Design

The 18-200mm feels reasonably well-made, with build quality fairly typical of Canon's mid-range zooms (similar to the popular 70-300mm F4-5.6 IS USM), and is fractionally larger and heavier than Nikon's equivalent. The mount is metal, and the barrel and extending sections are made from high-quality plastics. The bulk of the barrel is taken up by the large zoom ring, with a slim manual focus ring towards the front.

Two small switches on the top left of the barrel control autofocus and image stabilization, and diametrically opposite them at the lower right there's a zoom lock switch. This is extremely useful on a lens such as this with heavy front elements, which will tend to cause the lens to extend towards 200mm when the camera is carried over the shoulder. Like most such systems, it will only lock the lens at its shortest focal length (or perhaps more pertinently, its shortest physical length).

On the camera

The lens feels perfectly balanced on mid-range bodies such as the EOS 50D and, thanks to its relatively low weight, also fits well on lighter bodies such as the EOS 450D. The zoom ring falls naturally to hand, and the slim manual focus ring can be operated by the index finger when needed. The focus and IS mode switches are well-placed for operation by the left thumb, and special mention must also be made of the zoom lock switch, which is unusually positioned but works extremely well.

Despite the relatively large size of the lens (for a standard zoom), we encountered few problems with blocking of the built-in flash, helped in no small part by the high lift of the flash unit on current and recent Canon bodies. A little shadowing at the bottom edge of the frame can be observed at wideangle and close focusing distances of less than 1m, which is unlikely to be a problem in actual use.

Autofocus

Autofocus is driven via a micro motor system, which is louder and feels less refined than the ring-type ultrasonic focusing motors used in many other mid-range zooms. To be fair it is one of the better examples of its type, similar in focusing speed and noise level to Canon's ubiquitous EF-S 18-55mm F3.5-5.6 (IS) kit lenses (and much faster and quieter than some other micro motor lenses we've tested recently). The design does however mean that the focus ring spins around during autofocus, and manual focus can only be engaged by flicking the AF/M switch. This is a little disappointing for a lens of this level, but does help keep size, weight and cost down a bit.

In normal use the autofocus is fast and positive (tested mainly using the EOS 50D body), although with the usual tendency of slow maximum aperture zooms to hunt in low light. Its focus tracking abilities are reasonable, but not outstanding, and naturally no match for a fast, internal focusing USM lens. As always it must be noted that focus speed and accuracy is dependent upon a number of variables, including the camera body used, subject contrast, and light levels.

Lens body elements

	The lens uses Canon’s all-electronic EF-S mount. The ‘S’ designation means it is designed exclusively for APS-C digital SLRs, and the grey rubber ring on the rear prevents it from fitting on 1.3x or full-frame cameras. It's also not compatible with the older (pre-EF-S) D30, D60 and 10D APS-C models.
	The filter thread is 72mm. It does not rotate on autofocusing, which should please filter users. Behind the filter ring is a bayonet mount for the lens hood, although (as always for Canon) this is not supplied in the box, but instead is an expensive optional extra.
	The zoom ring rotates 80 degrees anti-clockwise from wide to telephoto. The ribbed rubber grip is 21mm wide, and the zoom action smooth and precise. The front element extends an impressive 60mm on zooming, and exhibits only a little lateral play at any focal length. Like the Nikon equivalent (and a common feature of internal-focus zooms), the angle of view increases significantly on focusing closer.
	The 7mm wide focus ring rotates approximately 50 degrees clockwise from infinity to 0.45m. It rotates during autofocus, and there's no option to tweak focus manually in AF mode. The manual focus action is a little loose, but perfectly usable and sufficiently precise for critical manual focus in live view.
	Two small but positive switches adorn the side of the lens barrel; the lower turns the image stabilizer on or off, and the upper switches between autofocus and manual focus. The IS switch is perfectly well-positioned, and has a slight ridge allowing it to be distinguished by feel alone. However the AF switch is distinctly towards the top of the barrel, and slightly out of reach with the camera up to your eye (only a minor inconvenience).
	A small zoom lock switch is placed at the base of the lens barrel (click through for a picture showing its position more clearly); this is a feature sure to make some Nikon 18-200mm owners green with envy. The positioning may at first appear to be mystifying, but with typical Canon attention to detail, it's perfectly located for a quick flick of the left index finder with the camera up to your eye.

Reported aperture vs focal length

Here we show the maximum and minimum apertures reported by the camera at the marked focal lengths.

The 18-200mm is approximately one third of a stop faster than the Canon EF-S 17-85mm F4-5.6 IS USM through their shared range, and (unsurprisingly) almost identical in speed to the Nikon AF-S 18-200mm F3.5-5.6 VR throughout the range.