Focusing rails are by far one of the most useful tools available to the macro photographer. They allow you to make several different changes to your focusing point and image composition without having to move the tripod or camera stand. You can tweak little aspects of your images without having to start from scratch because you moved the tripod head or legs. These tools will save you much time and frustration as you are working.
A focusing rail is a piece of machinery with rails on both side and a threaded shaft down the center or a geared mechanism along the side of the rail. A platform is attached to the moving portion of the rail and is the point where the camera or other accessories connect. There will be a knob to move the stage along the length of the rail. This movement of the camera is where you gain your advantages.
When working with macro images your depth of field is usually millimeters deep making a change in the focusing point nearly impossible by moving the head or legs you have the camera mounted on. This is where the invention of the focusing rail comes in. By having the camera on a rail you can adjust your focus point by moving the camera forward or back along the rail while not having to adjusting anything else. The adjustments you are making can be very small and very precise.
Since the focusing rail moves in a straight line you can use more than one to enable changes on more than one axis. You have one rail facing directly at the subject. This allows you to make changes to your focusing point throughout the depth of your image. The second rail is oriented parallel to the subject. Having movement from side to side will allow you to change your composition without having to reposition the rest of your setup.
Having the focusing rail setup parallel to the subject will give you the ability to make macro panoramic shots. When working with 1:1 reproductions you are limited by the size of the cameras sensor. By making a panoramic image you can maintain the 1:1 image while increasing the size of the subject you can work with. You want to allow 10% overlap on the edges of your images with most panoramic stitching programs.
Focus stacking is the technique of taking several images of your subject at different focusing distances and compressing them in one post to get one image with much greater depth of field. You can have the entire subject in focus if you would like. Focusing rails are an invaluable tool for doing this. You can take an image every couple of millimeters to compress. Having a focusing rail with a scale on it will be necessary to achieve this goal.
I have two different focusing rails currently. The first rail I every bought was the Velbon focusing rail. I picked it up about 7 years ago when I started getting serious about my macro photography. It was a hundred dollars back then and is still a great bang for the buck. You get good traveling distance from front to back. The unit also has a function for a little bit of side-to-side movement, a great feature when you want to change the composition of your images. Today I use this rail for a few different things in the studio.
During the past year I picked up the Really Right Stuff ultimate rail package. I would argue for a number of reasons these are the best focusing rails on the market and I will in an upcoming review. This kit includes two of their focusing rails, giving you both front to back and side-to-side traveling distances of about 6in. They are incredibly well made and have integrated Arch style quick releases. These rails allow you to make the tiniest of sub millimeter adjustments with ease. They also have an integrated scale allowing use for several different techniques.
There are other companies making rails besides the two I have mentioned. Another of the most popular is the Novaflex line. They are well made and have a reputation for being a precision piece of machinery. Kirk Enterprise is another company with a great reputation making focusing rails. A company names Hejnar Photo also makes some nice looking ones. I have never had the chance to try their products but hope to in the future. They have a line of Micrometer rails, which looks really interesting.
There will be a time when every macro photographer is going to wish they had one of these tools. If you are serious about the craft it is only a matter of time before you are going to need to get one. My advice is buying the best one you can afford. You want your rail to be rock solid with no play. When you turn the knob make sure there is no travel other than what you intend. These are precision made tools and you will get what you pay for.