Sigma versus Tamron 18-200mm lenses.

18-200 lenses

A review of the Sigma and Tamron 18-200 lenses, with samples

Overview

18-200mm lenses promise to be "all-in-one" lenses for SLR amateur photographers. But don't believe the hype - you can't make them your only lenses, unless you want to miss out on all a DSLR can do for you. Still, the 18-200 lenses are probably good upgrades to the starter "kit" lens that came with your camera. If you are looking into these lenses, you should be aware that the Sigma and Tamron versions are surprisingly different, despite having almost identical specifications. In a nutshell, the Sigma is vastly superior optically than the Tamron, but it has one achilles heel, at least in its Canon version. This is that the autofocus consistently front-focuses at 200mm for distant objects, unlike the Tamron which focuses perfectly at all focal lengths.

Fortunately, the Sigma's focus problem can be worked around. Because it is parfocal (meaning the focus doesn't change as you zoom), you can focus at around 135mm (where the autofocus is still dead-on accurate), then zoom in to 200mm to take the shot. This takes a bit longer, but works at least for me it works like a charm.

Sigma 18-200

Optically, Sigma's 18-200 lens is surprisingly sharp, even at wide open apertures, and noticeably sharper than Tamron's version. Red-green chromatic aberration and purple fringing, while present, are much better than the Tamron. This lens' main optical shortcomings (soft corners at 18mm wide open, and a tendency toward harsh bokeh) are comparable to Canon's 18-55mm kit lens.

Because the Sigma's optics were surprisingly good, it was a shame to discover that the autofocus was not accurate at 200mm for distant objects (>30 feet away). I had this problem with 3 copies of this lens on 3 different DSLR bodies (rebel 300D and XT), so it is definitely not a fluke. Basically the lens would focus in front of the intended object, even when using center-spot focus on high-contrast targets in broad daylight, with no other objects to distract the camera's autofocus. Strangely enough, the autofocus at 200mm was fine for relatively close objects (under 30 feet away).

As I noted above, I work around this problem by auto-focusing at 135mm, then zooming in to 200mm to take the shot. This simple trick made a day-and-night improvement in photos at 200mm.

One well-respected photography website reports the same front-focus issue at 200mm: http://www.photozone.de/8Reviews/lenses/sigma_18200_3563/index.htm

You can see the focus issue for yourself in the following example photos I took:

100% center crop of distant object. 200mm zoom, center spot focus:
This helicopter was 1/4 mile away, and directly on my active focus point. It is badly out of focus, whereas closer objects in the same photo (not shown) are in focus:

The next 100% crop was also taken at 200mm. This object (a large metal bell) was only 10 feet away, and much sharper:

As I mentioned above, I can focus at 135mm and then zoom in to 200mm to take the shot. I did that in the following example, which is quite sharp:

Leaf detail in this 100% crop is quite good, almost as good as my Canon "L" lenses. The next shot is the upper left corner (100% crop) of the same photo:

Detail-wise, the corner crop is still pretty good, although red-green chromatic aberration is apparent (e.g. on the leaf veins and the fence in the background).

If you can work around the focus issue, this lens is impressively sharp throughout its huge range. When correctly focused, the Sigma is almost as sharp as my Canon "L" lenses in the center (though of course the "L" lenses hold their sharpness all the way to the edges, and have less chromatic aberration).

By the way, if you were hoping to avoid the focus problem by buying Sigma's 18-125mm lens, you'll find reports of focus problems with that lens too. There is a spotlight review at Amazon.com of the 18-125 that sounds eerily similar to my experience with the 18-200. The review is titled "4 stars f/ quality (if good sample); 1 star f/ Sigma repairs":

http://www.amazon.com/gp/product/B0002CNYL8/103-5498228-1888643?v=glance&n=502394

Tamron 18-200 lens

The Tamron lens focuses accurately on my camera but its optical flaws are worse than the Sigma. With effort, they can be partly, but not completely, corrected in Photoshop. In my opinion, this lens is not worth the trouble. You should get this lens only if you really want the hassle of lugging around an SLR lens that has more optical aberrations than many point-and-shoots.

Chromatic aberration
Below is an example of the Tamron's chromatic aberration, and its removal with CAFree, a free plugin for Photoshop. As you can see, I was able to do pretty well, although it took considerable time to optimize the settings:

Purple Fringing
Below are 100% crops of the Tamron's purple fringing, which is quite horrendous. The sky isn't even overexposed by very much, yet the utility wires are covered with purple haze. For comparison, an unprocessed Sigma shot of the same object is shown, showing much less purple fringing. The Sigma is also sharper.

These shots are at 200mm f/6.3, where both lenses show their worst purple fringing. Purple fringing can be removed with this free Photoshop plugin: PFree.

Philosophical issues

The whole point of the 18-200 lens is to help the SLR user to "travel light". But think about what that means - the smallest SLR is still a behemoth compared to some very capable point-and-shoots. For most outdoor uses, a pocket-sized camera will take pictures almost as good as the SLR. And some point-and-shoots, like the Panasonic TZ3, even have 10x zoom lenses that are roughly equivalent to the Sigma 18-200 lens. So personally, when I want to travel light, I use the point-and-shoot for 90% of my shots, and pull out the SLR only when I need extreme telephoto or low light photography. Those demanding situations are where the DSLR shines, but they are also where the 18-200 lenses are at their worst. Hence, 18-200 lenses occupy a neglected middle ground for me - not small enough to be truly convenient, and not powerful enough to take full advantage of the SLR. However, as I said above, they are at least as good as most camera's kit lenses (albeit much costlier) so if you have the cash to spare, they are worth a look.

Personally, after all the trouble figuring out which 18-200mm lens to use, I ended up using it very rarely. Most of the time I used my point-and-shoot Ricoh R3 because its 28-200mm lens has almost as much zoom range (7x), it weighs 8 times less than the DSLR, and fits into my shirt pocket with room to spare!

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