Image Qualityshow all test shots
Note: the test shots for each of the image quality tests can be compared directly amongst all tested lenses. Within the test box click on "Choose lens to compare with..." to add a lens to the comparison. Then use your mouse to hover over the lens thumbnails. If all those test shots are a little too overwhelming consider clicking on "hide all test shots" above this paragraph.
Sharpness / Resolution
The EF-S 15-85mm f/3.5-5.6 IS USM is a very sharp lens. The resolution is highest at the tele end of the focal range but it's good throughout. It is a lot better than the kit lens EF-S 18-55mm f/3.5-5.6 IS II and also better than the EF-S 18-135mm f/3.5-5.6 IS. If you consider full frame EF lenses it has a resolution that's very much like the EF 24-105mm f/4L IS USM (on APS-C cameras) - which is an L series lens! Note that Canon (so far) do not give the L series tag to any of their EF-S lenses (probably to suggest that pros should go with full frame bodies).
At 24 mm effective focal length the EF-S 15-85mm f/3.5-5.6 IS USM shows a massive amount of barrel distortion. That's not nice but consider that this lens is wider by a couple of mm than say the kit lens EF-S 18-55mm f/3.5-5.6 IS II. But of course even at 24 mm lenses can perform much better in regard to distortion. Compare with the similarly priced EF-S 10-22mm f/3.5-4.5 USM at the same focal length to see how well distortion could be under control. Unfortunately distortion cannot be reduced by stopping down so if you need a distortion free picture go for longer focal lengths (or correct it in post). At 35 mm (effective) and upwards the EF-S 15-85mm f/3.5-5.6 IS USM shows only slight barrel (and from 50 mm pincushion) distortion.
Crop from top of image (downsized)
Crop from bottom of image (downsized)
Color Errors Transverse Chromatic Aberrations
At 50 mm and 70 mm effective focal length the EF-S 15-85mm f/3.5-5.6 IS USM shows moderate color errors. At all other focal lengths the color errors are quite intense and they cannot be reduced by stopping-down. The performance is better than that of the EF-S 18-55mm f/3.5-5.6 IS II and the EF-S 18-135mm f/3.5-5.6 IS but it is not good.
Beauty of blurred image parts Bokeh and Axial CA
Fortunately the EF-S 15-85mm f/3.5-5.6 IS USM has an evenly circular aperture shape. This is ideal to get nice and even out-of-focus blur. Canon achieve that by using 7 aperture blades of the circular type. Even more important the lens's performance in regard to color fringes in those out-of-focus areas of the image is outstanding. Even at maximum aperture hardly any color fringes are noticeable. Only at 50 mm and 70 mm effective focal length the EF-S 15-85mm f/3.5-5.6 IS USM shows slight color fringes that already fade after stopping down a single stop.
Curvature of the focal plane Field Curvature
From 24 mm to around 50 mm effective focal length the EF-S 15-85mm f/3.5-5.6 IS USM struggles with curvature of the focal plane. To reduce it you will have to use very high f-stops like f/11. Whether or not curvature is a problem for you depends a lot on what your subject is. If it's a straight wall that you'd like to capture from the left to the right image border you are in trouble (on the other hand a shot like this might be done using high f-stops anyway). In most real-life applications though you will use the focus more selectively and since the lens doesn't suffer from soft corners you'll have no trouble positioning your subject anywhere in the image. But getting everything in focus from the corner to the center at once will be difficult with this lens if you aim for the low f-stops.
Corner Shadow Vignetting
At wide focal lengths such as 24 mm (effective) any lens shows intense corner shadows - that's plain physics and cannot be avoided. Having said that the EF-S 15-85mm f/3.5-5.6 IS USM actually performs pretty well at 24 mm. Compare with the EF-S 10-22mm f/3.5-4.5 USM at the same focal length or the full frame L series lens EF 24-105mm f/4L IS USM (compare that one with the full frame setting to be fair) and you'll notice that the EF-S 15-85mm f/3.5-5.6 IS USM's corner shadows are less pronounced. At longer focal lengths the lens shows less and less corner shadow until from around 70 mm it is no longer noticeable.