Paul Howell
Subscriber
Vivitar sold several 28mm f2.8 lenses with a Canon FD mount over the years. They were made by different lens manufacturers at different times and produce different results. Probably the best is the 28mm f2.5, made by Kiron with serial numbers starting with 22XXXX. It is larger and heavier than the others and has a 67mm filter thread.
I had a Vivitar 28 2.5, in Nikon non AI, mount, found it at yard sale in the late 70s, I shot few rolls with it, IQ was pretty good. As I had a Nikon with the standard 52mm filter thread I gave it to a friend who's gear had gone up in a fire and was cobbling togeather a kit while he waited on an insurance settlement.