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[[Image:lighterstill.jpg]][[Image:focalpointflame.jpg|right|frame]]
[[Image:lighterstill.jpg]][[Image:focalpointflame.jpg|right|frame]]
In geometrical [[optics]], a '''focus''', also called an '''image point''', is the point where [[light]] rays originating from a point on the object converge [http://www.charfac.umn.edu/glossary/f.html]. 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 apertur]. Aberrations tend to get worse as the aperture diameter increases, while the Airy circle is smallest for large apertures.
In geometrical [[optics]], a '''focus''', also called an '''image point''', is the point where [[light]] rays originating from a point on the object converge [http://www.charfac.umn.edu/glossary/f.html]. 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 apertur]. 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.
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.
==Focal Point==
==Focal Point==
A '''principal focus''' or '''focal point''' is a special focus:
A '''principal focus''' or '''focal point''' is a special focus:
