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:
“From whichever angle one looks at it, the application of racial theories remains a striking proof of the lowered demands of public opinion upon the purity of critical judgment.”
—Johan Huizinga (1872–1945)
“While the focus in the landscape of Old World cities was commonly government structures, churches, or the residences of rulers, the landscape and the skyline of American cities have boasted their hotels, department stores, office buildings, apartments, and skyscrapers. In this grandeur, Americans have expressed their Booster Pride, their hopes for visitors and new settlers, and customers, for thriving commerce and industry.”
—Daniel J. Boorstin (b. 1914)