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This was one of the first of the 'superzoom' lenses, and has been on the market for four years now. It has since been surpassed (from a zoom range point of view) by other lenses including Tamron's new 18-270mm. I've owned this lens for over two years now, its proved time and time again, how useful a lens with this focal length range can be.
The lens is made up of 15 'elements' in 13 groups, including two 'low dispersion' lenses to reduce chromatic aberration. This complexity is required to achieve the massive 11.1x focal length range. The range of 18-200mm, when 'adjusted' for the Canon and Nikon 1.5/1.6x crop factor cameras gives a 35mm SLR equivalent of about 28-320mm. This covers wide angle to medium telephoto. This range means that, for many users, this one lens could stay permanently on the camera.
As well as the wide focal length range, this lens's versatility extends to limited 'macro' use. I've used this to take pictures of dragonflies and butterflies, and achieved frame filling pictures showing incredible detail. This is a definite bonus.
The build quality of this lens is more than acceptable. The lens is quite light, due to it's plastic construction, but does not feel flimsy (I have not been gentle when using this lens, but there are little signs of wear on the body). The lens doubles in length when adjusting the focal length from wide angle to telephoto, with a smooth telescopic action. The knurled grips for holding when adjusting for manual focus and focal length feel slightly rubberised and are therefore easy to move.
The autofocus capabilities of this lens are acceptable, but not exceptional. The lens is noisy when focusing, and nowhere near as fast as the best lenses on the market. The autofocus is accurate in good light, but can 'hunt' severely in low light conditions.
Sharpness is acceptable when wide open (at both ends of the range), and much better stopped down. I've found that f/11 gives the sharpest images, albeit with a huge loss in light gathering ability. Contrast is also very good. Lens flare is well controlled, too even without the supplied hood.
Lenses with this focal length range must come with some compromises, as a lot is being asked of a lens like this. There are two main problems, distortion and chromatic aberration. Distortion is very evident at the wide angle end, with straight edges (e.g. buildings) appearing curved. This is not too much of an issue when shooting natural landscapes, however.
Chromatic aberration (colour fringing at high contrast edges) is noticeable, particularly with the lens wide open. At the centre of the image, the effect is limited, but at the corners of the picture, it can be distracting. Software such as Photoshop Elements can reduce this effect considerably, however.
Overall, this is a competent and versatile lens which is quite good value for money. Many users of digital SLR's could buy this lens and never need another. Other, more demanding users might prefer to purchase more than one lens to cover the same range of focal lengths, and thus have a lens collection with less compromises made during their construction.