Close-up: Focusing Rails
In order to focus a lens, the elements are moved in and out by means of the focusing helicoids. What this is actually doing is changing the extension in the lens which, as we now know, changes the magnification the lens is producing. While such changes are negligible for normal sized subjects, they can complicate matters for macro. What works far better is to prefocus the lens manually at the approximate distance and find a means of moving the entire camera and lens in and out to fine tune things. A focusing rail can come in very handy for this. Without, you would be forced to constantly reposition your tripod, a tedious process guaranteed to frustrate.
Focusing rails can also be used to help with camera positioning, something that is important in macro work in order to control background coverage and optimize your usage of depth of field.
The standard focusing rail features a rack-and-pinion gear mechanism to move the camera. Others ingeniously make use of the open-ended Arca-Swiss quick release format and merely slide in and out through the jaws of the tripod head's clamp. In effect, this kind is merely an extremely long quick release plate with dovetails along its entire length.
Focusing rails range from relatively affordable to more expensive. A quality rail though is one of those things that, while it may seem extravagant, can make your life much easier and could easily become something you can't work without.
The first rail I purchased many years ago is pictured below on the left. While it offered movement both forward and back as well as side to side, its movements were anything but smooth and the thing was far to big to carry in the field. The image on the right shows a couple of better options from Kirk Enterprises and Really Right Stuff. Both of these will fit in your pocket if need be, and even the simple non-geared variety moves more easily than my old Velbon did.
The Kirk Long Rail Plate features a clamp that can be rotated 90 degrees to accommodate either body plates or tripod collar lens plates. It does not provide side to side movement though, but I have seen people stacking two of these before. The orientation of the RRS Geared Focusing Rail can not be changed, but fastening a small lens plate to the bottom of their B2-FAB clamp oriented crosswise to the main clamp. This also gives you a limited amount of side to side movement, but if you need more their LMT plate can provide a good solution. Kirk also offers a geared rail but the RRS one is nicer, in my opinion at least. And for completeness, Really Right Stuff now offers Camera Bar focusing rails in lengths up to 28 inches (wow!) that can be used for macro, but they require you to add your own clamp.
Some bellows such as the Nikon PB-4 and PB-6 also feature integral focusing rails. If you are already employing extension to achieve your desired reproduction ratio, this makes a nice feature.