The Sigma 17-50 mm f/ 2.8 EX DC OS HSM
Diagonal angle of view 72.4° - 27.9° (1.7х Foveon)
Equivalent focal length 25-75 mm. for Nikon
Equivalent focal length 27-80 mm. for Canon
Max. magnification 1:5 (0.20)
Angle of focusing ring rotation approx. 30-45°
Max. focal length 115 mm.
Diameter 83.5 mm.
Petal hood and carrying case are included with the lens.
The Sigma 17-50mm F2.8 EX DC OS HSMis a standard fast zoom lens with a switchable system of optical stabilization which is designed for digital SLR cameras with crop factor not less than 1.5. The 4-stop optical stabilizer is compatible with the Sony and Pentax bayonet mounts on Sigma lenses only even if the lens body has the Anti-Shake function . In practice there are no more than 3 f/ stops of optical image stabilization instead of 4 claimed by Sigma .
The optical scheme of the Sigma 17-50 mm f/ 2.8 EX DC OS HSM includes three aspherical elements (one hybrid and two molded) and two SLD elements of a new FLD type (Sigma's own product, according to its optic characteristics it is very close to fluorite) for correction of all types of aberrations. The super multi-coating of optic elements reduces flare and halation, and increases contrast.
The pictures taken with fluorite lenses are sharply different from any other: high density of black, and extremely pure colors, with grdual tonal transitions of density percentage(yet, contrast coefficient is almost 0.99, or 1% of diffusion, not 5-6 as other have), everything is perfect and clearly seen, especially for wide-angle f/8-16 lenses. And what is more, optical schemes themselves are very good. Fluorite glass imposes less constraints when designed, which makes it possible to make use of rather "daring" solutions, unachievable with SLD or ELD. The only thing is that color temperature is slightly shifted to - 300 ° K, therefore they are antireflected with a characteristic "yellow" coating, still absolute 0 is not reached.
Hyper Sonic Motor provides quiet and high-speed autofocusing. Internal system of focusing eliminates the front lens rotation, which makes it possible to use the petal hood and polarized filters. The size of the Sigma 17-50 mm f/ 2.8 EX DC OS HSM changes while zooming in. The focus ring is coupled to the AF mechanism so it rotates during focus operations. The lens is equipped with a zoom lock mechanism for preventing undesired barrel extension.
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Posted by: dv Date of publication: 27.10.2010