ETYM Cf. French focal. Related to Focus.
1. Having or localized centrally at a focus.
2. Of or relating to a focus.

Or focal distance; The distance from the center of a lens or curved mirror to the focal point. For a concave mirror or convex lens, it is the distance at which parallel rays of light are brought to a focus to form a real image (for a mirror, this is half the radius of curvature). For a convex mirror or concave lens, it is the distance from the center to the point at which a virtual image (an image produced by diverging rays of light) is formed.
With lenses, the greater the power (measured in diopters) of the lens the shorter its focal length. The human eye has a lens of adjustable focal length to allow the light from objects of varying distance to be focused on the retina.
The distance from a lens to its focus.
Distance from the focal point to the center of a lens or vertex of a mirror. + prikaži više