I am well aware of the arguments regarding UV/clear filters to protect a lens. When looking for a UV/clear glass filter I noticed that Tiffen, which was really a leader in filter manufacturing in the 1960s, still seems to have filters for sale, at this time. I found some of my old Tiffen filters from the 1960s and they look just great. They new Tiffen filters are far less expensive then other manufacturers' brands. I was surprised to find that they actually claim they make their filters in the USA.
I just wondered if anyone has had any good or bad experiences with their filters?

To respond to both Bill_de and Architect 1776, what you are saying is correct.

And after doing some more research, I see that I was incorrect about the Tokina lenses They do appear to render angles accurately. They also do not appear to straighten edges in images. That research revealed a field of view of 104-72 degrees for their 11-20 DX fisheye, compared to a field of view of 180-100 degrees for their 10-17 full frame fisheye lens. This is certainly consistent with the focal length ranges for these lenses. The problem I see is that because of this truncated image size, the possibility of realizing much true fisheye effect is going to be limited, even if the lens does treat distortion as a "real" fisheye lens.