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To compare sharpness at a mid-telephoto focal length of 135mm, we shot this scene with the Canon EF-S 18-135mm IS, using an EOS 7D body at all apertures. The 7D was set to RAW to avoid in-camera JPEG corrections; all files were processed in Canon DPP with lens corrections disabled and the default sharpness of 3. IS was disabled on each lens, and magnified assistance in Live View used to confirm the focusing.

You'll also notice the f5.6 crop is a little darker than the one at f8, due to vignetting. As you'll see in our vignetting results page, the EF-S 18-135mm IS suffers quite badly from darkening in the corners when zoomed-in and at its widest aperture of f5.6, although closed one stop to f8 greatly improves matters. This is also something which can be effectively corrected either in-camera with models which support Peripheral Illumination Correction, or after the event in the DPP software.

Since fringing and vignetting can both be corrected quite effectively in software, we're not going to berate the lens too much for suffering in both respects. Of greater concern though is the actual sharpness of the image in the corners. At f5.6, details in the corners are quite soft, and while there's some improvements at f8, the extreme corners remain relatively blurred. You can see this in the crops below, where the upper right corner improves a little on the f8 sample, but the lower left corners remain quite soft at both f5.6 and f8.

This softness can't be corrected after the event, so if you want a pin-sharp image across the frame when zoomed-in with the EF-S 18-135mm IS, you'll need to crop the extreme edges away and accept a small loss in resolution. Now scroll down to see how the situation compares in the centre of the frame at 135mm.