Irix Macro Rail 180

At 1:1 the depth of field is thin enough that turning the focus ring changes magnification as well as focus. A focusing rail solves that by moving the whole camera along the optical axis, which is how focus stacks are built and how a macro frame gets composed without recomposing.

The Irix Macro Rail 180 gives 180 mm of travel and is intended to work with the Irix 150mm f/2.8 Macro 1:1, though it takes any camera on a standard tripod thread. It is also sold as part of the Irix macro kit alongside the 150mm lens and the Arca Swiss key ring.