I have been playing with other lens with extension tubes, and I like the 50mm 1.8 the best. Tubes will work with most any lens, prime or zoom, except most wide angle lens. I've also been playing with reversing the lens, which gives a greater magnification by mounting the lens backwards on the camera. I bought an adapter to do this from B&H Photo for $7. Works very well, but exposes the back of the lens, which makes me a little uncomfortable.
As far as depth of field, Ruard is correct when he says he gets a greater depth of field at f8. With most lens, there is a "sweet spot", usually about 2 stops back from the minimum aperature (say f22) that gives the maximum depth of field in macro. Don't ask me why; beyond my understanding, but it's true.
Lighting seems to be a "biggie" with macro. I prefer off-camera flash to ring lighting. Ring lights are fine, but will give you a "flatter" looking image. I just use a flash with a 3 ft cord to the flash shoe on the camera. Costs a lot less than a ring flash, too.
Macro is a really interesting area to explore, always something to learn - just like tying.
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