De Oude Delft 50mm f/0.75

[squirrel$$$bandit]

on eBay, you have to be signed in to see the photos. It's not converted. If you absolutely have to have the fastest thing out there...

 

[xxloverxx]

It seems they can't be focused at infinity; would be an amazing portrait/macro lens though!

 

[bobkatz]

Friends...same the kowa lens: 50mm f/0.75m and rodenstock XR heligon lenses...I just converted one for Nikon mount...if interested let me know..of course not at ridiculous high price....
something like this: http://www.daifukusensei.com/?m=200910
a lot of funny...
Bob.

 

[Sonnar Brian]

There are a lot of these types of lenses around, used for CRT and X-Ray machines. I'd go with the CRT lenses! They can only be used close-up.

I would not plunk down $1500 on one, more like $50.