The ‘independents’ offer some interesting alternatives to the standard zoom and Sigma has made its 17-70mm model even more appealing by adding optical stabilisation. Report by Paul Burrows.

The standard zoom is arguably one of the most important lens purchases you’ll make as it has to perform a variety of duties. It’s probably the lens that’s on the camera the most and is asked to handle the widest range of subjects and situations. Consequently, it’s the lens purchase that needs a lot more consideration beyond merely accepting what’s packaged with a D-SLR. And this can mean looking beyond the camera system offerings.

Sigma-17-70mm-S1

Sigma-17-70mm-S2The barrel extension tube is marked with magnification ratios as the minimum focusing distance of 22 centimetres is maintained across the zooming range.

In order to make sure they get noticed, the independent lens makers often just go that little bit further in terms of focal range, maximum aperture, minimum focusing distance, features, size and weight. Sigma’s 17-70mm DC-series zoom is a very good example. This is a lens that was first introduced in 2006 and has since been updated, first to incorporate Sigma’s HSM (Hyper Sonic Motor) autofocusing drive and now to add an optical image stabiliser. Sigma’s DC-series lenses are designed for D-SLRs with the ‘APS-C’ size sensors so 17-70mm translates to 25.5-105mm at a focal length magnification factor of 1.5x (i.e. Nikon, Pentax and Sony) or 27.2-112mm at 1.6x (Canon).

Sigma-17-70mm-S3Zoom lock operates at the 17mm setting to prevent the lens extending when it’s being carried on a camera body. In use, though, there isn’t any zoom creep.

In practical terms, these are extremely versatile focal ranges, spanning wide-angle to short telephoto with the so-called standard focal lengths in between. A minimum focusing distance of 22 centimetres – maintained across the focal range – further adds to this versatility by providing reasonably useful close-up capabilities. At 70mm, the magnification factor is 1:2.7 which is just a shade smaller than half life-size reproduction… so the “Macro” part of the model designation isn’t quite so fanciful as it is on some zooms.

Sigma-17-70mm-S4The stabiliser operates in both the horizontal and vertical planes, but doesn’t offer a panning mode. It can still be used with D-SLRs bodies which have built-in sensor-shift stabilisation (i.e. Pentax and Sony), but with the camera based system switched off.
Sigma-17-70mm-S5 The 17-70mm OS zoom performs very well across its entire focal range and aperture range, although some slight fall-off in both sharpness and brightness is evident when the lens is used wide-open at 17mm. Stopping down fixes the problem.

Working Glass

Of course, there are plenty of longer range zooms around, but the Sigma 17-70mm is also designed to be manageable in terms of its size and weight. At 535 grams, it’s still a fairly substantial chunk of lens – especially compared to the typical ‘plastic fantastic’ 18-55mm kit zoom – but this is a good thing as it signifies not just a better quality mechanical construction, but also the use of glass elements in the optical construction. In fact, there’s a total of 17 elements (in 13 groups) packed inside the

Sigma’s polycarbonate barrel tubes so it’s perhaps not so surprising that it tips the scales at over half a kilo. One of these elements is made from ELD (Extraordinary Low Dispersion) optical glass which is designed to minimise chromatic aberrations. There are three aspherical elements which perform a number of duties, among them correcting for distortion, but also helping to maintain image quality across the zooming range. Two of the asphericals are created from the glass moulding process – which makes it easier to form the non-spherical surfaces – while the third is a hybrid type. Hybrid aspherical elements are made by coating a spherical glass core with optical-quality resin which is a technique employed when both surfaces are to be made aspherical (i.e. neither will be bonded as part of a group). The HSM autofocusing drive is built into all the mount versions now that Pentax has adopted lens-based motors to join Canon, Nikon, Sony and Sigma itself. However, some of the earlier Pentax D-SLRs don’t support lens-based focusing motors or, for that matter, lens-based image stabilisation. The stabiliser is also incorporated into all versions, but obviously one or the other system has to be turned off in the D-SLR bodies which have sensor-shift stabilisation.

Holding Steady

While Sigma describes its ‘OS’ or ‘Optical Stabiliser’ system as being “unique”, it operates along the same basic principles as those employed by Canon’s IS, Nikon’s VR and Tamron’s VC.

Gyroscopic-based sensors detect camera shake movement and this is translated via a powerful processor into compensatory adjustments of a special lens group which subsequently keeps the lens’s optical axis precisely centred. Sigma claims that up to four stops of extra ‘speed’ is subsequently available for hand-held shooting which gives the 17- 70mm zoom some pretty handy low-light capabilities… especially since the maximum aperture range is f2.8-4.0. Of course, the reality is that at some point the exposure time becomes so long that it simply isn’t possible to hold a lens still enough for the image stabiliser to be effective. Still, the appeal of the stabiliser is that, in quite a few situations, you can still use slower shutter speeds than would normally be possible without resorting to a tripod. In the hand, the Sigma 17-70mm OS zoom feels like a well-built lens and it has the distinctive ‘Zen’ matte black finish (which gives the ‘sparkly’ look in illustrations). Both control collars have ribbed rubber grips and are quite smooth in their actions. The focusing collar has distances marked in both feet and metres while the inner tube which extends during zooming is marked with the magnification ratios from 28mm to 70mm. The zooming collar has a lock, but it’s merely to keep the lens fully compressed when it’s being carried on a camera body. The bad old days of zoom ‘creep’ occurring when a lens was pointed downwards during shooting are well and truly gone.

The screwthread filter fitting is 72 mm while the supplied petal-type lens hood bayonets to the outer rim of the barrel’s leading edge.

Performance

For testing the Sigma 17-70mm was used on a Canon EOS 50D and, physically, the lens is a very good match for a mid-sized, mid-weight D-SLR body like this. The HSM drive coupled seamlessly with the Canon’s autofocusing so AF operations were fast, accurate and near-silent.

The 17-70mm focal range is also close to what’s possible before a swag of optical performance compromises come into play in order to enable a longer zooming ratio. Consequently, the Sigma is a strong performer across its entire focal range with excellent uniformity of sharpness and brightness. There’s some slight softening towards the corners at 17mm and the maximum aperture of f2.8, but closing down a stop or two significantly reduces the fall-off. The aperture ‘sweet spot’ is between f5.6 and f11 where the edge-to-edge sharpness is impressive, but there’s little to complain about at the larger or smaller f-stops. The story with vignetting is pretty much the same – it’s noticeable at 17mm and f2.8, and, frankly, we’d be surprised if it wasn’t – but disappears at the longer focal lengths and smaller apertures. Chromatic aberration is also well controlled throughout the focal range, but some slight colour fringing is evident towards the edges in big enlargements from image files made at 17mm. There is a small amount of barrel-type bending in the focal range from 17mm to 24mm, but from then on the lens is exceptionally well corrected for distortion. And you really won’t notice anything at the widest angle focal lengths if you compose so that any straight lines in a scene are kept away from the edges of the frame. A seven-bladed diaphragm gives nicely smooth out-of-focus effects and the internal anti-reflection measures – plus Sigma’s ‘Super Multi-Layer Coating’ – keep flare and ghosting at bay unless very strong light is falling across the front element which, of course, is when you fit the lens hood.