Everything about Focus Optics totally explained
In
geometrical optics, a
focus, also called an
image point, is the point where
light rays originating from a point on the object converge . 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 isn't 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's 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.
A diverging (negative) lens, or a convex mirror doesn't focus a collimated beam to a point. Instead, the focus is the point from which the light appears to be emanating, after it travels through the lens or reflects from the mirror. A convex parabolic mirror will reflect a beam of collimated light to make it appear as if it were radiating from the focal point or conversely, reflect rays directed toward the
focus as a collimated beam. A convex
elliptical mirror will reflect light directed towards one focus as if it were radiating from the other focus, both of which are behind the mirror. A convex
hyperbolic mirror will reflect rays emanating from the
focal point in front of the mirror as if they were emanating from the focal point behind the mirror. Conversely, it can focus rays directed at the focal point that's behind the mirror towards the focal point that's front of the mirror as in a
Cassegrain telescope.
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