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
Step down rings not a good idea.
Have you considered a small flash on an extendable arm?
If you are a Nikon user, you can use their macro flash which comes with a 72mm adapter. Otherwise a couple of flash units on a bracket would do the trick. Assuming you don't need the flat ring flash lighting.