Sigma 8mm f/3.5 Circular Fisheye, Sigma 15mm f/2.8 Fisheye, Sigma 12-24 EX DG. Click here to enlarge.
Fisheyes are unique lenses. Unlike all other lenses, they don't try to correct the distortion, so they can achieve an amazing angle of view of 180 degrees! Actually, there have been even fisheye lenses with an angle of view of 220 degrees, as the incredible Nikkor 6mm f/2.8, but the majority of fisheye lenses currently on the market at 180 degrees fisheyes.
Sigma produces four fisheye lenses: the 8mm and 15mm, for FF cameras, and the 4.5mm and 10mm, for APS-C cameras. In this article, I have tested the fullframe fisheyes, on my 21 megapixels 1DsIII; for comparison, I have taken some shots even with my 12-24, that is the widest rectilinear (non-fisheye) wideangle currently in production.
Phisically, the two fisheyes are very similar, except for the lens hood of the 15mm: the 8mm has a 180 degrees angle of view in every direction, so it has no lens hood, otheriwise it would inevitably be included in every photo. They are pretty small - much smaller than the 12-24, they are light and well built, as all Sigma EX lenses. Unlike the Sigma 12-24 and the newer 4.5 and 10mm, the two fullframe fisheyes have a traditional AF motor: it is not silent as HSM and it has not full time manual focus, but it is as fast as the other wide-angle's AF. Nearly all super wide-angles have fast autofocus, and the 8mm and 15mm are no exception!
 
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