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I'm looking at a Canon 17-40 mm wide angle lens, and I don't understand why anyone would buy it. EFS18-55mm II is the standard Canon 18-55 mm lens that DSLRs come with, and it seems almost as wide – 18 mm vs. 17 mm. Does the wide angle lens have a feature I'm missing, or is that extra 1 mm worth it?

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You are assuming the only measurement of a lens is the focal length(measured in mm). That is not the case. See: What characteristics make a good lens good? –  dpollitt Sep 13 '13 at 18:22
    
@dpollitt, that's not my reading. IMO, Vlad seems to be asking (in part) if the extra width is worthwhile. No need for dupe lawyering here. –  Reid Sep 20 '13 at 19:14
    
@Reid - I believe you can see based on your rep that I didn't vote to close this as a duplicate. I was specifically replying to the piece that notes "I don't understand why anyone would buy it.". And the answer to that question is in the linked to question I provided. Clearly very few if any people buy a lens based on a 1mm difference, they buy it for the other reasons outlined in the linked to question. –  dpollitt Sep 20 '13 at 21:59
    
@dpollitt: (a) If you were replying to a specific portion of the question, you should say so. (b) "Clearly" is in the eye of the beholder. A newbie might not know that 1mm isn't much different. (c) Do you genuinely expect me to waste time digging up whether you voted to close? How is that even relevant? –  Reid Sep 21 '13 at 19:14

4 Answers 4

up vote 15 down vote accepted

I don't understand why anyone would buy it

Optical quality, build quality, and overall durability. The EF 17-40mm f/4L USM is an "L series" lens -- essentially professional grade, while the EF-S 18-55mm f/3.5-5.6 is a consumer grade "kit" lens. L lenses are made with better materials, better designs, and more features. They're weather sealed to keep out moisture and dust. You'll notice that the maximum aperture for the 17-40mm is f/4 across the entire zoom range, whereas it varies between f/3.5 and f/5.6 on the 18-55mm. Most importantly, L lenses tend to have better optics -- sharper focus and fewer optical flaws such as chromatic aberration. Also, the "USM" designation on the 17-40mm stands for "ultrasonic motor", i.e. the autofocus motor is nearly silent yet very fast.

One other important difference is that the 18-55mm is an EF-S lens. The "-S" means "small image circle" -- lenses designated "EF-S" instead of "EF" produce a smaller image at the back end, inside the camera. For that reason, and also because EF-S lenses may project a bit further into the camera body, EF-S lenses only work on crop-sensor bodies (the Digital Rebel series, the x0D cameras, and the 7D. The smaller sensor doesn't require as large an image as larger sensors, and the smaller image in turn allows lenses to use smaller components. That means that EF-S lenses can be smaller, lighter, and less expensive than their EF counterparts. Note that the 18-55mm also includes image stabilization, while the 17-40mm doesn't.

To answer your title question, the difference between focal lengths of 17mm and 18mm is small enough that you could consider the two lenses to be about the same, focal-length-wise, when used on the same body. (On the other hand, 17mm on a full frame sensor is a lot wider than the same lens on a crop sensor. 17mm on a crop sensor gives the same effect as a 27mm lens on a full frame sensor.)

By the way, you might also want to compare the EF-S 17-55mm f/2.8 IS USM to your kit lens. The focal range on that lens is almost exactly the same as the 18-55mm, neither lens is designated "L", both offer image stabilization, and yet the 17-55mm costs about the same (a little more, actually) than the 17-40mm. How can that be? It's pretty much the same answer -- despite some similar specs, they're different lenses. The 17-55mm has a constant f/2.8 aperture, so it needs larger lens elements that collect more light. More glass means a heavier, more expensive lens even though it's an EF-S lens. If you want a similar lens at f/2.8 on a full frame sensor you need even more light and bigger lens elements, so the closest equivalents would be one of the two EF 24-70mm f/2.8L lenses, at $1800 or $2300, respectively.

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Wow, thank you for a very thorough answer! Also thank you AJ Henderson and Patrick Hurley...this is a very helpful community! –  Vlad the Impala Sep 13 '13 at 21:10
    
It doesn't make a huge difference as long as you remain within one ecosystem (Canon DSLRs, say) but it's worth noting that "crop sensor" is not some specific sensor size and thus crop factor (relative to some other sensor size). Canon APS-C sensor sizes lead to a 1.6× effective focal length increase in terms of relative projection size, I think Nikon's design gives you 1.5×, and other cameras may have yet other characteristics. "Crop sensor" here simply refers to the fact that the sensor is smaller than the traditional "small format" 24×36 mm film. –  Michael Kjörling Sep 14 '13 at 10:01

The first thing worth mentioning is that the EF-s lens won't even work on a full frame camera, so it isn't an option for anyone using a 6d or up. The optical quality and barrel distortion are also far better on the L series lens. The build quality is more solid, it is a fixed volume lens, so it can be better weather sealed (or weather sealed at all for that matter). It has a fixed aperture range for the entire zoom range.

Overall, it is simply a better lens in addition to going 1mm wider. Focal length isn't the only factor and a large range of focal lengths isn't generally a strength. The 18-55 is basically a plastic toy with minimally capable optics while the 17-40 is a "real" lens, though I'll admit it is the lens I reach for least in my bag (preferring the much more expensive 24-70 f/2.8 II) but I do use it when I need those extra few mm of angle.

(Personal note, I own a 17-40 f/4L and have an old EF-s 18-55 from on my xTi, they don't even remotely compare in terms of quality.)

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Assuming you are using an APC crop, the difference from 18 to 17 is a 3° wider field of view on the horizontal. On a full frame it would be a little more.

But in this case you are also paying for better glass. The 17-40mm is going to be a lot sharper and has a constant aperture across its focal range (of course it is slightly slower at f/4 vs f/3.5 wide open). It also has better build quality and weather sealing (with a front filter).

It is also worth noting that us crazy photographers will pay a lot for a few mm of focal length and good quality, check out the price on the 14mm f/2.8.

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An even closer comparison is between the Canon EF-S 18-55 kit lens and the Canon 17-55 F2.8 lens. The kit lens sells for under $200, and the 17-55 F2.8 sells for about $1000. Five times the money, and nearly exactly the same zoom range.

Obviously, the price difference is not about the difference in zoom range.

For these specific lenses, the kit is much slower. While its rated at F3.5-5.6, its only F3.5 at the very end of its range, it quickly drops to F5.6. The difference between F5.6 and F2.8 is huge. The F2.8 lens has much more glass, and its much higher image quality.

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