In the 80's Schneider Kreuznach made an ultra wide attachment for Nizo 6080 & Nizo Pro Super-8 cameras. I've got one in pristine condition, it's got a 67/62mm thread, so I've stuck it on the V1.

http://farm2.static.flickr.com/1439/1474447750_0209c8df76.jpg?v=0

It's one uncoated (?) element with a large surface of glass, so there are reflections flying around everywhere; catch the sun and it explodes. I'm primarily thinking of using it for evocative beautiful enhancement rather than transparent increase of the angle. From images previously posted I'm estimating similar sharpness, CA and barrel distortion to the Century .65... but I'm only guessing.

Has anyone used one or done a direct comparison with a Sony/Century/Raynox wide angle?

The Raynox claims 540 lines/mm resolution.

Does anyone know how this compares to the Century Optics or Sony?
And how big a difference does this mean, really? I would assume that even if the Sony or Century Optics are lower that the glass quality would be much higher and thus provide more of a PQ advantage.

What do people usually keep in mind when investing in a WA lens? (Aside from chromatic aberration, barrel distortion, etc.)

Craig,

Thanks for the info about the Raynox.
Here's a link here to an article that explains how to measure lens resolution (that I think can be adapted to work for the uw + V1 lens...)

http://members.aol.com/floatinghabitats/resolve/lpmm.html

For me the main factors are angle of vision, resolution, fringing & light transmission (loss). I'm not worried about barrel distortion, but coatings and element design make a big difference re internal reflections. Size & weight are an issue, as is the ablility to zoom through, only if I'm in a documentary situation.