Sigma DC 18-200mm 1:3.5-6.3 Lens
Sigma’s new 18-200mm F3.5-6.3 DC zoom lens is marginally heavier than the similarly-featured Tamron lens we reviewed in issue 25 but also somewhat smaller in size and considerably cheaper. Designed specifically for current DSLR cameras with ‘APS-C’ sized sensors (1.5-2.0 field of view crop), this lens has internal focusing, which prevents the front from rotating, making it ideal for use with angle-sensitive accessories like polarisers and graduated filters. . . [more]
Sigma’s new 18-200mm F3.5-6.3 DC zoom lens is marginally heavier than the similarly-featured Tamron lens we reviewed in issue 25, but also somewhat smaller in size and considerably cheaper. Designed specifically for current DSLR cameras with ‘APS-C’ sized sensors, this lens has internal focusing which prevents the front from rotating, making it ideal for use with angle-sensitive accessories like polarisers and graduated filters.
The optical system consists of 15 elements in 13 groups and contains two Special Low Dispersion (SLD) glass elements and two hybrid aspherical lenses to counteract chromatic and spherical aberrations and reduce distortion. Optical surfaces are coated to minimise flare and ghosting (both relatively common with long lenses on digital cameras) and ensure accurate colour reproduction. The lens barrel extends roughly 5.5cm with zooming and the zoom movement is positive and controllable. A lock on the lens barrel keeps it in place when it is carried head down, and a slider allows users to switch between auto and manual focus.
During zooming, the maximum aperture changes from f3.5 at 18mm to f6.3 at 200mm, while the minimum aperture is adjusted from f22 to just over f40. The lens focuses to 45cm at all focal lengths and the 7-bladed diaphragm creates an attractive looking blur (‘bokeh’) in out-of-focus areas in compositions at wide lens apertures. The zoom ring carries settings for 18, 24, 35, 50, 80, 135 and 200mm focal lengths, while the focus ring has measurements in metres and feet. Macro reproduction ratios are engraved on the inner lens barrel.
We tested the Sigma 18-200mm lens on a Canon EOS-300D camera with the Adobe RGB colour space and all parameters set to normal. ISO 100 sensitivity was used for all shots. Overall, the performance of this lens was very similar to that of the Tamron 18-200mm lens. This is not surprising as the two lenses have almost identical specifications.
We found greatest image sharpness in shots taken at f8 with the 18mm focal length, while the poorest performance was recorded in shots taken at the minimum aperture throughout the focal length range. Diffraction probably played a significant role in the deterioration in image quality, especially since at the 200mm setting the minimum aperture was a tiny f40.2!
Not unexpectedly, chromatic aberration (CA) was lowest in the middle of the zoom range, where it was barely visible. Here, again, mid-range apertures delivered lower CA than the widest aperture and stopping down to the minimum aperture resulted in CA that was likely to be highly visible. We found no evidence of flare or ghosting in backlit shots and colours and tones were, in the main, accurately reproduced. Bokeh (the rendition of out-of-focus backgrounds and foregrounds) was generally attractive, as you can see in the illustration.
Slight barrelling was observed at the 18mm focal length setting but otherwise the lens was relatively distortion free. We estimate a maximum print size of 20.3 x 15.2cm would keep imaging flaws to a minimum and you should be able to make prints that look acceptably sharp when enlarged to A4 size from shots taken with mid-range apertures and most focal length settings. 
Focal length range: 18-200mm (picture angle = 28-300mm in 35mm format)
Maximum aperture: f3.5-6.3
Lens construction: 15 elements in 13 groups
Minimum focus: 0.45m
Filter size: 62mm diameter
Compatible mounts: Canon, Minolta, Nikon and Pentax
Dimensions (Diameter x L): 70 mm x 78.1mm
Weight: Approx. 405g
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