In optics, particularly as it relates to film and photography, depth of field (DOF), also called focus range or effective focus range, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.

In some cases, it may be desirable to have the entire image sharp, and a large DOF is appropriate. In other cases, a small DOF may be more effective, emphasizing the subject while de-emphasizing the foreground and background. In cinematography, a large DOF is often called deep focus, and a small DOF is often called shallow focus

Precise focus is possible at only one distance; at that distance, a point object will produce a point image.[1] At any other distance, a point object is defocused, and will produce a blur spot shaped like the aperture, which for the purpose of analysis is usually assumed to be circular. When this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus; it is rendered as "acceptably sharp". The diameter of the circle increases with distance from the point of focus; the largest circle that is indistinguishable from a point is known as the acceptable circle of confusion, or informally, simply as the circle of confusion. The acceptable circle of confusion is influenced by visual acuity, viewing conditions, and the amount by which the image is enlarged (Ray 2000, 52–53). The increase of the circle diameter with defocus is gradual, so the limits of depth of field are not hard boundaries between sharp and unsharp.

Motion picture

For 35 mm motion pictures, the image area on the film is roughly 22 mm by 16 mm. The limit of tolerable error was traditionally set at 0.05 mm (0.002 in) diameter, while for 16 mm film, where the size is about half as large, the tolerance is stricter, 0.025 mm (0.001 in).[2] More modern practice for 35 mm productions set the circle of confusion limit at 0.025 mm (0.001 in).

Still photography

For full-frame 35mm still photography, the circle of confusion is usually chosen to be about 1/30 mm. Because the human eye is capable of resolving a spot with diameter about 1/4 mm at 25 cm distance from the viewing eye, and the 35 mm negative needs about an 8X enlargement to make an 8x10 inch print, it is sometimes argued that the criterion should be about 1/32 mm on the 35mm negative, but 1/30 mm is close enough.[4]

For 6x6 cm format enlarged to 8x8 inches and viewed at 25 cm, the enlargement is 3.4X, hence the circle of confusion criterion is about 1/(3.4 x 4) = 0.07 mm.

Similarly, for subminiature photography (for example the Tessina) with a frame format of 14x21mm, 8x12 inches corresponds to 14.5X enlargement, hence circle of confusion limit about 0.017 mm.

Many sources propose CoC limits as a fraction of the film format diagonal, typically 1/1000 in the early twentieth century to 1/1500 more recently. The three formats above at fraction 1/1500 would use 0.029 (about 1/32), 0.056, and 0.017 mm.

Object field methods

Traditional depth-of-field formulas and tables assume equal circles of confusion for near and far objects. Some authors, such as Merklinger (1992),[6] have suggested that distant objects often need to be much sharper to be clearly recognizable, whereas closer objects, being larger on the film, do not need to be so sharp. The loss of detail in distant objects may be particularly noticeable with extreme enlargements. Achieving this additional sharpness in distant objects usually requires focusing beyond the hyperfocal distance, sometimes almost at infinity. For example, if photographing a cityscape with a traffic bollard in the foreground, this approach, termed the object field method by Merklinger, would recommend focusing very close to infinity, and stopping down to make the bollard sharp enough. With this approach, foreground objects cannot always be made perfectly sharp, but the loss of sharpness in near objects may be acceptable if recognizability of distant objects is paramount.

Other authors (Adams 1980, 51) have taken the opposite position, maintaining that slight unsharpness in foreground objects is usually more disturbing than slight unsharpness in distant parts of a scene.

Moritz von Rohr also used an object field method, but unlike Merklinger, he used the conventional criterion of a maximum circle of confusion diameter in the image plane, leading to unequal front and rear depths of field.

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