In a photograph, the area of critical focus is the portion of the picture that is optically

in focus IN, In or in may refer to: Places * India (country code IN) * Indiana, United States (postal code IN) * Ingolstadt, Germany (license plate code IN) * In, Russia, a town in the Jewish Autonomous Oblast Businesses and organizations * Indepen ...

. This does not relate to depth of field which describes apparent sharpness. Reducing the size of the

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

will increase the depth of field but the plane of critical focus will not change. Depth of field extends away from the plane of critical sharpness. The image is only critically in focus within a plane. The formula that describes the relationship between plane of sharpness,

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

and film is :

1/I+1/O = 1/F

, where

I

is the film to lens distance,

O

is the distance from the lens to the plane of critical focus, and

F

is the focal length of the lens. 'Critical Focus' is also the title of a regular column by Brian J. Ford in the American magazin
''The Microscope''

See also