Focus (optics)

An image that is partially in focus, but mostly out of focus in varying degrees.

In geometrical optics, a focus (also called an image point) is the point where light rays that come from a point on the object converge (come together).^[1]

In reality the focus is never a point but a small spot called the blur circle.^[2]

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.

Focus (optics) Media

Eye focusing ideally collects all light rays from a point on an object into a corresponding point on the retina.
A demonstration of camera focus on different distances, showing a bamboo rooftop

References

↑ "Standard Microscopy Terminology". University of Minnesota Characterization Facility website. Retrieved 2006-04-21.
↑ 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.

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[1] "Standard Microscopy Terminology". University of Minnesota Characterization Facility website. Retrieved 2006-04-21.

[2] 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.

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