Quote Originally Posted by Bob F.
http://www.dennisonbertram.com/hackm...ring-flash.htm (*** WARNING *** Article contains the phrase "digital photography". Persons under the statutory age or of a nervous disposition should seek advice before continuing)...

or, somewhat more practically, http://www.intro2020.co.uk/pages/sunpak3.htm but 77mm seems to be at its limit so you might want to try one first.

Cheers, Bob.
I have one of these Sunpack Ringlight units and it works ok for me - but I've never tried it on anything larger than 77mm.