In What are the best practices for DOF stacking? I noticed a mention of a "focusing rail". I've never heard of such device. What is it, and in what circumstances is it useful?
Focusing rail is a type of camera mount that allows you to move the camera forwards and backward on a very small scale.
(image taken from article at Earthbound Light)
The reason for using them is that in macro photography you have such small depth of field that it's easier to focus lens to given distance and then move the camera to get the parts you want in focus than the other way round (and it's also ensures you always use maximal magnification, if you need to).