mm, i.e. under-correction in the vertical direction. Overall, the OS doesn't seem to be as effective as on competing lenses from the likes of Canon Nikon or Tamron.