Focusing rails are extremely useful in macro photography since your depth of field is so shallow you need precise placement of your camera to get your subject in focus. They are rails that attach to your tripod or ballhead, and have a rack and pinion system of gears that let you move the camera back and forth in precise movements. You don’t usually want to turn your focus ring because that often doesn’t allow for the precise focusing a good focus rail can provide, and also whenever you turn your focus ring, you are also changing the magnification. Focus rails basically come in two varieties, a single axis rail like the one pictured here, and a two axis variety that allows for left to right movement as well as forward and back. A lot of manufactures make focusing rails already equipped with quick release plates (like the one pictured here) that are compatible with the various quick release systems out there such as the Really Right Stuff, and Arca-Swiss.
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