How do I know what size reversing ring to get?

[AprilArchambeau]

I am not even sure if I am posting this in the right forum. But I called for some last minute things at Adorama and he had no clue what a reversing ring was, so I am asking here before I order, I am getting my T1i tomorrow morning, my husband got me the 100mm 2.8 macro for Christmas and I am buying a 50mm 1.8 with my kit and I wanted to turn that around backwards and then put my lens on *I think I worded that right*

Anyways, I believe the 100mm macro has a 58mm filter but the 50mm 1.8 has 52mm filter, so is there a reversing ring size?

Thanks so much and sorry if that is confusing

April :)

[k_wakasugi]

52-58mm reversing ring.

http://www.amazon.com/Fotodiox-52mm-52-58mm-Close-up-Anodized/dp/B001G445RI

[AprilArchambeau]

Awesome, thanks so much :)