The ‘fisheye’ designation tells you immediately that this optic is what people will regard as being either a special purpose or gimmick lens: in reality it is both because the images that it records can either complement or detract from the subject respectively.

Sigma’s 15mm lens is comparatively compact yet still finds room for both a focusing ring and, on the Nikon-fit lens, an aperture scale that can be viewed by older Nikon bodies. The aperture scale must be set to its f/22 position for automatic aperture control and a lock is provided to maintain this position.

Missing from the lens is a focused-distance window. More serious, however, is the non-IF focusing mechanism that forces the user to keep his/her fingers away from the focusing ring in AF mode. This is very hard to do as even the slightest forward movement of the hand risks intruding into the fisheye’s field of view, which is 180° across the frame diagonal on a full-frame camera.

Despite its marginally longer focal length, Sigma’s 15mm Fisheye has a wider field of view than is offered by a 14mm rectilinear lens but, of course, the trade-off comes in the form of severe barrel distortion that causes straight lines to curve around the centre of the frame. This is in the nature of a ‘fisheye’ lens and can be minimised by confining straight lines to the middle of the frame.
Extreme edge distortion of this type makes technical evaluation difficult but consistent results were recorded through careful use of the test target.

The MTF data obtained indicate good sharpness, with resolution figures above the critical 0.25 cycles-per-pixel threshold between f/5.6 and f/16. There was minor chromatic aberration and also some slight signs softness in the far corners, beyond the reach of the test target.