So, here's the deal. I'd like to create a DIY extension tube to use my 50mm f/1.4 as a macro lens. How much additional space will I need to create to make that happen? I see that Canon has a couple of electronic ones for sale, more than what I want to spend on one, but I'm not sure of the math calculations involved to get to the point of using it as a macro. I understand this won't utilize AF, and will be strictly MF, but I'm ok with that. Thanks in advance for whatever advice you can give.