Thanks for the link.Use this simple calculator to determine your magnification.
Mine is 72mm so you will need an adapter ring to use it on your lens. I'll let you know when the next shoot is scheduled. That could be fun!Understood Mike, looks good. The magnafication factor you listed is with it at 1:1? I had forgotten you mount the 500D, what filter size do you have? My tamron 90 is 55mm. I might have to swing by one of your aquarium shoots and just see what it looks like at various distances with and without if your ok with it. Thanks!