i have the sigma 180 macro lens which has a 72mm filter thread i want to buy a ring flash to suit this lens,all the flash's i have looked at have smaller threads 52mm etc would these be suitable with step up rings or would they chop the corners of the image and cut out light any help and any recomendations would be very helpful thanks paul
I know this doesn't answer your question directly, but I wouldn't get a ring flash. If you use Nikon, look at the RC1. There are many other choices where you have two small flashes on either side of the lens pointing in. It just takes a bracket.
On my Nikon, I use the Sigma 150 APO F2.8 Macro with the RC1 kit. I don't use the included attachment to the lens anymore. I use a after market bracket that holds the two flashes. I can adjust the direction much better. I have one flash on full and one at .5 ev or less to give it a studio effect. Sometimes I use a third flash behind the subject. Nikon's Creative Lighting system offers to wirelessly trigger several flashes in several groups. Other brands have similar systems, I think.
Have fun.