= Principal plane
{title2=$H,H'$}
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>.<Refracting systems (details), University of Tennessee physics teaching resource>[Refracting systems (details), definition of principal planes, unit lateral magnification, and final imaging equation]{ref}
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