My friend recently purchased a Nikon D90 dSLR, and went the same route as I did and purchased a 18-200mm as her first lens. The Sigma 18-200mm f/3.5-6.3 DC OS HSM is an all around lens with a huge zoom range (11.1x). It offers a lot of the newest technology all with a pretty low price point.
In my case I purchased the Nikon equivalent, the 18-200mm f/3.5-5.6mm DX AF-S VR lens, which is very similar to this lens. I've used the Nikon lens for over a year, so I have a good lens to compare this to.
I recently got to play with her D90 and Sigma lens for a few days and it is pretty comparable to the lens I purchased. Although my lens retails for about $750, the Sigma costs almost half at $450. And let me tell you, after purchasing my Tamron 17-50mm f/2.8 I'm never going to say no third-party lenses without doing my research.
So first of all, for those unfamiliar with the Sigma acronyms, I'll go over what each of them in the name means.
- DC - made for digital crop cameras
- OS - optical stabilization which is their equivalent to IS for Canon and VR for Nikon
- HSM - hyper sonic motor which allows for fast and quiet focusing. It is equivalent to USM for Canon and AF-S/SWM for Nikon
- IF - inner focus, meaning the lens length won't change as it focuses.
Basically the features are pretty much equivalent to the Nikon counterpart. They both a 18-200mm zoom range, they are both made for Digital crop cameras, they both have quick and fast autofocus, and they both have image stabilization. From looking at the name, the main difference is the aperture range. The Sigma copy has a maximum aperture of f/6.3 at the far end and it actually comes into play at about the 135mm mark, so the performance there isn't as high as the Nikon which has a maximum aperture of f/5.6.
Like the Nikon, this lens comes with a small petal shaped hood to block out stray light. It takes a 72mm filter. The front lens cap is a center pinch cap which allows you to take the cap on and off easier. The rear lens cap of the Sigma lens locks into place with the Nikon camera body sensor cover so that you can keep these two pieces together.
Now as for the body of the lens, it has a different finish to it, but it's typical for a Sigma lens. The finish has a slight problem where dust and particles easily stick to the lens body and the zoom/focus rings. The overall build of the lens is good. It's very comparable to my Nikon in bulid quality, but it is tighter.
The zoom and focus rings are for some reason side by side, meaning there is no gap between them. It feels a little bit odd this way. The focusing is internal, but for some reason the focus ring rotates when the lens focuses. This isn't a good thing especially if your fingers are on the focus ring and it moves automatically when it auto focuses. Being that the focus ring is right beside the zoom ring, your fingers may be touching the focus ring. The focus is quite quick but not as quick as the SWM in my Nikon lens. It is also not nearly as silent. I also don't like how the focus ring is locked when the focus switch is on auto.
Given that it is an 18-200mm lens, the lens extends out quite a bit at 200mm. The copy that I used had no zoom creep which is very good. There is even a locking mechanism that prevents the lens from zooming in and out unintentionally. I wish my Nikon had this, and I wish my Nikon did not have any zoom creep. That being said, the zoom and focus rings are very tight, and they don't feel loose. Now my biggest peeve with this lens. I'm a Nikon user, and I used the Nikon version of this Sigma lens. So being a Nikon user, I'm used to turning the zoom ring clock wise to zoom-in. With this Sigma lens, it is opposite so you have to turn counter clock wise to zoom in. I always get confused when zooming in and out with this lens!
The Sigma 18-200mm is also listed as a Macro lens, so it has lines on the lens as you zoom in to help you figure out what the macro reproduction ratio is.
Now as for the Optical Stabilization. It is very loud and makes a quite disturbing sound when it starts and stops spinning. It's kind of hard to explain, but to me it sounds like someone grinding their gears in a stick-shift car, though not as loud. It does work though, and can be turned on or off with the switch.
Now how is the picture quality. Well of course being that the aperture range is 3.5-6.3 be prepared to bump up the ISO when you are in doors or in low-light. I found that I had to do this quite a bit to get my shots properly exposed. The quality of the pictures were fairly good. Though, a lot of people complain that the lens is not sharp when wide open (or when the aperture is at its max), I did not notice this in the pictures I took.
I'm not sure if I would recommend this over the Nikon equivalent. It is a few hundred dollars cheaper, but I don't believe the performance and actual mechanics of its features are as good. The main advantage it has is the locking mechanism. Other than that, you can still find Nikon 18-200mm lenses that do not have zoom creep, for example if you can find ones not made in Thailand (but you really have to try out the lens first. This isn't always the case.). Other than the locking mechanism, the overall feel is better on the Nikon, the AF-S is quicker and quieter, the VR is much more quieter, and the zooming goes the proper way.
This lens will also work with the Nikon D40/x, and D60 dSLRs which require AF-S lenses.
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