Design

The 17-70mm uses Sigma's latest cosmetic design and finishing, introduced alongside its rearrangement of its new lenses into 'Contemporary', 'Art' and 'Sports' categories. The changes aren't in fact huge - the lens name has moved from the zoom ring to the barrel, and lettering that used to be gold is now white - but the resultant entirely monochrome look is somewhat more understated and elegant than before. It's rounded off by the silver 'C' (for 'Contemporary') badge, that's inlaid into the barrel.

In terms of construction though, the 17-70mm is much like other recent Sigma lenses at this price level, which means a lightweight but solid-feeling black plastic barrel, and relatively smoothly-operating zoom and focus rings. It uses a 'double trombone' design to extend to its longest setting, with impressively little play of the barrel when set to 70mm. As we'd expect of a lens at this price, the mount is metal.

Two large, positive switches on the side of the barrel control the autofocus and image stabilization mechanisms. The AF switch is distinctly the larger of the two, making it easier to distinguish by touch alone while shooting. It also has a white inlay that's visible when the lens is set to AF, which provides a quick visual check for the focus mode in poor light when the lettering can be hard to see.

Compared to Canon EF-S 18-55mm f/3.5-5.6 IS STM

Here's the Sigma alongside a fairly typical SLR kit zoom, Canon's latest EF-S 18-55mm f/3.5-5.6 IS STM. This view shows how impressively Sigma has managed to minimise the size of the 17-70mm; despite its longer zoom range and faster aperture, it's really not very much bigger. The real difference, though, lies in the weight - the Sigma is over twice as heavy.

On the camera

These views reiterate the 17-70mm's size, or rather, lack of it. On the EOS 700D shown left, it's perfectly well balanced and doesn't feel bulky at all. Even on the diminutive EOS 100D it's not hugely out of proportion. The zoom ring is placed towards the centre of the of the barrel where it falls naturally to hand, and the AF and OS switches operate with satisfyingly positive clicks.

About the only operational criticism lies with the manual focus ring; it's slightly 'loose' in feel, and coupled with its short travel, this can make critical manual focusing slightly tricky. Then again, we suspect the majority of users will use autofocus almost all of the time, so this won't matter much. The ring also rotates during autofocus, but that's true of many lenses at this price level.

Dependence of effective focal length on focus distance

This lens's angle of view widens substantially on focusing from infinity to 0.22m, particularly at the telephoto end. This isn't unusual with internal focusing zooms, and in normal use isn't really noticeable. Its biggest impact is for close-up work, where the effective focal length is closer to 50mm than 70mm. This means it feels like a rather 'short' macro lens, with a wider field of view than you'd get with a 60mm macro prime.

Autofocus

The 17-70mm uses Sigma's Hypersonic Motor for autofocus, and when shooting with the optical viewfinder our Canon mount sample was pretty quiet in use, and impressively fast and decisive. As always, though, it must be noted that focus speed and accuracy is dependent upon a number of variables, including the camera body used, subject contrast, and light levels.

Switch from the optical viewfinder to live view, though, and the story changes. Focusing slows right down (although the exact speed is highly dependent on the camera used), and if you refocus during recording the clicking of the AF motor will be audible on your soundtrack as the camera fine-tunes focus. To be fair most SLR lenses behave much the same, as they're just not optimised for video work (the honourable exceptions being Canon's STM models).

Lens body elements

The lens comes in Canon, Nikon, Pentax, Sigma and Sony versions; our review sample was in the Canon EF mount.

This view shows the somewhat unusual 'double trombone' mechanism which is used for moving the rear element on zooming.
The filter thread is 72mm, which means rather more expensive filters compared to the typical kit zoom. It does not rotate on autofocusing, though.
The bayonet-mount hood is provided as standard, and clicks positively into place on the front of the lens. It's made from thick plastic, and features ribbed moldings on the inside to minimize reflections of stray light into the lens. Sigma has even added a ribbed grip to make it easier to remove.

A white dot on the outside of the hood aids alignment for mounting, and the hood reverses neatly for storage.
The zoom ring has a 21mm wide rubber grip, and rotates 60 degrees anti-clockwise from wide to telephoto (the same way as Canon lenses, but opposite to Nikon, Pentax and Sony's). In typical Sigma fashion the action is smooth and even.

The front element extends 40mm on zooming and feels impressively solid when fully extended, with just a little lateral play.
The focus ring is 15mm wide grip, although the ridged grip makes up just 4mm of that. It rotates 50 degrees clockwise from infinity to 0.22m, matching Canon and Sony lenses but opposite to those from Pentax and Nikon. The focusing action is smooth, but a little loose; the ring also rotates during autofocus.

A basic distance scale is marked in feet and meters. The imperial markings are in mid-grey on a dark grey background, which isn't especially legible.
Two chunky, positive switches on the side of the lens barrel set the focus and image stabilization modes. When the focus switch is set to AF a white inlay is visible behind it; this provides a quick, positive visual confirmation of its position.

You can't adjust focus manually when the lens is set to AF (or at least, you shouldn't try).
A slightly curious scale on the outer sleeve of the 'double trombone' zoom mechanism shows the image magnification when the lens is set to its minimum focus distance of 0.35m. The numbers correspond to each of the focal lengths marked on the zoom ring, except for 17mm.
As on other recent Sigma lenses, a deeply-ridged grip covers most of the underside of the barrel, and provides positive traction for changing lenses. It's a small touch and easily overlooked, but genuinely worthwhile.
An inset silver-coloured 'C' on the lens barrel denotes that this lens is part of Sigma's 'Contemporary' range.

Sigma USB dock compatibility

The 17-70mm is compatible with Sigma's unique USB dock, which allows you to plug the lens into your computer and apply detailed adjustments to its autofocus calibration. You can also update the firmware, which in principle provides a greater degree of 'future proofing' and reassurance that the lens will continue to work properly with future camera models.

The USB dock fits onto the lens mount, and plugs into your computer. Lens settings can be changed using Sigma Optimisation Pro software, downloadable from the company's website. The main option available for the 17-70mm is AF microadjustment, allowing correction of any systematic misfocusing you may experience. You can also update the lens's firmware.

The AF microadjustment options are more detailed than you'll find on any camera body, allowing you to define separate corrections for four different focal lengths at each of four focus distances. So if (for example) you find your copy of the lens tends to set focus slightly in front of distant subjects and slightly behind closer ones, but only at the telephoto end, then no problem: you can fix that. However it may well take quite a lot of time and experimentation to get it right.

The 17-70mm will also be eligible for Sigma's Mount Conversion Service, which means that you'll be able to pay to have your lens converted to a different mount should you choose to change camera brand. We're not sure whether this will offer a useful saving compared to simply selling your lens and buying another in the new mount, but the option's there if you want it.

Reported aperture vs focal length

Here we show the maximum and minimum apertures reported by the camera at the marked focal lengths.

Focal length 17mm 24mm 28mm 35mm 50mm 70mm
Max aperture
F2.8
F3.2
F3.5
F3.5
F4.0
F4.0
Min aperture
F22
F22
F22
F22
F22
F22

The 17-70mm is unusually fast for a small normal zoom. Compared to a typical 18-55mm kit zoom it's 0.7 stop faster at wideangle, and a whole stop faster at telephoto. This extra light-gathering ability is genuinely useful.

Sensibly, Sigma has limited the minimum aperture to F22 throughout (at least on our Canon mount sample) rather than allow smaller apertures at longer focal lengths, which would show excessive image quality degradation due to diffraction.