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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?

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The 17-40 L being an older misliked lens aside, its also worth noting that in your example the 17-40 is wide open at f4 while the 18-55 is stopped down 1/3 stop which should noticeably improve sharpness. The same comparison at f3.5 favors the 17-40. –  Shizam Apr 4 '11 at 1:01
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@Shizam, "The same comparison at f3.5 favors the 17-40." - well not really, you wont get any photo at 3.5 from 17-40 ;) –  kristof Apr 4 '11 at 9:47
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Of the 18-55 @f3.5, ie both of them wide open. –  Shizam Apr 4 '11 at 15:33
    
Also remember that part of the benefit (price) of the L lens is fixed aperture and sturdiness of the build. –  Michael Nielsen Feb 27 '13 at 19:20
    
@MichaelNielsen: Not all "L" lenses have constant aperture. The EF 100-400mm f/4-5.6L IS USM and the EF 70-300mm f/4-5.6L IS USM are two examples. –  Michael Clark Feb 28 '13 at 10:03

4 Answers 4

up vote 16 down vote accepted

"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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I find this answer somewhat malformed and misleading "some of them are very much softer, often as a consequence of being two or three stops faster than the cheaper lenses" makes no sense, do you mean if both are shot at max aperture? "...can afford the sometimes fiddly and fragile nature of consumer lenses, then the newer, cheaper optics are often a much better option"... what? Picking an old red-headed stepchild L lens as an example vs a very very new consumer lens then maybe, but as a rule this is not a good guideline. –  Shizam Apr 4 '11 at 0:59
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No, you don't necessarily have to shoot an expensive, fast lens wide-open to experience softness. "L" (or a gold line, or whatever the maker's distinguishing mark might be) is not a guarantee of sharpness. My answer says precisely what I meant it to say. –  user2719 Apr 4 '11 at 3:18
    
Thank you for giving me a new understanding of what the "L" actually means! –  eWolf Apr 5 '11 at 16:33
    
a photographer once asked me "is the sharpest picture always the best?" –  Michael Nielsen Feb 27 '13 at 19:22

Especially when comparing older full frame "L" lenses to newer EF-S lenses there are several things to consider.

  • Image circle size: AN EF lens must project an image circle large enough to cover a "full frame" sensor or film negative (roughly 36X24MM). An EF-S lens must only project an image circle large enough to cover the Canon APS-C sized sensors that are approximately 22.5X15mm. Since the APS-C image circle is only about 40% the area of an EF image circle, this allows the lens design to be smaller and lighter. Another benefit is that the lens elements themselves can be smaller, and thus use smaller amounts of the expensive materials used in modern lens design. In the case of these two lenses, the 18-55 can use smaller amounts of high quality materials and still be manufactured for a very reasonable cost.

  • Optics are only part of the equation: The "L" lens designation also carries an expectation of better build quality, rock solid durability, and resistance to the elements like rain, dust, and extreme temperatures. The Ultra Sonic Motor (USM) focus of the "L" lens line means they can be manually focused without moving the AF/MF switch out of AF, and they have an actual usable rubber focus ring on the lens barrel instead of ridges molded into the plastic of the front element housing of the lens. The USM is also faster and quieter when focusing. The front element does not rotate on "L" lenses so use of polarizers and graduated ND filters are much easier to set without having to be repositioned every time the focus is changed.

  • Age of the design: The EF 17-40mm f/4L USM was introduced in 2003. The EF-S 18-55mm f/3.5-5.6 IS was introduced almost 5 years later. That doesn't sound like much until you compare the digital bodies current when each was introduced. The 1Ds, the first Canon FF digital body, was only a year old and the 10D was introduced about the same time as the EF 17-40mm f/4L. The original Digital Rebel was still in development. By the time the EF-S 18-55mm IS was introduced, the current bodies were the new 1Ds mkIII, the 40D, and the third generation Rebel XTi was over halfway through its life cycle. At the time we were just beginning to discover how much more demanding digital imaging is of lenses than film. No one used to blow every frame of film up to 60X40 to look at it, yet that's what I do every time I pixel peep at 100% om my monitor.

As others have noted, the EF 17-40mm f/4L is the smallest, lightest, and least expensive "L" series lens currently sold. I own one and in terms of optical performance it is a step below my other "L" glass. There are many lenses available for much less money that have comparable optical performance on EF-S sized bodies. For full frame cameras the options are much more limited and the EF 17-40mm f/4L becomes a "value" ultrawide zoom lens for them.

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Generally speaking, one must consider the age of the technology when doing fair comparisons. The 18-55 IS is a fairly new lens (replaced the 18-55 non-IS only a couple of years ago), while the 17-40 is an older lens (by itself, though, it does not necessarily mean that a newer lens is better/sharper).

Then, like any mechanical device, there are manufacturing tolerances among batch samples. It is not impossible that the L sample in the test is not the best example of this line.

Third, it can be that the lens just needs calibration after some use and mounting on the specific camera body.

Fourth, the "L" stands for more than just image quality. It also counts for build quality, weather resistance, etc.

There are many more aspect contributing to the image quality that are not seen in this focus test. Resistance to flare, color rendering, geometry distortions, aberrations, and more. All these can count towards a better IQ even with lesser sharpness.

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A lot of on-line commentators express disappointment with the 17-40's optics and consistency in particular. And it's smallest, lightest L lens available. It's among the cheapest as well. The L primes cost and weigh more.

The reviews all seem to agree that the 18-55 IS has really good optics for the price.

Some observers of photo tech trends argue that modern high-pixel-density sensors are quite a lot more demanding of lenses than lower-density sensors from just a few years ago. The 17-40L was released in 2003, so it was probably designed during 2002 or before, when it was obvious that digital photography was the future but it perhaps wasn't as obvious that SLR sensors would become as dense as they have.

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+1 for a factual reply with references. –  labnut Apr 4 '11 at 5:48

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