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[cjo1]

Hi, can anyone tell me how to work out the minimum and maximum distance at which i can focus given any focal length; using extension tubes and dSLR with a sub-full frame sensor? What i mean is, if i use extension tubes and a lens of any given focal length, how do i work out what the focusing distance is going to be taking into acount the conversion factor of the dSLR sensor? eg 100mm lens and a 1 cm tube, what is the focusing distance using a canon 30d?

I hope one of these questions makes sense to someone!