Originally posted by carpents Can you post a photo of the adapter?
I have never heard of your problem.
I take it back - it sounds like you're trying to fit a T-mount adapter (42mm x 1mm thread pitch) onto an M42 lens (42mm x .75mm thread pitch). The other brand lens is probably a T-mount.
Thanks for the response, I was hoping that this was one of those common easy to fix issues. The adapter is in fact a M42 to Pentax K mount adapter (brand is Kalt), and the cheap lens is an M42 (I took it off my old Spotmatic).
I wonder if I should spend the $30 and get a genuine Pentax adapter? It's just bugs me that the adapter works on a cheap off brand lens but not the genuine article.