Purpose:Illustrate the idea of depth of field of a (camera) lens, and to locate the near and far points.
Description:The beams passing through lenses 1 and 2 are focussed to produce an object for lens 3, the "camera" lens, which is imaged on a distant screen at the right as a straight line. Adjust the screen position so that the image is as sharp as possible, and mark the object point on the optical board with black tape. By moving lens 2, the position of the object is changed, eventually causing the image to become blurred. Mark the near and far points on the board for an acceptably clear image line. The result as obtained here is somewhat arbitrary, but in fact there is a mathematical definition for what constitutes a sharp image.