I've been itching to see how well the 50mm Ultron does on an FX sensor, but unfortunately the flange focal distance of the M42 mount is a little over 1mm longer than that of the Nikon F-mount. Add the extra mm or so from a converter, and the Ultron turns into a macro lens.
The adapter I bought did have a glass element to compensate for this, and it did give me infinity focus, but the IQ was really, really bad. So off came the glass element (well, more like hammered away). You would think that 2mm or so wouldn't make much of a difference, but the Ultron was barely able to focus to four feet.
I took a few photos, and that was that. Maybe one day I'll find a Canon FX body at a bargain price and give it another go.
The 52mm filter adapter for the Ultron from eBay finally arrived after a false start (wrong item shipped, had to get a refund and re-order). It's a nicely machined piece, and fits well into the B50 mount. The tabs are a bit on the thin side, something to keep in mind. I'm now able to mount 52mm screw-in hoods and filters.
I tried several hoods, and unfortunately it takes a pretty deep one to minimize the appearance of the rainbow flare. Note that it isn't totally eliminated; I don't have a 52mm screw mount hood that's deep enough for that. Adding the adapter and a hood on the lens makes it pretty big, and doesn't quite look right on the X-E1. I've also covered that nice chrome finish on the nose of the lens. Function over form.