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Concept
Depth of field
Summary
The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera.
For cameras that can only focus on one object distance at a time, depth of field is the distance between the nearest and the farthest objects that are in acceptably sharp focus. "Acceptably sharp focus" is defined using a property called the "circle of confusion".
The depth of field can be determined by focal length, distance to subject, the acceptable circle of confusion size, and aperture. Limitations of depth of field can sometimes be overcome with various techniques and equipment. The approximate depth of field can be given by:
for a given circle of confusion (c), focal length (f), f-number (N), and distance to subject (u).
As distance or the size of the acceptable circle of confusion increases, the depth of field increases; however, increasing the size of the aperture or increasing the focal length reduces the depth of field. Depth of field changes linearly with F-number and circle of confusion, but changes in proportion to the square of the focal length and the distance to the subject. As a result, photos taken at extremely close range have a proportionally much smaller depth of field.
Sensor size affects DOF in counterintuitive ways. Because the circle of confusion is directly tied to the sensor size, decreasing the size of the sensor while holding focal length and aperture constant will decrease the depth of field (by the crop factor). The resulting image however will have a different field of view. If the focal length is altered to maintain the field of view, the change in focal length will counter the decrease of DOF from the smaller sensor and increase the depth of field (also by the crop factor).
For a given subject framing and camera position, the DOF is controlled by the lens aperture diameter, which is usually specified as the f-number (the ratio of lens focal length to aperture diameter).
Official source
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