OK, here are a couple of links that talk about the lens. http://www.adaptall-2.org/lenses/52B.html and http://www.fredmiranda.com/forum/topic/838477. I use Olympus OM equipment and own the Zuiko 90mm f2 lens and two of the Tamron SP 90mm f2.5 52B lenses. Why, I take the Tamron when I know conditions might be a little iffy and I also teach a basic photo classs and have my students used it sometimes. Great lens. Bill Barber