Sigma’s DC 17-70mm features image stabilisation.
This is a new version of a useful short macro zoom with a large maximum aperture designed for APS-C/DX sensor format cameras. It has undergone a complete barrel redesign and now comes with an optical cell arrangement of 17 elements in 13 groups, up from the original 15 in 12.
It has been said on more than one occasion that the more elements that are crammed into a zoom lens, the higher the risk of image ghosting caused by internal glass reflections. What is evident on close inspection of the opto mechanics of this lens is that interior elements are well shrouded from stray light. The problem is further addressed in the maker’s choice of Special Low Dispersion (SLD) glass and aspherical elements combined with selective element multicoating.
As is also usual with Sigma products, the lens comes with its own bayonet fitting hood, which is petal-shaped for maximum efficiency through the focal length range. Images obtained when using the lens against strong point source light benefit from using the hood; it more or less eliminates the risk of flare while pushing image contrast a little higher than when not using it.
One of the main new features of this lens is its integral optical image stabiliser function, gaining as much as an extra four stops of hand-held usability with a reasonable guarantee of image sharpness for far lower shutter times. Sensors inside the lens barrel detect vertical and horizontal camera movement, activating a compensating optical cell group to counter shake.
The OS unit gets its power feed from the camera, but it takes nearly a full minute for the system to settle down into “active status” mode. Once the camera is switched on, a quick look through the viewfinder may show an image vibrating as the stabiliser is initiated; only after it has settled will the feature be usable. A halfway depress of the shutter release activates OS through a half to one second period. A small and fiddly button adjacent to the on/off OS switch can be used to lock the function at either setting, although this seemed a little irrelevant to me.
Leaving OS turned on soon drained the battery on my Nikon D2xs; it is one of several issues Sigma comprehensively covers in its lens instructions. It also advises not using the feature when the camera and lens are tripod-mounted or when using bulb shutter timing, as well as warning of odd noises when the lens is removed from a camera with the OS unit switched on.
Zooming from the wide to tele end of the lens is smoothly and firmly accomplished, and focal length selection is finely adjusted and holds position without slacking. Focus is accomplished with the lens in either AF or MF mode, and the switch on the barrel side needs to be set correctly; users cannot use the distance ring to adjust AF manually, but the switch is well placed for more or less instantaneous selection. Overall, handling is improved with a new parallel sided and slightly fatter barrel design. Components are well made using polycarbonate, partially coated with Sigma’s recognisable black matt flock.
As we have seen in previous reviews of Sigma lenses, an established record for high level imaging performance makes them a cost effective alternative to camera manufacturer products. The image circle of DC labelled lenses is too small to be used efficiently on full frame cameras and at the wide end of this sample, some residual barrel distortion was evident in test images with the lens used at maximum aperture. Documented specification lists the minimum focus distance at 22cm, but it forgets to advise this figure is established from the sensor plane; at all focal lengths it is possible to approach the subject more closely, down to just 50mm at the longest end from the lens front element when the hood should be removed to permit improved camera manipulation. Magnification ratios at the minimum distances range from 1:2.7 at 17mm to 1:4.8 at 70mm, with fine details of small objects sharply rendered.
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