I'm trying to figure out what the 500d will give me in magnification. With my Sigma 70-300 APO DG Macro, I am at 1:3 at 200mm and 1:2 at 300mm. Ideally I'd like to use this closeup filter at 200mm since the lens is considerably sharper at 200mm than 300mm. So, is there anyway to figure out what my magnification ratio will be with the Sigma at 200mm and 300mm with the 500d attached? Also considering getting rid of the Sigma to get a Nikon 55-200VR to do this with....it is 1:4, but again, I have no way of knowing if the 55-200VR with 500d will be greater than the 1:2 I'm getting now with my Sigma 70-300 APO. For what it's worth, Canon lists the 500d as a diopter +2, but I have no idea what that means. I appreciate any help.