focal plane

noun Optics.
a plane through a focal point and normal to the axis of a lens, mirror, or other optical system. Compare principal plane.
the transverse plane in a telescope where the real image of a distant view is in focus.

1890–95 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
focal plane
1.  the plane that is perpendicular to the axis of a lens or mirror and passes through the focal point
2.  the plane in a telescope, camera, or other optical instrument in which a real image is in focus

Collins English Dictionary - Complete & Unabridged 10th Edition
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