Macro Photography, using extension tubes

Extension tubes

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The cheapest tube set with electric contacts is about $60 to $70 I understand. At least, it was that a couple of months ago, although prices may have gone up by now. Just check Amazon or the B&H site. For a good set but more expensive, get a Kenko 3 tube set (plastic), and if you want metal the only option is Canon. However, with a 40 mm you could just get away with a 12 mm tube to begin with anyway. As this lens has an MFD of 25 cm, a 12 mm tube at MFD will give you a new MFD of approximately 175 mm with 113 mm WD and a magnification of 1:0.55, or slightly better than half size. A 25 mm tube 161 mm MFD, 86 mm WD, and magnification of 1:0.875. A 30 mm tube will provide you with 1:1 for this lens.
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The most important aspect of having tubes with contacts is the ability to control the aperture on an EF or EF-S lens."