Abstract: An adapter for an optical instrument to control the angle of the plane of focus of the instrument. The adapter has front and rear coupling faces, a secondary lens and a tilt-swing mechanism or a fixed tilt. The adapter's rear coupling face is connected to the optical instrument, an interchangeable primary lens is connected to the front coupling face of the adapter and the tilt-swing mechanism tilts or swings the primary lens relative to the image plane. The tilt-swing mechanism, which controls the angle of the plane of focus, would allow, among other things, for the utilization of Scheimflug's principle.