I have actually used that Rollei E36 flash with the 16mm fisheye but the light falloff from the flash is kind of an ugly rectangle with the bottom top and sides each having a different abruptness in cutoff.
I happened to come across a Nikon SB-28 flash for $80, but did not buy it yet. It looks like that will go as wide as 20mm and will work fine in Auto mode on my non-TTL cameras. I'll goggle around a little more tonight on this flash and may stop by and pick it up tomorrow.
Turns out the SB-28 was gone from the photo shop and a SB-24 was now there at $90. I kind of had my mind set on the SB-28 as it can go wider, so I got one off ebay. The SB-28 has a pull-down diffuser that extends the range to 18mm coverage. The SB-28 has an external sensor, so it can be used on all my cameras.
Ken, I couldn't tell you… and the nature of the beast makes it feel a little moot to me - if you measured directly, you'd see a drop of at least a stop I'm sure, but then you get more spread-out, soft light in the overall scene. Plus, I can't remember a time when on-camera flash didn't have enough oomph for me, even in the film days, shooting EPP at 80 iso. But for, say, a wedding guy or someone who does a ton of events, I'm sure there's a more thought-out approach. I use this for meet & greet, minor celebs in town kind gigs, and I'll likely whip it out at a party or two - all digital though.
It's essentially like those $60 Gary Phong tupperware-looking things, just a lot cheaper and seems to spread the light much more. It is the same all around… I've thought about putting foil against the back interior of it, but I don't do enough on-camera stuff to need to play with it too much. $19 and Amazon's return policies, maybe worth a try if you're not psyched with your current approach. I used a lumiquest reflector-thing and a rubber band for the last 15 years, so for me, huge improvement.
Just gutted a Speedotron head to put my Speedo reflectors on a Vivitar 285 - MUCH more excited about that, with a radio slave it's a cheap & portable deal for more of a beauty dish look, but I'm sure it could take a good sized strip box or softbox.