Abstract: A copying machine provided with an optical system having a variable imaging ratio and a fixed distance between the image plane and the object plane and a lens whose focal length can be adjusted to the selected imaging ratio by rotation of a toothed ring coupled to the lens. Displacement of the lens with respect to the object plane and the image plane, in order to change the imaging ratio, is converted into a rotation of the toothed ring on the lens by means of a system of cooperating gearwheels. One of these gearwheels has at least one toothed ring segment which is pivotable about an axis; situated almost at the circumference of the gearwheel and extending axially of said gearwheel. The toothed ring segment can be caused to pivot about the axis and be fixed in any position.