Canon EF-S 17-85mm f/4-5.6 USM IS - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

The lens broke a new record here - at 17mm it shows the most pronounced barrel distortion of all (supposedly) corrected lenses tested to date and that's by quite a margin. As you may observe in the sample shot below this is not only theory ...

Typical for all zoom lenses the barrel distortion changes to pincushion distortion when zooming towards the long end of the zoom range. At about 24mm both forces even out technically though there is still some wavy-like distortion left here. At 50mm and 85mm the level of pincushion distortion remains relatively slight so for most of the zoom range the performance is actually pretty decent.

Move the mouse cursor over the focal length text marks below to observe the respective distortions
17mm 24mm 50mm 85mm

The chart above has a real-world size of about 120x80cm or 60x the focal length. Expect higher distortion levels towards closer focus distances.

Vignetting

So far most of the tested lenses were full-frame variants. APS-C DSLRs such as the EOS 350D can take advantage of the sweet spot of the lens here usually resulting in pretty decent vignetting figures. However, the EF-S 17-85mm is a native APS-C lens with a reduced image circle so it is not surprising that vignetting is far more pronounced exceeding 1.3EV at 17mm @ f/4 and 0.7EV at 85mm @ f/5.6. As usual stopping down a little helps to reduce the problem significantly.

MTF (resolution)

The lens produced a relatively decent performance in the lab but there're clearly some flaws here. Throughout the zoom range the center performance is generally great approaching the resolution limits of the EOS 350D (8 MP). The corners at about 70% off the center can be considered as good. Due to the reduced image circle a closer look at the extreme corners at about 90% off the center revealed some problems. At 17mm these extreme corners showed only an okayish quality and stopping down did not really improve the issue. At 24mm the extreme corners recover towards medium apertures.

The 1st sample of the lens suffered from a centering defect and a test of a 2nd sample did produce better results specifically regarding the center performance and the border performance towards the tele end.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) can be a significant problem from 17mm to 50mm where the average CA pixel width at the image corners can exceed 1.4 pixels. Interestingly the 2nd tested sample of the lens performed better here specifically at 17mm. At 85mm CAs are generally a non-issue.




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