Focus (optics)

A point where light rays converge, or from which they appear to diverge, after reflection or refraction. Optical systems can have real or virtual foci.

Real image: An image formed where light rays actually converge after passing through an optical system. A converging beam meets at a real focus, where a real image can form.
Refraction: The change in a wave's direction and speed as it enters a medium with a different refractive index. Lenses create foci by bending rays through refraction.
Camera lens: An optical assembly that forms an image on a camera's sensor or film. Its elements bring scene light to a focused image plane.
Optical aberration: A departure of an optical system's image from the ideal predicted by paraxial optics. Aberrations spread rays instead of bringing them to one exact point.
Virtual image: An image formed where light rays appear to originate, although they do not actually meet there. A diverging beam traces back to a virtual focus, producing a virtual image.
Reflection: The return of a wave from a surface into the medium from which it came. Mirrors create foci by redirecting rays through reflection.
Autofocus: A system that adjusts an optical device to bring a subject into sharp focus automatically. It changes lens position or configuration until the image plane is focused.
Spherical aberration: An aberration in which rays at different distances from a spherical surface's axis focus at different positions. It makes marginal and paraxial rays have different focal locations.
Focal length: The distance from a lens or mirror's principal plane or vertex to its principal focus. It quantifies how far the principal focus lies from the optical element.
Paraxial approximation: An approximation that treats rays close to an optical axis and at small angles to it. The familiar single-focus description is most accurate for paraxial rays.

Linked from 1 page

Shutter speedCompared with: Defocus blur comes from optical alignment, not from the duration of exposure.