On a whim at the end of last week I bought a set of Vivitar macro extension tubes for my Canon XTi. These are pretty neat: they pass through the auto-focus and auto-exposure signals from the camera to the lens, making them easier to use. There are three in the set: a 13mm, 21mm, and 31mm tube. If my mathematics is correct, this makes seven possible tube lengths since you can combine them in any order (call them A, B, and C, then the possible combinations are A, B, C, AB, AC, BC, and ABC). […]
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