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
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