Optically sharp, but focus errors at 200mm on distant objects
Written: Jul 02 '08 (Updated Jul 22 '08)
Product Rating:
Pros: Small, compact. Good optics for such a wide range.
Cons: Soft corners at 18mm wide open. Canon version must be focused parfocally above 135mm.
The Bottom Line: Optically good despite vast zoom range; much better than Tamron version. At 200mm, I must focus parfocally (focus at 135mm, shoot at 200mm) to get correct focus.
tcchou71's Full Review: Sigma 18-200mm f/3.5-6.3 for Canon
I am resubmitting this review, with some revisions, because for some reason, my original review no longer appears under the product listing for this lens. This has happened at least a half-dozen times over the last year, and seems related to Epinion's website reorganizations.
In case the review gets lost again, there is a permanent version of the review (with sample photos) on my Geocities site:
The Review:
This lens is optically good for its wide range, and much better than the Tamron 18-200, which has more chromatic aberration, purple fringing, and corner softness. Optical quality is as good or even slightly better than the Canon 18-55mm kit lens, yet with almost 4 times the range! As a general-purpose lens, this is pretty good, though of course it will cost you more than the kit lens. However, this lens had one big flaw for me: autofocus accuracy at full zoom for distant objects.
Summary of misfocus issue
I tried three copies of this lens, on three different Canon DSLR bodies, with the same results: the Sigma lens at 200mm focuses in front of the intended object, even under ideal lighting conditions. All my other lenses focus fine, as does the Tamron 18-200 lens.
Fortunately, there is a simple workaround. The lens is parfocal, meaning the focus does not change as you zoom in or out. Hence, I can focus at about 135mm, where the autofocus is still accurate, then zoom in to 200mm to shoot. It's troublesome, but works reliably, yielding sharp photos at 200mm, that come surprisingly close to the performance of Canon "L" lenses I've used, at least in the center.
Interestingly, the autofocus at 200mm is accurate for relatively close objects, e.g. less than 20 feet away, so at least your macro photos will turn out OK.
At least one respected lens review site, photozone.de, also reports a focus problem at 200mm:
That site conjectures that the focus errors are due to the small (f/6.3) aperture at 200mm, but this doesn't explain why my Tamron 18-200 lens focuses perfectly at 200mm, despite also being f/6.3: (http://www.epinions.com/content_192627838596). Sigma's 18-125 lens also has exactly the same focus problem, despite a larger f/5.6 aperture.
The long end of the zoom is often useful for shooting distant objects, so it's pretty problematic that the focus errors are worst for distant objects. To show how bad the focus can get at 200mm, I posted a 100% center crop of a sample image on my website:
For good measure, here are someone else's photos illustrating a similar front-focusing problem, albeit at all focal lengths:
http://www.pbase.com/ingor/sigma_18200_test
Many people say the Sigma gets soft at 200mm. When manually focused, I don't find this to be true, so I suspect the softness is actually due to misfocusing. When manually focused, my Sigma lens at 200mm is almost as sharp as my Canon 100-400L in the center (though obviously the "L" is much better at the edges).
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. I spoke on the phone to a Sigma representative, who basically 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.
On the bright side, this lens is still pretty useful, because it focuses fine up to about 135mm, and can be coaxed into working above that with the aid of its parfocal property.
Minor flaws
This lens' optical flaws are no worse than other consumer-grade zoom lenses, and are generally much better than Tamron's 18-200mm lens:
1. At 18mm, f/3.5, this lens gets soft in the corners. It's somewhat worse than the 18-55 kit lens, but much better when stopped down. This doesn't bother me, as I rarely have important subject material in the corner.
Incidentally, at 18mm and f/3.5, images from my copy are 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. Interesingly, the 18-200 has much less purple fringing than either Canon lens, so overall the mega-zoom 18-200 has fewer aberrations than other lenses with much less range. The CA is only visible on large (8.5"x11" or larger) prints, and can be removed with the following Photoshop plugin:
http://www.sd3.info/pf828/CAfree/CAfree0-1.html
The above site also has a plugin for removing purple fringing.
Nice build quality
Having owned the 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 is pleasant to touch. The old 28-300 had a soft squishy barrel component (only exposed while zooming), while the new lens is sturdier. Incidentally, the Sigma lens has a sturdy metal mount, unlike the Tamron 18-200's plastic mount.
This lens is also slightly smaller than the 28-300, even though it uses the same size filters (62mm). I happen to like smaller, lighter gear, and the 18-200 goes nicely with the rebel XT, which is also astonishingly small.
Alternatives
A lot of people considering this lens will also consider Sigma's 18-125 lens, and Tamron's 18-200mm lens. As I mentioned above, some people have reported focusing issues with the Sigma 18-125 also, though many have not. In my own experience, 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 severe optical shortcomings.
Summary:
I consider this lens to be among the best of the 10x super-zooms, as it is much better than Tamron's 18-200mm lens. It is a worthy upgrade from the kit lens, and if you could only own one lens, this would be my recommendation. However, the whole point of owning an SLR is to use multiple lenses, depending on the purpose. After taking a lot of trouble to test this lens against its Tamron counterpart, I found that I didn't use either lens very much. In bright sunny conditions, where this lens works best, I tend to use a point-and-shoot, for portability. In dim light, I choose faster-aperture lenses, and for telephoto work, I choose dedicated telephoto lenses.
Other lens and camera reviews
I have used and reviewed the following lenses and cameras:
Canon 100-400 - fast autofocus, good optics make very good wildlife lens. But loses some sharpness at 400mm, and becomes difficult to use with 1.4x teleconverter.
http://www.epinions.com/content_171720740484
Sigma 18-200 - good optics, versatile range, but my copies misfocus near 200mm (though I can coax it into working with parfocal technique).
http://www.epinions.com/content_185158569604
Sigma 18-125. My first attempt to use a Sigma DC lens. Had front-focus problem at all focal lengths. Did not try a second copy as I switched to 18-200.
http://www.epinions.com/content_152606051972
Sigma 28-300. My first lens for my Canon 300D, purchased before Sigma released any DC lenses. It was pretty good considering its wide range, and focused quite accurately. But 28mm isn't very wide on a DSLR.
http://www.epinions.com/content_153463459460
Digital Camera reviews
I have also used a handful of small digicams, in my quest to find the perfect travel pocket camera. Here are my reviews:
Ricoh Caplio R3 - my favorite small camera, with its 7x optical zoom, 28mm wide angle, astonishingly small size, very good image color and detail, image stabilization, and voice recorder. Camera's image-processing is not as good as Canon models, but can be corrected in Photoshop (which is rarely actually needed).
http://www.epinions.com/content_217558847108
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