This lens was part of a camera bundle I bought recently. I could find very little info on the web about this lens (apparently, it's almost identical to Vivitar 70-150mm f/3.8). The lens has an Adaptall adapter for Nikon. Here is a snapshot of the lens:

I'm curious whether this lens is any good. Also, there is a A/M switch on the lens. Any idea what it's for?

Thank you!

Kind regards,