[the following is a modified repost from Myrmecos blog]
Dedicated insect photographers normally employ specialized macro lenses to focus on their tiny subjects. These can be pricey. My high powered MP-E lens costs $1,000, for example, and my 100mm f/2.8 is $500.
But macro does not have to be expensive. Consider the effect of a single extension tube mated to a simple 35mm prime lens:
What is an extension tube?
It's a simple ring that mounts between the lens and the camera, holding the lens farther out from the sensor than usual. There's not much to it: no glass, no moving parts. The tube pictured here is a Canon model, but some enterprising photographers make their own from whatever is lying around. Say, a toilet-paper tube. Or an old pringles can.
Extension tubes work by shifting the minimum focus distance towards the camera. With the lens able to form images closer in, the subject is effectively magnified. Since the tubes don't have any glass- they just add some extra space- their use does not degrade image quality.
Extension tubes can be used with any lens. Short lenses do well with short tubes, but longer lenses require longer tubes for a similar effect. For instance, the 35mm lens is a good match to a 12mm tube, but a 50mm lens will require 25mm tube for similar magnification. With the right length tube any lens can be converted for macro duty.
There is a catch, of course. With a tube in place, lenses no longer focus farther than a few inches away. No more distant mountains- it's macro or nothing. But that is a small price to pay for inexpensive macro.