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Canon EF-S 15-85mm f/3.5-5.6 IS USM

Review Date: Oct 11, 2014 Recommend? yes | Price paid: Not Indicated | Rating: 5 

Pros: Great IS, little flare, fast & silent AF
high field curvature, blurry corners

After using this lens for about 2 years I don't quite understand all the raving reviews on the web. I used the EF 17-85 for about 3 years until it stopped working due to the widely known "flex cable" problem. I switched to the 15-85 based on the many positive reviews. Here is my comparison of the two lenses:
- The 17-85 is sharper at the long (telephoto) end of the zoom range.
- The 15-85 is sharper in the corners at the wide end *if* you manage to get center and corners focused simultaneously. Doing so is not as easy as it may sound because the lens exhibits a strong field curvature, meaning that the image field is not flat but strongly curved. So unless you stop down to f11 or more, depth of field will often be too shallow to bring both center and corners properly into focus. This effect accumulates with the inherent inaccuracy of phase detection AF. As a result, corners and edges of images shot with this lens are often *very* blurry and in no way better than what I got from the 17-85 in terms of sharpness.

On the positive side the 15-85 renders images with beautiful color and contrast, the AF and IS work great and the lens is rather resistant to flare for a complex zoom.

The field curvature on the other hand prevents it from reliably exceeding the 17-85 in terms of edge/corner sharpness. The field curvature also seems to be distance-dependent. It appears stronger at large distances. When shooting test charts indoor at short distances the problem is less pronounced. This could be a reason why the lens performs well in formal lens tests. Note also that some sites (like focus the lens separately for each corner which is something you can't do in real shooting.

Note that I had the opportunity to check several copies of this lens and they all performed similarly. I rated sharpness by viewing images from a 10 mega pixel camera at 100% on an LCD screen.

Bottom line, a nice versatile lens, but if you mind having significantly blurry edges and corners in many of your wide angle photos, you better look somewhere else.