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 17-70mm f/2.8-4 DC Macro OS HSM turned out to be a generally reliable performer, with decent optics and effective AF and OS systems. We used it on the Canon EOS 100D and EOS 700D.

Wideangle image quality

The 17-70mm is an excellent performer across most of its zoom range, but it's relatively weak at wideangle, due to a combination of softness off-centre, colour fringing from lateral chromatic aberration, and quite strong barrel distortion. This means that if you spend most of your time shooting at wideangle, it may not be the best choice.

Colour fringing can be readily corrected if you shoot RAW, and if recent Nikon SLRs will remove it in JPEG processing too. Softness towards the edges is less easy to deal with, but some software can make the images look better by progressively increasing the sharpening as you move off-centre. This is shown below, comparing JPEGs from the Canon EOS 700D, to RAW files processed using DxO Optics Pro 8.3 with corrections for CA and lens softness.

Out-of-camera JPEG
RAW + DxO Optics Pro 8.3
Canon EOS 700D, 17mm F8, 1/160sec ISO 100
100% crops, bottom centre
100% crops, left edge of frame

Here you can see strong green/magenta fringing in the JPEGs, and the crops are also distinctly soft. DxO Optics Pro has done a very good job of removing the CA, and its lens softness correction has pulled out plenty of detail too. But this does come at the cost of some fairly strong sharpening artefacts.

Distortion

The 17-70mm also shows quite obvious barrel distortion at wideangle, but to be fair it's not really any worse than other lenses of similar range, and as usual is only likely to be noticeable in shots with straight lines towards the edges of the frame. Again it can be corrected easily enough in post-processing, if you're prepared to take the time. This is shown in the example below; roll your cursor over the labels to compare corrected and uncorrected versions. See how the pier straightens out, making the image look much more natural.

17mm, barrel distortion uncorrected
17mm, corrected using DxO Optics Pro

Flare

The 17-70mm generally deals pretty well with flare, and isn't excessively fazed by strong light sources (usually the sun) in or just outside the frame. In the first example below, with the sun at the edge of the frame, contrast is maintained pretty well, and flare patterns aren't excessive even at F22 (there's also a rather pretty 14-ray sun star).

The second example shows what happens when pointing the lens directly into the light at the telephoto end, so the sun is just outside the frame (above and dead centre), shining directly onto the front element. There's a slight loss of contrast at the top of the frame, and if you look carefully, some coloured patterns at the bottom. But overall the lens has handled a difficult situation pretty well.

17mm, F22, sun at edge of frame 70mm, F5.6, 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 17-70mm does particularly well here, especially towards the long end of the zoom where background blur is most pronounced. It won't match a 17-50mm F2.8 zoom for the degree of blurring, of course, but it'll do rather better than an 18-55mm F3.5-5.6. Most importantly the blurred backgrounds are generally smooth and attractive, so don't distract from the main subject.

Distant Bokeh
Close-up Bokeh
70mm F4, Canon EOS 100D 70mm F5.6, Canon EOS 100D
Background detail, lower right Background detail, lower left

Macro

The 17-70mm's 'Macro' tag reflects its pretty impressive 0.36x maximum magnification. But in practice things aren't so straightforward; this comes at an extremely close working distance, with just over 4cm (less than 2") between the front of the lens and the subject. This means that for close-up work you'll often find yourself getting right on top of your subject, with the large diameter front element frequently casting a visible shadow within the image area. At such close focus distances the lens feels rather 'wide' too (as the the lens's angle of view increases as you focus closer), which results in slightly 'busier' backgrounds than you'd get from a 60mm macro prime, for example.

This example gives an idea of the results you can get out of the 17-70mm, in controlled indoor shooting using a tripod. As usual there's a trade-off between stopping down for sufficient depth of field, and avoiding excessive diffraction softening; in general we found that shooting at around F11 to F16 gave best results. But you can also see just how close to the subject the lens ends up when shooting at minimum focus.

Canon EOS 100D, 0.8sec F16 ISO 200 Shooting set-up

There's another catch for macro work, too. Like almost all image stabilization systems, Sigma's OS becomes progressively less effective the closer you focus. This means you still need to keep shutter speeds up quite high to get properly-sharp images, and therefore will often need to use high ISOs (at least if you're not using a tripod). With the high ISO capabilities of modern DSLRs you can still get pretty decent results this way.

Flash shadowing

One problem we might expect to see, given the lens's large front diameter, is shadowing of the built-in flash at wideangle. But with all the recent Canon SLRs we tried, this only became visible at very short focus distances (less than 0.4m), which would be unlikely to cause any problem in normal use. However it's certainly possible the effect could be much worse with DSLRs that don't lift their flash units so high above the lens axis.