Why does the cheap kit lens seem to be sharper than the expensive L wide-angle?

Question

So recently, I was just browsing through Canon's product lists and found the 17-40 f/4 L to be quite interesting, although I will not buy it anytime in the near future. Still, I decided to make some comparisons, just for fun:

EF 17-40 f/4 L USM vs EF-S 18-55 f/3.5-5.6 IS

These shots were, according to the website, done with the same camera, aperture and almost the same focal length. What struck me is that the L lens seems to be only a little bit sharper in the center, and, even worse, the kit lens seems to beat it in the corner areas!

How can this be? Is the test flawed? Anything I missed out?

Of course, the 17-40 needs to draw a bigger image circle in order to suit FF cameras, and the 18-55 is optimized for an APS-C sensor. But until now I assumed I could still expect clearly better image quality even on an APS-C camera?

Answer

"L" lenses aren't always necessarily sharper than their non-L cousins. In fact, some of them are very much softer, often as a consequence of being two or three stops faster than the cheaper lenses, but sometimes just because they're a decade or two older in design.

What you do get with "L" lenses is professional features and the build quality of a light armoured vehicle. For instance, you might not care so much about a constant aperture while zooming, but I can guarantee that somebody who is using separately-metered studio-type flashes either as main or auxiliary lights would. And the manual focus ring on kit lenses (so far true of both Canon and Nikon) is just a little less than useful.

There have been a couple of "L" lenses that were, frankly, lousy lenses by most objective standards, the most notorious being the now-discontinued 50mm f/1.0. Considering it as a "standard" lens and using it at f/5.6 or f/8, you'd have to be an idiot to spend the extra two grand on that lens (and that was in early-'90s dollars) -- but when TMax P3200 was that fastest thing around, being able to go to f/1.0 for "available darkness" shooting made the difference between being able to get the shot and not being able to get the shot. It didn't matter that the $50 (at the time) f/1.8 version could shoot rings around it at other apertures -- that stop and a half meant everything to the people who needed a lens that fast. Most of the "L" series for the old FD mount were like that -- they filled a niche that pros needed to do their jobs, but were actually worse for general photography than the non-"L" variety. Things got better for the AF EOS era, but there have been a couple of stinkers along the way too. The worst of them have already been replaced by updated designs, but some adequate performers are still in need of an update.

If you want (or need) the build quality, usability and environmental sealing of a pro lens, then the "L" is almost always worth the money. If you're making pictures for your own pleasure and can afford the sometimes fiddly and fragile nature of consumer lenses, then the newer, cheaper optics are often a much better option.

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