Back in the manual focus era, Vivitar were known as a great third-party lens company. Though based in the USA, they sourced their glass from the best Japanese manufacturers: Komine, Kiron and Tokina. Even now their lenses can provide excellent optics for those who do not mind -- or actually prefer -- to focus manually. When I became interested in the 28mm focal length, which provides a comfortable "normal equivalent" on the APS-C sensor, I started cataloguing the different varieties available. Little did I know then I'd end up with 38 of them!
The latest update to this work is the largest yet. I have greatly expanded the introductory comments and added a section of advice and recommendations. Each lens now links to a second page which has a standard lens database template along with pictures of the items I have sourced from wherever I could.
If you have better pictures or more information, please send it along! This is a community effort that started with Pentax Forums and has now spread much further afield.