Depending on how bad a dinge it is, you can pound it out with a piece of wood, and a light hammer.
Take a piece of wood, the size used for wall framing studs, and using a hole saw cut an indent into it close to the size of the outside lens diameter. Lay the lens in this indent, and using a small piece of wood dowel and the light hammer, tap back and forth along the section that is dinged until it is back in round.
You won't hurt the threads on the lens as the wood dowel absorbs all the damage, and as long as you don't get too agressive with the tapping you shouldn't have to worry about cracking the ring.
If it's bent clean over, then you probably will have to hack out a section. Good luck.
thanks for the advice. i had not thought about it a great deal but that is a little gem of a hint ....i have a board that i use to chop herbs with(always comes around to food doesn't it, no matter what the thread, :p :rolleyes: :eek:)
and the board has machined concave....i reminded me of when i did metal work and we used similar shaped wood to beat and shape copper sheet into a vessel or rounded shape
i just might have the right shaped timber laying about ....