The 18-55mm STM lens is a low-cost low-weight general-purpose zoom for Canon's APS-C cameras. It is often supplied with a new camera purchase as a "kit" lens.
Since being welcomed by the photographic community about 2 years ago, it has garnered variable reviews, with some rating it as ho-hum whereas others consider it to be fantastic. I purchased my copy about a year ago, and it has performed marvelously for me. In fact, I have been using the 18-55mm STM routinely as a replacement for the highly regarded 17-55mm/2.8, which despite its excellent reputation was giving me serious autofocus problems. (Those who are interested can find a thread on the 17-55mm/2.8 problems here.)
Eventually I became curious about the actual abilities of the 18-55mm STM and how it compared to the revered 17-55/2.8. What was I missing by using the STM?
So I did some comparison shots with the two lenses. For the purpose, I selected two focal lengths, wide (20mm) and slight tele (35mm). For apertures, I looked at f/8, f/5.6 and f/4, or whatever the widest aperture was for the 18-55mm STM. Focusing was done manually using Live View. That got around the AF problems with the 17-55mm lens. The tests were done on a tripod.
When reviewing the results, I noticed a couple of things immediately: the STM lens makes slightly brighter pictures at identical settings (by about 1/6 of a stop), and the STM photos are a bit bluer (with a temperature difference of about 150-200K). Both lenses displayed chromatic aberration off-center, but there is no question that the STM lens showed more. My tests were not set up to compare barrel and pincushion distortion, but I believe the 18-55mm lens has more, depending on focal length.
Anyway, chromatic aberration and distortion are both easy to fix in post-processing. Accordingly, lens corrections were applied to all the test shots for the sharpness comparison.
The two lenses were very close in sharpness. Sometimes one would win by a slight margin, and sometimes the other. Several times I couldn't tell which was better. At a focal length of 20mm, the 17-55 won by a small amount at f/3.5 for center and edge sharpness. At the other apertures, sometimes one lens or the other would be slightly better, and sometimes I couldn't tell.
At 35mm, the widest aperture common to both lenses was f/4.5. At that aperture, the 17-55 was slightly better in the center and the 18-55 slightly better at the edge. At f/5.6 and 8, the lenses were essentially tied, although to my eye the 18-55 had a slight edge at times.
So the two lenses were essentially equivalent at common apertures after routine lens corrections were made in software.
To help you decide for yourself, I am reproducing two of the comparisons here. Both were taken at 35mm and f/4.5. Lens corrections were made for CA and distortion in Lightroom, but not for brightness and color. Note that the shutter speeds differed slightly. These are 100% crops.
The first is the CENTER of the image, and I rate the 17-55mm as slightly better.
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The second is the EDGE of the image, where in my opinion the 18-55mm wins.
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The 18-55mm STM lens is an excellent general purpose zoom that is essentially the equal of the much more expensive 17-55mm/2.8 after doing lens corrections in software. Considering the AF problems that some experience with the 17-55mm lens, I would avoid the latter for outdoor use. It is still a good choice indoors where its large aperture can be used to advantage, and where there is no AF problem because of the shorter range.
As for the 15-85mm lens, the humble 18-55mm STM is its equal in the center and outperforms it in the edges and corners. I would avoid the 15-85mm lens entirely.
