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fo'cal point"




1. Also called principal focus. Optics.either of two points on the axis of a mirror, lens, or other optical system, one point being such that rays diverging from it are deviated parallel to the axis upon refraction or reflection by the system and the other point being such that rays parallel to the axis of the system converge to the point upon refraction or reflection by the system.
2. the point at which all elements or aspects converge; center of activity or attention: The focal point of our discussion was the need for action.
3. the central or principal point of focus.

Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.

focal-plane shutterfocal ratio
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