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Get information and user reviews for this lens from Amazon: Sigma 15mm f/2.8 EX DG Diagonal Fisheye Lens for Canon SLR Cameras
Full-frame fisheye lenses (of which there are two, the Nikkor 16mm and the Sigma 15mm described here) provide a 180 degree angle of view across the 35mm frame diagonal. These lenses don’t attempt to correct for rectilinear distortion (as do the 14mm lenses) and thus produce a very unique “look” to a shot. The Sigma 15mm isn’t exactly a lens suitable for architectural photography, but it can be used for dramatic effect. Focusing is quick and somewhat quiet. Overall sharpness is excellent, though I do note a bit of softness in the corners, especially wide open. Depth of field is phenomenal even wide open, which tends to increase the perception of sharpness of the lens. Chromatic aberration is minimal (and not an issue on the digital bodies; a trivial issue on the 35mm bodies). Light falloff is substantial wide open in the corners of 35mm shots, only a minor issue for digital. By f/5.6, light falloff is minimal and ignorable. Straight lines that run directly through the center of the frame are rendered straight, while straight lines near the edges have considerable barrel distortion (outward curve). READ FULL REVIEW