Studio Tests

NOTE the line marked 'Nyquist Frequency' indicates the maximum theoretical resolution of the camera body used for testing. Whenever the measured numbers exceed this value, this simply indicates that the lens out-resolves the sensor at this point - the calculated MTF values themselves become meaningless.

The 17-85 gives what can only be described as mixed results in our studio tests. Its performance is rather disappointing at wideangle, where it serves up a witch’s brew of corner softness, green/magenta chromatic aberration, barrel distortion and light falloff; however it becomes very much better at telephoto, and in the 35-70mm range is really very good indeed.

Macro Focus

A decidedly average performance here, with an image area of 10.5cm x 7cm, equating to a maximum magnification of 0.21x, close to Canon’s specified 0.2x. This is achieved at 85mm and the closest focus distance of 35cm, giving a working distance of about 19cm between the subject and the front of the lens. The image shows strong barrel distortion, soft corners and yellow/blue chromatic aberration; this is a poor result, far worse than the kit 18-55mm and certainly no substitute for a true macro lens.

Macro - 105 x 70 mm coverage Distortion: Strong barrel Corner softness: Poor Focal length: 85mm (135 mm equiv)

Specific image quality issues

Chromatic Aberration

By far the most problematic aspect of this lens’s performance in real-life shooting is chromatic aberration at wide angle. Strong green/magenta fringing is visible in 17mm shots even at modest magnifications, and stopping down the aperture has no effect. It remains a problem at 24mm, but is much less of an issue at longer focal lengths. This type of chromatic aberration can be ‘corrected’ in software, and particularly efficiently if you’re prepared to shoot RAW. For instance, using our standard converter, Adobe Camera RAW, processing with -30 Red/Cyan and +30 Blue/Yellow effectively eliminated CA.

100% Crop, left of frame	17mm F8.0, EOS 400D

Aberration correction using Digital Photo Pro (updated July 2008)

With version 3.4 of their free RAW conversion software Digital Photo Pro (DPP), Canon have added lens aberration correction support for the 17-85mm IS. This allows RAW shooters to eliminate (or at least reduce) the effects of chromatic aberration, falloff (which Canon refer to as 'peripheral illumination'), geometric distortion, and 'color blur' (which appears to be Canon's term for axial chromatic aberration).

DPP's aberration correction works by using profiles specific for each lens, making the software fairly straightforward to use, and with no major need to play with the slider settings (except perhaps for the occasional small tweak). Here's an example of chromatic aberration correction using default settings for the image above; DPP has effectively removed the ugly green and magenta fringing completely.

100% Crop, left of frame	17mm F8.0, EOS 400D

The corrections for peripheral illumination and geometric distortion are equally impressive; in the example below we can see clear correction for falloff even at F8 (however in more extreme cases, this will come at some cost to image noise). More importantly, the strong barrel distortion at 17mm is also well corrected; this is most obvious in the bowed line along the lower edge of the frame, but verticals also become properly parallel. Of course there's no such thing as a free lunch, and this kind of software distortion correction will require some localized 'stretching' of the image data resulting in a loss of resolution in affected regions, but overall this is a small price to pay when distortion is otherwise objectionable.

EOS 400D, 17mm F8
Original	Peripheral Illumination	PI + Distortion

Optical Image Stabilization

The 17-85mm features Canon's second generation optical image stabilization, which claims to allow handholding at shutter speeds 3 stops lower than usual before blur from camera shake becomes apparent. It's near-silent in use, with just a quiet whirring noise when operational, and with distinct clicks when it activates and deactivates, presumably due to the lens elements moving in and out of the 'at rest' position.

We've generally found the units in SLR lenses to be pretty effective in real-world use, and to quantify this, we subjected the 17-85mm to our studio image stabilization test at both wideangle and telephoto. With its effective focal length range of 27-136mm, we'd normally expect to be able to get good results handheld at 1/30 sec at wideangle, and 1/160 sec at telephoto without image stabilization. The subject distance for these tests was approximately 2.5m.

17mm IS OFF	85mm IS OFF
17mm IS ON	85mm IS ON

Here we can see clearly the effectiveness of Canon's in-lens IS unit. At the wideangle end and shutter speeds of 1/6 sec, IS is delivering 70% critically sharp shots, compared to just 20% without. Even at very slow shutter speeds of 0.3 sec, IS is giving usable images with no more than mild blur 80% of the time, compared to just 10% without.

Performance is even better at the telephoto end; at 1/20 sec, IS gives 100% sharp images, in contrast to just 10% without IS. Even at 1/10 sec, 90% of images are usable (just mild blur) with IS on, compared to 10% with it switched off. Impressive stuff, the image stabilization is clearly delivering the goods here.