The focusing rail: millimetric adjustment in macro photography

In macro photography, focusing is done by moving the camera and lens assembly. Rails are the best instrument when we need millimetric adjustment.

What is a Focus Rail

A standard focus rail is a tool that allows a camera and lens assembly to be moved along one or two axes with millimeter control. It consists of a fixed platform that screws into the shoe of a tripod and a movable platform that slides along a metal rod or a pair of parallel metal rods. A rack and pinion gear or screw mechanism allows this second platform to move smoothly back and forth. This is a mechanism very similar to that used by most macro photography bellows, although the use of the latter is very limited to the studio.

While a focus rail is an extra item to carry around when you go into the field, it can also save you a lot of time and effort and guarantee better results.

Desirable features of a focusing rail

If macrophotography is our main area of work, we will want a focusing rail that is strong, durable and reliable, and we will avoid those equipment that hinder the work instead of facilitating it. A quality tool should meet a series of requirements:

The platform should be solid and stable, with no play between it and the rails, either when moving or locking it.
The composition and field of view should not change noticeably when the mechanism is locked. This should be able to be done using minimal force.
The platform should be able to support the weight of the camera and lens without slipping. This can be tested by mounting the frame vertically. With the mechanism unlocked, check to see if the platform slides. If it does, the mechanism is too loose.
The platform should move smoothly, without jumping or jamming.
The focus wheel should turn without applying excessive force.
The movement of the platform should be precise but fast enough. Platforms that move too fast or those that require many turns to move a short distance are not practical.

The quality of focusing rails sold as photography accessories varies widely, and the really good ones are also really expensive.

The best focusing rails are those made for microscopes and laboratory optical equipment, although their price can be very high. An alternative is to find second-hand ones on eBay and adapt them by hand for use with tripods, although most are too heavy to carry around the field. Another drawback is that their range of movement is limited to about 30-50mm, but if you can use them within that range, you will be enjoying a first-class focusing rail.