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:
“You took my heart in your hand
With a friendly smile,
With a critical eye you scanned,
Then set it down,
And said: It is still unripe,
Better wait awhile;”
—Christina Georgina Rossetti (1830–1894)
“If we focus mostly on how we might have been partly or wholly to blame for what might have been less than a perfect, problem- free childhood, our guilt will overwhelm their pain. It becomes a story about us, not them. . . . When we listen, accept, and acknowledge, we feel regret instead, which is simply guilt without neurosis.”
—Jane Adams (20th century)