Quoted By:
Let me tell you about this lens.
This is not the Superwide Sigma released in 1981, or the refresh in 1986. It might be related to the 1975 24mm Filtermatic, as the typeface on the lens suggests it was released around the same time, and there is a 24mm f/2.8 listed on that year in the Sigma Imaging UK site's timeline. It's hard to find evidence of this lens existing outside of the example I have, and it probably didn't sell well.
The first thing that'll be obvious is that this lens flares up. Specular light sources inside the image circle will produce internal reflections that appear inside the frame, which recognizably match the shape of the light / aperture with a light blue ghost (in-frame sunlight produces 2x14 distinct ghost hexagons). Side-entering light just fogs up the image and reduces contrast. This lens should really have a hood, but the body design makes that impossible for full frame, so it makes sense that Sigma didn't bother.
This lens has a 62mm filter mount, which is too small for how long the body of the lens is. There is a heavy gradual vignette in the full frame image, and even an APS-C sensor will get clipped by the filter mount at minimum focus. I wouldn't bother mounting a filter to the front of this, as it'll just make things worse. This lens body may have been repurposed later for the more common 28mm f/2.8 Sigma-Z on M42 (1978?), and the relative abundance of that lens suggests that the body may have worked better with that focal length.
When shot in a way that eliminates the vignette, you're left with a lens that has a distinct character in its flares and achromatic aberrations. It isn't as difficult to use as the Helios, because it actually behaves itself at small apertures aside from flare, but it is a restrictive tool that can be a little spicy at times.
