Focus (optics)
This article is about the focus in geometrical optics. For other meanings, see Focus (disambiguation).
Brennfleck is a redirect to this article. For the German entrepreneur, founder of Libertas Hotelgesellschaft, see Manfred Brennfleck.
The focus (from Latin focus for "fireplace", "hearth") or focal point is that particular point of an imaging optical device where the rays incident parallel to the optical axis intersect. It is located at the distance of the focal length on the optical axis. The plane perpendicular to the axis at the focal point is called the focal plane (sometimes also Fourier plane, focal plane, or focal plane). For distant objects, such as the sun, the focal plane is also the image plane - from the hot image of the sun, where its radiant energy is concentrated, the focal point gets its name. Here and in practice, it is therefore also called the focal spot. The weaker the focus of the optics, the further away the focus is.
In diverging lenses or convex mirrors, it is not the refracted or reflected rays themselves, but their backward extensions that intersect at the focus. This type of focus is also called a focal point, although no radiant energy is concentrated here.
A lens or lens group has a second focal point for the rays incident from the opposite direction. A parabolic mirror, on the other hand, has the same focal point for both directions.
The mathematical foci of an ellipse, parabola or hyperbola are not foci in the sense of this article, but their name is derived from the physical focal point, since real ellipsoids and paraboloids can also focus light rays in the (mathematical) focal point, e.g. in the parabolic mirror or the parabolic antenna.
Focal points F and focal lengths f of some optical elements
