In a
photograph
A photograph (also known as a photo, or more generically referred to as an ''image'' or ''picture'') is an image created by light falling on a photosensitivity, photosensitive surface, usually photographic film or an electronic image sensor. Th ...
, the area of critical focus is the portion of the picture that is optically
in focus. This does not relate to
depth of field
The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus.
Factors affecting depth ...
which describes apparent sharpness.
Reducing the size of the
aperture
In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...
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 that 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
A film, also known as a movie or motion picture, is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, emotions, or atmosphere through the use of moving images that are generally, sinc ...
is
:
,
where
is the film to lens distance,
is the distance from the lens to the plane of critical focus (in the object space), and
is the
focal length
The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...
of the lens.
'Critical Focus' is also the title of a regular column by
Brian J. Ford
Brian J. Ford HonFLS HonFRMS (born on May 13, 1939 in Corsham, Wiltshire) is an independent research biologist, author, and lecturer, who publishes on scientific issues for the general public. He has also been a television personality for mo ...
in the American magazin
''The Microscope''
See also
*
Sharpness (visual)
In photography, acutance describes a subjective perception of visual acuity that is related to the edge contrast of an image. Acutance is related to the magnitude of the gradient of brightness. Due to the nature of the human visual system, ...
References
Science of photography
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