The 18-200mm OS is a typical upper-mid-range Sigma design, which means a solid-feeling lens with an attractive matte-black finish and smooth operation of controls. In common with other lenses it in its class, the mount is metal and the barrel is constructed from lightweight but good-quality plastics. Like other superzooms the Sigma features a 'double-trombone' design for extension of the front lens group on zooming, but unusually it also uses a similar mechanism for moving the rear group internally. The lens is very similar in size and weight to both the Canon and Nikon 18-200mm zooms and Tamron's 18-270mm F3.5-6.3 VC (although the latter of course extends longer, due to its greater telephoto range).

Three switches are placed down the side of the barrel within easy reach of the right thumb. At the top is a zoom lock which stops the lens from extending under its own weight; this is slightly better-placed than that on the Tamron 18-270mm, but like the other lenses in this class only works at 18mm. On our review sample we found zoom creep was simply not a problem, but it's worth bearing in mind that lenses often loosen up in their zoom action with age, so once the lens is 'run in' this may well be a geniunely useful feature. Below the zoom lock are the controls for the focus and image stabilisation mechanisms; these two switches are large and easy to operate, especially in comparison to those on the Canon and Nikon 18-200mm lenses.

On the camera

The lens feels ideally balanced on larger DSLRs such as the D300, but can feel a little front-heavy on smaller bodies such as the D40/D60 range (although still perfectly usable). Controls are well-placed, with both the zoom ring and the unusually broad manual focus ring falling readily to hand. The zoom lock, AF and OS switches are also well-placed within easy reach on the side of the lens barrel; overall the Sigma scores highly in terms of usability.

One note of caution though; on smaller SLR bodies such as the D60, the lens will block the built-in flash at focal lengths wider than 24mm, resulting in a shadow in the lower center of the image. On bodies with flashes which lift higher above the lens axis (e.g. the D300 and D90) you'll only see flash shadowing at focus distances close to the minimum (0.45m), and at more normal distances of 1m or longer there's really nothing to worry about.


The Sigma 18-200mm F3.6-6.3 OS is unusual in that it comes in two flavours, with the Nikon version under test sporting an HSM badge (which signifies an ultrasonic-type 'HyperSonic Motor'), but the Canon and Sigma mount variants having to make do with a standard micromotor instead. The hypersonic motor on the Nikon mount model is however of the micro- rather than ring-type, so the two approaches are operationally very similar; in both cases the focus ring rotates during autofocus, and no full-time manual AF override is available (only the Nikon 18-200mm F3.5-5.6 VR provides such an option in this class of lenses).

Autofocus was generally positive and near-silent in operation and, although not as fast as Nikon's own AF-S equivalent, not terribly slow either. However even on the normally ultra-reliable Nikon D300 body used for testing, focusing could occasionally be inconsistent and inaccurate, especially at 200mm (presumably due to that F6.3 maximum aperture). The lens would also occasionally 'freeze' and simply refuse to focus; this could be easily remedied by switching from AF to manual and back, but was obviously an irritation when it occurred (it's also entirely possible this issue was limited to the specific sample we tested). 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.

Dependence of apparent focal length on focus distance

This lens's angle of view widens dramatically on focusing from infinity to 0.45m, especially at the telephoto end. This is a common trait with superzooms (the Canon and Nikon 18-200s and the Tamron 18-270mm behave in just the same way), but at a focus distance of 2m that 200mm telephoto end has an effective focal length closer to 135mm. In context, it is worth bearing in mind that long telephotos generally tend to used more for distant subjects, in which case the lens naturally behaves as a 'true' 200mm.

Lens body elements

The lens comes in versions for Canon Nikon and Sigma DSLRs; our sample was in the Nikon F mount.

This view shows the somewhat unusual 'double trombone' mechanism which is used for moving the rear element on zooming.
The filter thread is 72mm. It does not rotate on autofocusing, which should please filter users.
The bayonet-mount hood is provided as standard, and clicks positively into place on the front of the lens. It's made from thick plastic, and features ribbed mouldings on the inside to minimize reflections of stray light into the lens.

A white dot on the outside of the hood aids alignment for mounting, and the hood reverses neatly for storage.
The zoom ring has a 20mm wide ribbed rubber grip, and rotates 80 degrees anti-clockwise from wide to telephoto (the same way as Canon lenses, but opposite to the majority of Nikon's). The action is smooth and even, in marked contrast to Tamron's 18-270mm VC. The front element extends 62mm on zooming and has just a little lateral play when fully extended.
The focus ring is the widest in this class at 17mm, but rotates just 45 degrees clockwise from infinity to 0.45m, again matching Canon lenses and opposite to Nikons. Its action is nice and smooth, however the limited travel makes precise manual focus rather difficult.

A basic distance scale is marked in feet and meters. The focus ring travels slightly past the infinity position, and rotates during autofocus.
An array of switches are placed on the side of the lens barrel within easyreach of the left thumb. At the top we have the zoom lock (which works at 18mm only), below that the auto/manual focus mode selector, with the image stabilisation on-off switch at the bottom.
A slightly curious scale on the outer sleeve of the 'double trombone' zoom mechanism shows the image magnification when the lens is set to 0.45m (i.e. the minimum focus distance). The numbers correspond to each of the focal lengths marked on the zoom ring except for 18mm (at which position the barrel is fully retracted, and this sleeve is not visible).

Reported aperture vs focal length

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

Focal length 18mm 28mm 35mm 50mm 80mm 135mm 200mm
Max aperture
Min aperture

The 18-200mm is just marginally slower than the Canon or Nikon equivalents at the telephoto end, by about one sixth of a stop at 135mm and one third of a stop at 200mm - not really a significant difference in real world use.