Review: 18-200 mm. f/3.5-6.3D IF DC Sigma. You can't have it All.

For starters, anybody who expects stellar optical performance from a 400 dollar 11,1x zoom, better stop reading right now. It is not going to happen, not with this one, nor with the Nikkor, even if it costs $ 300 more.

Summary:
I got this lens in a hurry, because I needed an all-purpose lens to cover a spur-of-the-moment client event. At the time, this was the only 18-200 mm. available in Santiago.
I also took a look at the (now discontinued) 18-125 mm. f/3.5-5.6D Sigma, but decided against it because it lacked the built-in focus motor necessary on the Nikon D40, my target camera.

Tamron offers a similar lens, and so does Nikon: the AF-S DX VR Zoom-NIKKOR 18-200mm f/3.5-5.6G IF-ED comes with optical stabilization (VR in Nikon speak) and a built-in Silent Wave Motor (the S in AF-S), which presently streets for about $ 700 at Amazon.
I expected this Sigma to focus slow compared to my HSM and AF-S lenses, but it’s surprisingly fast on the D40, the only camera I’m using it on. Build quality is also unexpectedly good, with a metal (instead of plastic) F-Mount, a partially metal body and a nice - EX type - crinkle finish.

The AF/MF switch is placed where I need it to be, and the lens also includes an anti-creep button that locks it in the 18 mm. position for trouble-free transport.
Thanks to what Sigma calls "inner focus" the front end does not rotate, facilitating the use of a petal-shaped lens hood (included), polarizers and other orientation sensitive filters such as grads.
However, it is important to point out that this "inner focusing system" is not the same as IF, as it is slow compared to AF-S or HSM and a bit noisy, while the front end extends considerably - making it look a bit “amateur” at the long focal end (see image).

At 18 mm., this lens suffers from pronounced barrel distortion and vignetting (wide open), but produces very good to excellent center sharpness, combined with good sharpness at the extremes.
At 35 - 85 mm., the center is still very good, but the borders suffer at large (if you can call f/5.6-6.3 "large") apertures.
However, stop down just a little, and border quality reaches again very respectable levels.
From 100 mm. on, the lens is pretty good stopped down to f/8, but interestingly, testers found that resolution (MTF) starts dropping off as soon as f/11.
You'd want to be extremely carefull with your aperture selection to get optimal results.

I have found myself avoiding the 200 mm. end, where my sample is decidedly soft, even stopped down. Lay off a little, though – to 150-180 mm. – and the problem is gone.
Because of it’s rather complicated optical construction, this lens suffers (as expected) from moderate chromatic aberration throughout it’s focal range.
On the other hand, it’s not as bad as other, even more expensive, lenses of comparable focal range.
In short, pretty decent for a budget 11,1 times zoom, which seems to reach its sweet-spot early on: only 1 to 1,5 stops down from its f/max. throughout the entire focal range.

People who have been destroying this lens (and others like it) in their critiques, seem to forget that for some purposes - like social events, weddings and general light travel - an all-purpose 11x 400-dollar-zoom-lens comes in extremely handy, and therefore, ought to be forgiven for some of its weaker points.
Lest of course, you are prepared to lose *the* shot while changing lenses back and forth (which I am not) or are planning to mount it on a 8.000 dollar D3x, which is plain out stupid.
Lets face it, this Sigma makes for a fairly campact & light package, which, combined with any half decent DSLR blows the pants (plus underwear) off of any 10x zoom P&S camera out there.

Strengths:
• Solid build quality & finish, good focus speed and accuracy.
• Overall pretty decent optical performance, except at the very long end (in my sample).
• Pretty unobtrusive internal focus, no rotating front-end. You can use petal shaped lens hoods, polarizers and grad filters without problems.
• Quite heavy, especially compared to the D40’s (very good) 18-55 mm. AF-S plastic kit-lens. But, compact enough to be well balanced and not overly heavy - it is the lightest among its peers; you can use it hand-held and without penalty down to 1/125 or even 1/60 sec..
• Very flare resistant. I have not seen any in my 4.000 takes with this lens, sofar.
• Well placed AF-MF switch, useful creep-lock button for transport.

Weaknesses:
• Chromatic aberration and purple fringing on highlight edges throughout the focal range, especially wide open. This can, however, be easily corrected in post or even in-camera (select Nikon models).
• Pronounced barrel distortions at the wide end, some pin cushion in the medium to long focal ranges.
• No built-in image stabilization. Users who are prepared to pay for an expected 1-2 stop gain, might want to consider the optically stabilized OS-version. (MSRP: $ 750) Combined with one of the clean high ISO Nikons, however, one ought to very carefully balance his options. You can get as much as 3 stops (ISO 1.600) from these cameras with minimal sacrifice, while 300 bucks extra is 300 bucks you can spend elsewhere.
• Can NOT be used with the Sigma EX 1.4x and 2x tele-converters. But then, I would not recommend using a lens of this level with a TC, anyway.
• The f/max. of f/6.3 at the long end is pushing towards the very extremes of AF capability; the lens may hunt on occasion, but mostly at the long end.
• Not exactly cheap for a "consumer" or kit lens.

Those who consider this lens, might also want to take a look at the AF-S 55-200 mm. Nikkor, which - although of a more limited focal range - pairs nicely with the 18-55, and brings better optical quality, less weight plus a lower price (± $250) to the equation.

Conclusion*:
Optical quality: 3,5
Build quality: 4
Design: 5 (no mayor flaws)
Value for money: 3,5
* 1 = poor, 5 = excellent
Recommended(for social events, weddings, light travel, pseudo P&S)

Manufacturer's Page:
http://www.sigmaphoto.com/lenses/lenses_all.asp

Pro Reviews:
http://www.photozone.de/canon-eos/320-sigma-af-18-200mm-f35-63-dc-lab-test-report--review
http://www.ephotozine.com/article/Sigma-18200mm-f3563-DC-4354
http://www.slrgear.com/reviews/showproduct.php/product/175/sort/8/cat/31/page/1

Pro 18-200 mm. Nikkor review:
http://www.dpreview.com/lensreviews/nikon_18-200_3p5-5p6_vr_afs_n15/