Note on 9/2/2008: A year after submitting this review, I'm noticing that it now appears under the wrong product listing. I wrote this review for the lens in CANON mount, but my text has somehow migrated into the product listing for PENTAX mount. This is the 9th or 10th time that a lens review of mine has either disappeared altogether, or migrated to a different product. Epinions - please get your act together! I'll move this review to its proper place when I have time, but I suspect it will just migrate again.
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Most camera-store salespeople will tell you that "all-in-one" lenses like this are optically poorer than lenses with less range. Hence, I was surprised to see that Sigma's optical sharpness is about as good as Canon's 18-55mm kit lens, and purple fringing is less, despite having 3-4 times the zoom range.
The Sigma 18-200 also has much better optics than the Tamron 18-200, whose chromatic aberration, purple fringing, and corner softness are all worse than the Sigma. I'd also tried the Sigma and Tamron 28-300mm lenses for full frame cameras, and results were very similar - the Sigma clearly outperformed the Tamron in sharpness, and also had less purple fringing. (By the way, Tamron has vastly improved their optics with their latest 18-250mm lens - my gripes only apply to their older 18-200mm model).
I had high hopes that the Sigma would be the perfect upgrade from the kit lens, but unfortunately, the Sigma has one major problem that at first appeared to be a deal-breaker: it does not autofocus properly at 200mm for distant objects. I tried three copies of the lens on three different DSLR bodies (rebel 300D and XT), and all combinations front-focused at 200mm for distant objects (>30 feet away). In other words, the sharpest objects in the photo were always objects in front of (closer to the camera) than the object I'd actually focused on. I had this problem even under ideal lighting conditions, and when using center-spot focus, which eliminates any uncertainty about the focus point.
Because I also owned the Tamron 18-200 lens, and because it focused perfectly at all focal lengths, I assume the problem is in Sigma's electronic interface with the camera. I was very close to abandoning this lens, when I noticed that it is parfocal, meaning its focus does not change as one zooms. Hence, I could autofocus accurately at about 135mm, then zoom in to 200mm to shoot. Though cumbersome, this gets me photos that are quite sharp at 200mm. In fact, when correctly focused, center sharpness at 200mm is close to that of Canon "L" lenses I've used (of course, the "L"s are much better all around, with better edge/corner sharpness, autofocus, and chromatic aberrations).
Additional thoughts on misfocus issue The lens only mis-focuses in the 150-200mm range, so at first I thought the occasional fuzzy shots were due to user error. But after trying 3 copies of the lens on 3 camera bodies, and comparing with the Tamron lens, I noticed the front-focus behavior was exactly the same across all three Sigma lenses, in every case occurring at 200mm for distant objects but not with close objects. The defect was so amazingly consistent that I concluded this was no fluke: Sigma apparently made a design mistake somewhere in this lens.
In real-life situations, the long end of the zoom is most useful for distant objects, so the focus errors at 200mm were pretty inconvenient. If you want to see the focus problem, I have posted a sample 100% crop at the following link. To see it, cut and paste the URL into your browser, then scroll down to the photo of the red helicopter:
http://www.geocities.com/tom_jhou/sigma_tamron_18_200.html
In online forums, many people say the Sigma gets soft at 200mm, but I've found the lens to be quite sharp when focused correctly, so I suspect the reported softness is actually due to misfocusing. As I said above, the Sigma should be almost as sharp as a Canon "L" lens in the center, even at 200mm.
A respected photography site, Photozone, also reports focus problems at 200mm, using that flaw to conclude that "there is no free lunch":
http://www.photozone.de/8Reviews/lenses/sigma_18200_3563/index.htm
The photozone site conjectures that the focus errors are due to the small (f/6.3) aperture at 200mm, but this can't be the whole story, because the
Tamron 18-200 focuses perfectly on my camera at 200mm, yet is also f/6.3. ). I have also had a similar focus problem with Sigma's 18-125 lens, also for distant objects, and its aperture is a larger f/5.6.
Sigma can't fix the focus problem After returning two misfocusing 18-200 lenses, I sent my 3rd copy to Sigma for calibration. I included a detailed description of the problem, along with photos. It came back with a note denying any focus problem. Very frustrating. It makes me wonder if they bothered to test it on very distant objects, something that would be difficult with an indoor lab.
I also spoke on the phone to a Sigma representative, who admitted that Sigma doesn't understand Canon's complicated autofocus system very well, and said he isn't sure Sigma can make my lens work with my camera. Not a huge vote of confidence, but at least it was honest.
Other minor flaws As you'd expect for such a large range, this lens has some optical flaws, but they are on par with (or even slightly better than) other consumer-grade zooms. You'd have to spend a lot more money to get better optical performance in a zoom lens.
1. At 18mm, f/3.5, this lens gets soft in the corners. It's slightly worse than the 18-55 kit lens, but it hasn't been a problem for me because the softness gets much better when stopped down, as usually happens in outdoor situations. Because of the corner softness, I would not recommend using the Sigma wide open in low-light, but very few consumer lenses perform well under those conditions.
