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:
“Probably more than youngsters at any age, early adolescents expect the adults they care about to demonstrate the virtues they want demonstrated. They also tend to expect adults they admire to be absolutely perfect. When adults disappoint them, they can be critical and intolerant.”
—The Lions Clubs International and the Quest Nation. The Surprising Years, I, ch.4 (1985)
“It’s sad but true that if you focus your attention on housework and meal preparation and diapers, raising children does start to look like drudgery pretty quickly. On the other hand, if you see yourself as nothing less than your child’s nurturer, role model, teacher, spiritual guide, and mentor, your days take on a very different cast.”
—Joyce Maynard (20th century)