Sigma produced this lens in manual focus mount from 1989-1995. It is a true macro lens capable of focusing down to 1:1 magnification. The lens is very large and heavy for a 50mm lens, 320 grams (12oz) or a bit more then a zuiko 50mm f/1.2 lens. The lens is claimed to be exceptionally sharp, sharper then most OE manufacturers. From my use so far the lens is extremely sharp with no evidence of chromatic aberration or distortion.
The lens consists of 10 elements in 9 groups and is multicoated. It has a filter diameter of 52mm. The lens has a maximum f/stop of 2.8 and a minimum f/stop of 22 with detentes every half stop. The diaphragm has 7 blades. The lens is 66mm in diameter and 57mm in length at infinity (significantly longer when focused to 1:1). The lens weighs 310 grams. As mentioned before the lens focuses down to 1:1 magnification (about 4 1/2″). Focusing from infinity to 1:1 takes about a 3/4 twist of the lens barrel and focusing is relatively easy.
I felt that it might be nice to compare the Sigma 50mm f/2.8 macro to the Zuiko 50mm f/3.5 and zuiko 50mm f/2 macros as the Sigma has the same focal length and falls almost in the middle for lens speed.
Sigma 50mm macro | Zuiko 50mm f/3.5 macro | Zuiko 50mm f/2 macro
Length: 57mm | 40mm | 55mm
Width/filter: 66/52mm | 60/49mm | 69/55mm
Weight: 310g | 200g | 320g
Reproduction size: 1:1 | 1:2 | 1:2
The Sigma is larger/heavier in all respects then the zuiko 50mm f/3.5 and only wins out in being 2/3rds of a stop faster and being able to achieve 1:1 reproduction size. The Sigma ends up being a full stop slower then the zuiko 50mm f/2 and very slightly longer (2mm), but it also ends up being slightly narrower and slightly lighter while also going to 1:1 instead of 1:2. If size and weight are an issue then the zuiko 50mm f/3.5 is definitely the champ, if speed is a requirements then the zuiko 50mm f/2 would win, but if it is reproduction size or possibly just a compromise between several different things then the Sigma 50mm f/2.8 wins. For image sharpness, distortion or CA, I have no idea until I can get copies of the zuiko lenses to test with (if I ever do).
As for cost, I managed to get my sigma 50mm f/2.8 for $50, about half or a little less then what the zuiko 50mm f/3.5 macro usually runs for and a very small fraction of what a zuiko 50mm f/2 macro generally runs for.