SIGMA 18-200mm F3.5-6.3 II DC OS HSM
The 18-200mm range is popular amongst those who own a camera with an APS-C sensor and wish to travel light, with the convenience of a large 11.1x zoom range.
The optically stabilised version of this lens, we are reviewing here, is available to fit Sigma, Nikon and Canon cameras. A non-stabilised version is also available for Pentax and Sony cameras. In addition to the optical stabilisation and huge zoom range, this lens sports a silent HSM focusing motor, although it doesn't support full time manual focus override.
Sigma 18-200mm f/3.5-6.3 II DC OS HSM Handling and features
Weighing only 495g, this optic is lighter and slightly more compact than equivalent lenses from Canon and Nikon. However, the maximum aperture drops to a fairly modest f/6.3 at 200mm, which may be limiting for shooting in low light, requiring slower shutter speeds and/or higher ISO sensitivities to be used to compensate. The lens balances well on the Nikon D300 used for testing, and should also feel right at home on more compact entry-level camera bodies too.
High quality plastics have been used for much of the lens' construction, with a matt finish that doesn't mark easily and the lens mount is made of metal. A wide rubberised zoom ring fills much of the lens barrel and the zoom action feels very smooth indeed, having just enough resistance to prevent the zoom creeping through the range when pointed downwards. A locking switch is provided to hold the lens at 18mm during transport.
A silent HSM focusing motor powers the focusing mechanism, resulting in reasonably quick focusing, although not as fast as some equivalent lenses. Moving subjects can be a challenge to accurately get in focus as a result. Focusing is performed internally, so the 62mm filter thread does not rotate, making this lens ideal for use with polarising and graduated filters.
Full time manual focus adjustments are not supported as the focus ring is solidly engaged with the focusing motor. Manual focus adjustments can be difficult to apply, as the focus ring is very loose, and has a very short travel from infinity to close focusing. The close focusing distance of 45cm is fairly typical of this kind of lens.
Thanks to the optical stabilisation, sharp hand-held images are possible around half the time at shutter speeds as low as 1/40sec at 200mm, which is roughly three stops slower than the usual rule of thumb would dictate is possible.
Sigma 18-200mm f/3.5-6.3 II DC OS HSM Performance
As is the case with many high zoom ratio lenses, sharpness levels are very high at shorter focal lengths, dropping as the lens is zoomed in.
At 18mm sharpness in the centre of the image area is outstanding from maximum aperture, dropping due to the effects of diffraction as the lens is stopped down. Sharpness towards the edges is very good at this focal length and remains so until the lens is stopped down to f/8.
Zooming to 50mm results in a reduction of overall sharpness. At maximum aperture the clarity in the centre is still very good, but the quality towards the edges falls below good levels. Sharpness towards the edges increases as the lens is stopped down with peak quality being recorded between f/5.6 and f/11 for this focal length.
Finally, at 200mm sharpness in the centre of the frame remains very good, with a further reduction in quality towards the edges of the frame. Peak quality for this focal length is achieved at f/11.
Chromatic aberrations rise and fall as the lens is zoomed through the range. At 18mm and 200mm CA levels are quite high towards the edges of the frame and may become apparent in large prints with areas of high contrast near the edges.
Falloff of illumination towards the corners is reasonably well controlled. At 18mm the corners are 1.49 stops darker than the image centre and at 200mm the corners are 1.3 stops darker. Visually uniform illumination is achieved with the lens stopped down by just over a stop from maximum aperture throughout the zoom range.
Distortion is often a weakness of high ratio zooms like this. Although the 3.45% barrel distortion at 18mm is quite strong, it isn't all that bad when compared to some similar lenses. Pincushion distortion of 1.61% is present at the telephoto end, which shouldn't pose too many issues for most. If you require completely straight lines, you'll be glad to know that the distortion pattern is uniform across the frame throughout the zoom range, which should make correction in image editing software relatively easy to apply.
A petal-shaped hood is supplied as standard with the lens, which does a reasonable job of protecting the front element from extraneous light that may cause unwanted flare and loss of contrast. During testing there were no issues with flare and contrast holds up well, even when shooting into the light.
