Telecentric optics for focus analysis

Magnification variations due to changes in focus setting pose problems for vision techniques, such as, depth from focus and defocus. The magnification of a conventional lens can be made invariant to defocus by simply adding an aperture at an analytically derived location. The resulting optical configuration is called "telecentric". It is shown that most commercially available lenses can be turned into telecentric ones. The procedure for calculating the position of the additional aperture and a detailed analysis of the photometric and geometric properties of telecentric lenses are presented. Experiments are reported that use a phase-based shift detection algorithm to demonstrate the magnification invariance of telecentric lenses.