Focus,

Focus, in Optics, is a point in which several rays meet and are collected after being reflected or refracted, while a virtual focus is a point from which rays tend after reflection or refraction. The principal focus is the focus of parallel rays after reflection or refraction. See LENS, MIRROR, OPTICS, PHOTOGRAPHY. For the focus of geometry, see ELLIPSE, HYPERBOLA, PARABOLA.—The Focimeter is an instrument for assisting in focusing an object in or before a photographic camera; it is usually a lens of small magnifying power.