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
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
Five instruments can be mounted at the focal plane of the telescope.
And the focal plane is almost perfectly flat, so the edges of your shots won't
  be too blurry.
Photographing with a point-and-shoot camera means an especially deep focal
The satellite's light detector has two focal plane arrays-one for star tracking
  and for the transit observations.
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