hmmm...would it be a synonym for a (ground-glass) lens?
quotes:
This smaller aperture can create a pretty nasty hot spot in your images... even with a plano-convex. If you get the hot spot or not (or how bad) will depend on how well your GG diffuses the light.
I'm using a Nikon D focusing screen as my GG. I'll probably just get a Canon fd lense, even if it is slower.