I have a couple of questions about extension tubes. First off....I have a Sigma 70-300 APO DG Macro that has 1:2 magnification at 300mm. I usually shoot at betwenn f12-f18 with a shutterspeed at between 1/250 to 1/500 to darken the background and always use an off camera diffused flash. Using this setup, I get suprisingly sharp and clear photos (considering the lens). So...question 1. Do extension tubes change the quality/sharpness of the lens? I understand that there is no glass and it's only moving my rear glass farther from the body.......but does that affect IQ and sharpness? Question 2 - I've searched and searched but haven't found it. I'd like to get as close to 1:1 as possible and only plan to buy 1 extension tube (this only to hold me over until I can afford a true Macro lens). How can I figure up what each mm of extension tube will change my magnification to? One place said you take the focal length (300mm) divided by the extension (say 25mm) which would make it 1:12 obviously isn't right since I'm already at 1:2. Any help is hugely appreciated.