principal-point

principal point

noun Optics.
the point where a principal plane intersects the axis.

Origin:
1695–1705

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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American Heritage
Medical Dictionary

principal point n.
One of the two points in an optical system where the axis is cut by the two principal planes. Lines drawn from these to corresponding points on the object and the image will be parallel.

The American Heritage® Stedman's Medical Dictionary
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