Not really an off brand as Sigma is a big player in the 3rd party lens world, Sigma made a number of manual focus K mount lens and a few auto focus as well, the SA 7 and 9. I bought a SA 7 at a Kitts Store, came with 2 kit lens. My F3P had been ripped off at LAX after 911, wasn't allowed to bring my camera bag as a carry on had to check it, Needless to say my F3 did not arrive in Phoenix. I limped along with my Pentax and Chinon M42 cameras and lens, did not know if I was going to get a F5 or jump to the dark side. The SA7 and lens was cheap, like $170 for the kit. Turned out I liked it so much I got an upper level Sigma body the SA9. The down side is over time both bodies developed yellowing in the viewfinder. Sigma told me it was the glue that held different layers of the pentaprism, others say it is tarnish. I have 2 SA7 with out much yellowing and I still use them. I have Minolta and Pentax film AF bodies, the Sigma is much easier to use. Has a shutter speed dial, the aperture is controlled by a wheel. Has sort of back button AF, on the top of the camera on the side rather on the back. The SA 9 shoots at 3.5 FPS the SA7 around 2, the SA9 has a top shutter speed of 1/8000 while the SA7 1/2000, the set up is identical, really easy to switch from one body to the next. Over the years I added SD bodies and better quality lens. One limitation is that you cant use Tamron or Tokina lens, only Sigma lens. The mount is a K mount, electronics is very similar to Canon,I've read that some have replaced the K mount with a Canon mount and most function works but no AF.