A thick lens retains the separation of its refracting surfaces when predicting image formation. In the paraxial approximation with air on both sides, object and image distances measured from the respective principal planes satisfy the familiar imaging equation. Measuring instead from one mechanical centre generally introduces an error.[ref]
Refracting systems (details), University of Tennessee physics teaching resource. University of Tennessee physics teaching resource, Refracting systems (details), final paragraph: object and image distances must refer to the principal planes
The two principal planes are reference planes that represent a centred imaging system in the paraxial approximation. In air on both sides, measure object distance from the front principal plane, image distance from the rear principal plane, and focal length from each principal plane to its corresponding optical focus. They need not coincide with a surface or mechanical centre. Their signed separation matters in the Bessel lens displacement method.[ref]
Refracting systems (details), University of Tennessee physics teaching resource. Refracting systems (details), definition of principal planes, unit lateral magnification, and final imaging equation
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