The Schneider 10mm Extension Tube is a component for use in the Schneider Macro System.
Unlike small and medium format photographic lenses, enlarging lenses have no fixed flange-to-image distances. The distance from the lens to the camera is different for each lens with the same imaging dimensions. Cameras differ also in their back focal lengths, which means that the extension length of the lenses on the image side must be adapted to the current image task.
This is done with extension tubes, which can be combined in different lengths. They are installed between the lens and the camera.