Lens centration tolerances are an essential consideration in optical design and assembly, and achieving them is a
fundamental phase of optical fabrication. The tolerances for centration may be expressed in any of several different
ways (including wedge, edge thickness variation, beam deviation, image decenter) by the designer or draftsperson; and
may then be measured in the shop according to a different set of parameters (including wedge, edge thickness variation,
beam deviation, surface reflection orbits, edge lateral runout) depending on available equipment and methods. The
necessary conversions between expressions are subtle, easily misunderstood, and not readily available. The available
literature is not entirely consistent. This paper provides definitions, diagrams, explanations, derivations, and conversions
between the various expressions. Aspheric lenses have additional datum features and are not addressed in this paper.