This is taken with the original 1980s Sigma Filtermatic 16mm f2.8 Focus Confirmation lens in Minolta AF mount. It is a very compact lens compared to later 16mm and 15mm Sigma fisheyes, but not far from Minolta's 16mm design which later became today's Sony SAL 16mm f2.8. Centre resolution is very high even at f2.8, but the edges are equally soft. It needs to be stopped down to at least f8 for acceptable results, ideally f11 or even f16, where the gain in sharpness to the corners of the frame probably outweighs the small loss in overall sharpness caused by diffraction. The exposure control works perfectly on the Alpha 900, and so does the manual focus confirmation.
Minimum focus is just 15cm/6" and that is measured from the film plane.