The point on the principal axis where incident parallel rays meet after reflection from the mirror is known as the focus. The focus of a mirror is at the center of the optical system.
The distance from the focus to the mirror end is called focal length, also known as flange distance. When light enters a lens, it travels in different directions. The shorter rays converge and meet at the lens’ focus.
The focal point of an object that has been reduced to its smallest angular size is its image and its real position in space (the geometric magnification).
In contrast, only one point on an illuminated object needs to be considered for ray tracing purposes: it should be located at a distance from which all outgoing rays fall within some specified cone (angular width) after they leave their respective starting points on that object.
A lens focuses light and forms an image of the object being illuminated. A lens forms an image by converging all light rays where they exit the surface of a material which is called an aperture.
The conical intersection of all rays inside this aperture, called the focal point, is the point at which they converge. The focal point can be moved along an optical axis.
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