Sigma 18-200mm f/3.5-6.3 II DC OS HSM

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.

Resolution at 18mm		Resolution at 50mm
Resolution at 200mm		How to read our charts The blue column represents readings from the centre of the picture frame at the various apertures and the green is from the edges. Averaging them out gives the red weighted column. The scale on the left side is an indication of actual image resolution. The taller the column, the better the lens performance. Simple. For this review, the lens was tested on a Nikon D300 using Imatest.

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.

Chromatic aberration at 18mm		Chromatic aberration at 50mm
Chromatic aberration at 200mm		How to read our charts Chromatic aberration is the lens' inability to focus on the sensor or film all colours of visible light at the same point. Severe chromatic aberration gives a noticeable fringing or a halo effect around sharp edges within the picture. It can be cured in software. Apochromatic lenses have special lens elements (aspheric, extra-low dispersion etc) to minimize the problem, hence they usually cost more. For this review, the lens was tested on a Nikon D300 using Imatest.

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.

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