In a photograph, the area of critical focus is the portion of the picture that is optically in focus. This does not relate to depth of field which describes apparent sharpness. Reducing the size of the aperture 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 and film is
- ,
where is the film to lens distance, is the distance from the lens to the plane of critical focus, and is the focal length of the lens.
'Critical Focus' is also the title of a regular column by Brian J. Ford in the American magazine The Microscope.
Famous quotes containing the words critical and/or focus:
“The male has been persuaded to assume a certain onerous and disagreeable rôle with the promise of rewards—material and psychological. Women may in the first place even have put it into his head. BE A MAN! may have been, metaphorically, what Eve uttered at the critical moment in the Garden of Eden.”
—Wyndham Lewis (1882–1957)
“Let us be aware that while they preach the supremacy of the state, declare its omnipotence over individual man, and predict its eventual domination of all peoples of the earth—they are the focus of evil in the modern world.”
—Ronald Reagan (b. 1911)