= Thick lens
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.<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]{ref}
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