physics---light ray

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge [1]. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by aberrations of the imaging optics. In the absence of significant aberrations, the smallest possible blur circle is the Airy disc, which is caused by diffraction from the optical system's aperture. Aberrations tend to get worse as the aperture diameter increases, while the Airy circle is smallest for large apertures.
An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged. The border between these is sometimes defined using a circle of confusion criterion.
A principal focus or focal point is a special focus:

For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal points—one on each side. The distance in air from the lens or mirror's principal plane to the focus is called the focal length.
Elliptical mirrors have two focal points: light that passes through one of these before striking the mirror is reflected such that it passes through the other.
The focus of a hyperbolic mirror is either of two points which have the property that light from one is reflected as if it came from the other.
請參考網:
http://en.wikipedia.org/wiki/Focus_(optics)
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