depth of field

Definition: Measure of zone or distance over which any object in front of lens will appear acceptably sharp: it lies in front of and behind the plane of best focus. * Depth of field is due to geometry of image formation plus inability of eye to distinguish a circle blur from a point in the image: it cannot be affected by lens design. * It is proportional to the circle of confusion, to f/number (i.e. increases with smaller apertures) and is inversely proportional to the square of focal length (i.e. depth of field decreases very rapidly as focal length increases) and is proportional to square of focus distance (i.e. depth of field decreases very rapidly at close object distances). * The other factor that affects depth of field is the angle of the plane of best focus: usually this is assumed to be at right angles to the optical axis but can be angled in tilt lenses or tilt movements of large-format cameras. * Do not confuse with depth of focus.
Deeply digital
Depth of field applies to digital images just as much as to analogue: the difference is that digital pre-processing to increase sharpness can give the appearance of increasing depth of field. Depth of field increases rapidly with smaller sensors because of the use of shorter focal length lenses to obtain the same magnification compared to images from larger sensors.
LINKS:  - a plain-language but quite technical and balanced explanation.  - depth of field calculator .

Related Terms: blur circle

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