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:
“It is a sign of our times, conspicuous to the coarsest observer, that many intelligent and religious persons withdraw themselves from the common labors and competitions of the market and the caucus, and betake themselves to a certain solitary and critical way of living, from which no solid fruit has yet appeared to justify their separation.”
—Ralph Waldo Emerson (1803–1882)
“Why is it so difficult—so degradingly difficult—to bring the notion of Time into mental focus and keep it there for inspection? What an effort, what fumbling, what irritating fatigue!”
—Vladimir Nabokov (1899–1977)