Camera Accessories

Sigma's versatile 18-200mm F/3.5-6.3 DC OS lens has been designed as an all-purpose lens for digital SLR cameras with 'APS-C size' image sensors. Offering an 11x zoom ratio and a minimum focusing distance of 45cm at all focal lengths, this lens will appeal to photographers who want a single, long-range zoom lens to fit on their camera body. A built-in Hyper-Sonic Motor (HSM) uses ultrasonic waves to drive the autofocusing controls, providing fast and quiet AF.

SLD (Special Low Dispersion) and aspherical glass components are incorporated in the lens design to minimise a wide range of aberrations. Consisting of 18 elements in 13 groups it features internal focusing, which allows angle-critical filters (such as polarisers and graduates) to be used without re-adjustment when focal length settings are changed. Petal-shaped lens hoods can also be fitted, although none was supplied with the review lens.


The structure of the Sigma 18-200mm lens.

Like most lenses in its category, the Sigma 18-200 lens has a seven-bladed iris, which opens to form a near circle at wide apertures. Maximum apertures range from f/3.5 at 18mm to f/6.3 at 200mm, while minimum apertures are f/22 at the 18mm focal length and f/40 at 200mm. These ranges are common in extended zoom lenses and the f/6.3 setting provides great scope for differential focusing at 200mm.

The lens is sturdily constructed and we had no difficulty fitting it to the Canon EOS 400D and EOS 40D cameras we used for our tests. Towards the rear of the lens barrel is a wide, rubberised zoom ring that is ridged to provide a secure grip. A slightly narrower focusing ring lies in front of it. This ring is also rubberised and has closer ridging.

A lock behind the zoom ring stops the lens from extending when it is carried facing downwards. Behind and slightly below it is the AF/MF switch and, a little further down, the on/off switch for the optical image stabiliser.
Sigma's OS (Optical Stabiliser) technology uses two sensors inside the lens to detect vertical and horizontal movement of the camera and then adjusts a dedicated lens element group to counteract camera shake.

The system claims to be capable of automatically detecting panning movement as well as normal camera shake. Close focusing is supported to 45cm throughout the zoom range, providing a maximum magnification ratio of 1:3.9. A magnification scale is displayed on the lens barrel as it is extended with six settings ranging from 1:3.9 to 1:12.8. 

Performance
Overall performance was good for an extended-zoom lens, although not outstanding. Subjective assessment showed most shots to be reasonably sharp and detailed, although we found some coloured fringing around the edges of shots taken in bright conditions, especially at 18mm. Edge softening was also observed in these shots. Both aberrations were only visible when shots were magnified and neither would interfere with normal photography.

Our Imatest tests showed overall resolution to be slightly below expectations - although not unusual for a lens with such an extended zoom range. Not surprisingly, the best sharpness was achieved at mid-range apertures. Imatest also revealed a steep decline in resolution at both ends of the aperture range with the 200mm setting and at f/22 at the 18mm focal length position. A graph showing the MTF50 results we obtained at different aperture and focal length settings is presented below.

 

Imatest also showed lateral chromatic aberration to be low at most focal length and aperture settings, with a minor slide into the lower end of the moderate range at the 18mm focal length setting. Some barrel distortion was also observed at 18mm but it had vanished by 28mm and switched to pincushioning at around 50mm. At no point was either distortion severe enough to affect normal picture-taking, although both could affect architectural shots, particularly with short camera-to-subject distances.

Bokeh was pleasing with wide lens apertures, especially at longer focal length settings. Flare was common - and quite obvious - in shots taken with the lens pointed towards the sun, even though the sun was well out of the shot. However, backlit subjects were handled reasonably well when the front element of the lens was totally shaded.

IMATEST GRAPHS

SAMPLE IMAGES

18mm focal length.

200mm focal length.

Close subject with 18mm focal length and f/3.5 aperture setting.

Close subject with 200mm focal length and f/6.3 aperture.

Depth-of-field comparisons: top at f/6.3, bottom at f/16.

Lens flare.

Backlit subject.

Focal length range: 18-200mm
Maximum aperture: f/3.5-6.3
Lens construction: 18 Elements in 13 Groups

Lens mount(s): Sigma, Canon, Nikon (including D40 and D40X models)

Diaphragm Blades: 7
Minimum focus: 45 cm
Filter size:  72mm
Dimensions (Diameter x L): 79 x 100 mm
Weight: 610 grams