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From Nordan Symposia
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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==
A '''principal focus''' or '''focal point''' is a special focus:
A '''principal focus''' or '''focal point''' is a special focus:
*For a lens, or a [[sphere|spherical]] or [[parabola|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''.
*For a lens, or a [[sphere|spherical]] or [[parabola|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''.
