Clarification: Can't work easily with macro, which I should have written. And by that I mainly meant with extension tubes. Turning from infinity to MFD on most of my widest lenses only moves the elements by 1-2mm away from the film plane at most. With my thinnest extension tube being something like 10mm thick or so, I've never been able to focus on anything with lenses wider than 28mm. (I've got my RokiBowYang 35/1.4 just working with 10mm tubes, and my Super Ozeck II Macro 28/2.8 works ok, but not the Mir 20/2.5 or Tokina 17/3.5, and I've just realised I don't have a 24mm prime. I'd best rectify that with GAS).
Not as nice as with the OM 20mm macro, which you can rack out to 12x and the working-distance doesn't change much with focussing, stays fairly constant at "bugger-all mm". Still I want one, for when I need to take photos of match-heads and all.

But yeah, I wasn't thinking about reversed, which I've also done a bit of (manual-lenses only, there are ways to use EF but it gets complicated). And with the whole thing of pupil factors and effective apertures and all that, I'm too lazy to work it all out so I don't even bother attempting macro without accurate TTL metering, and only use flash when I can chimp on the digital...

Anyway, the OP hasn't clarified exactly how close 'macro' will be, we're all presuming that the smaller aperture is to get bigger DOF, but there could be other reasons like to purposefully introduce diffraction (in which case, may I suggest a pinhole?)