500D for Up Close & Personal

I have a Sigma 70-300 and was thinking about getting a 500D diopter close up lens. Can anyone tell me how much magnification I would get with the 500D at 300mm?

Thanks

I find that I can focus to about 8 or 9 inches from the front of my 70-300 DO with the 500D close up lens. Not quite 1 to 1 but it is close. This is a photo of a ring I assembled take with a 70-300 mm DO and a 500D close up lens.

you can use the following link to calculate: http://eosdoc.com/jlca​lc/

It will give you an additional 0.6X magnification on top of what the 70-300 at 300 provides you. The 500D is a 500 mm lens, essentially, and the extra magnification is then 300 mm / 500 mm = 0.6X.

HTH, kind regards, Wim