Fine focus for overhead projector or enlarger - has lever running in spiral groove to operate axial drive for condenser lens

摘要

The fine focus control for an overhead projector has the condenser lens (21) mounted on a lever (18) pivotted about a horizontal axis (17) near the pillar mounting (13) and with a guide slot (20) in the projector head to set the angle of the . lens. A control knob (32) operates on a peg in the side of the lever via a spiral groove to provide a fine control on the tilt of the lever. The control provides a slow drive for the condenser focus with a controlled angular setting. Only the lens and controls are moved while the head and lamp housing remain fixed. This allows a lightweight drive to be used without danger of vibrations and of loosing focus.