Incidentally, at 18mm and wide open (f/3.5), images from my copy are slightly softer on the right edge than the left. This has also been reported by others, and also goes away when stopped down.
2. Slight chromatic aberration (CA) in edges and corners. Almost all consumer zooms have this problem, including the Canon 18-55 and 17-85 IS lenses. The Sigma's red-green CA is comparable to the 18-55, and it is actually better than the 17-85 IS at equivalent focal lengths.
3. Purple fringing. Again, almost all consumer zooms have noticeable purple fringing around very bright objects, and the Sigma is no exception. But it actually has
less purple fringing than the popular Canon 18-55 and 17-85 IS lenses. And it has MUCH less purple fringing than the Tamron 18-200 lens. You can see my Sigma vs Tamron purple fringing comparison shots at the following page (same link as above, and you need to scroll to the very bottom of page):
http://www.geocities.com/tom_jhou/sigma_tamron_18_200.html
As with most lenses, purple fringing is most pronounced at longer focal lengths. It can be partly removed with the following freeware Photoshop plugin:
http://www.sd3.info/pf828/PFree/PFree0-1.html
That site also has an excellent free plugin for removing red-green chromatic aberration:
http://www.sd3.info/pf828/CAfree/CAfree0-1.html
3. Does not actually reach 200mm with nearby objects. When focused on nearby objects, this lens's maximum reach seems to get about 20% shorter. Reportedly, this is due to the Sigma's internal-focus design. I'm not terribly bothered by this - with nearby objects, I'm rarely using the maximum zoom anyway, as it's easier to just walk closer to the subject.
Build quality Having owned Sigma's 28-300 lens, I expected this lens to be built similarly to that earlier lens. Surprisingly, it has a nicer feel than the older lens. The plastic has a matted soft finish that has a pleasant velvety texture. The new lens has sturdier barrels (only exposed while zooming) than the old.
I like the Sigma's build much better than the Tamron 18-200. For example, the Sigma has a metal mount, whereas the Tamron has a cheap-looking plastic mount. Internally, the Sigma's mechanism is all geared, whereas the Tamron also uses rubber belts, which are prone to wearing out or stretching.
This lens is slightly smaller than the Sigma 28-300, even though it uses the same size filters (62mm). I like smaller, lighter gear, and the 18-200 goes nicely with the rebel XT, which is also surprisingly small for an SLR.
Alternatives A lot of people considering this lens will also be interested in Sigma's 18-125 lens, and Tamron's 18-200mm lens. I recommend the 18-200 over both of those, but your mileage may vary. The Tamron's optical flaws are significantly worse, but it focused perfectly at all focal lengths, without resorting to any parfocal tricks. The 18-125 was optically good, but it also gave me some front-focus issues, also for distant objects.
My experience with dozens of Sigma/Canon/Tamron lenses is that Sigma is uniquely frustrating with its combination of great optics, uniquely useful focal ranges, and inconsistent focusing. Tamron lenses trend in the opposite direction for me - tending to focus well, but to have more softness and purple fringing.
Is it really "all-in-one" ? Although often described as an all-in-one lens, I prefer to view the 18-200 lens as a kit lens upgrade. I would not advise making it your only lens. In particular, the 18-200 lens (like the kit lens) will not perform well in low light, due to its small max aperture. For low light, you'll need faster lenses, preferably primes (I like the Canon 35mm f/2). Also, I happen to like nature photography, and I find the Sigma's autofocus to be too slow for things like flying birds. For serious telephoto, you'll want a bigger aperture and faster autofocus. For walkaround use, the 18-200 lens will serve you well, but if it's all you use, you'll miss out on the capabilities where a DSLR really shines.
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I have one more comment that is related to the Epinions site itself.
Over a year ago, I reviewed this exact product, in this lens mount (Canon), and many people found my review helpful. However, that review seems to have disappeared, forcing me to resubmit it afresh.
Interestingly, I can still see the original review via its original link (
http://www.epinions.com/content_185158569604) but the Epinions search engine no longer finds that page. Instead, if you search Epinions for "Canon Sigma 18-200",
two items show up:
http://www.epinions.com/Sigma_18_200mm_f_3_5_6_3_DC_Lens_for_Canon_Digital_SLR_Cameras_1 http://www.epinions.com/SIGMA_LENS_18_200mm_F3_5_6_3_DC_Lens_for_Canon_Cameras_777_101_411378010360_1 You can see that the products are identical, except for the UPC number. The first item's UPC matches that from the manufacturer (according to www.upcdatabase.com) while the second does not (see http://www.upcdatabase.com/dummyupc.asp?upc=0411378010360).
My original review seems to be linked to a
third page that now looks orphaned and is no longer reachable by the search engine. It still be reached by its original link, which is as follows:
http://www.epinions.com/Sigma_Zoom_Super_Wide_Angle_18_200mm_f_3_5_6_3_DC_Aspherical_IF_Lens_for_Canon_Digital_EOS_777101 I hope the Epinions' gurus can help sort out what is going on.
