Focal length
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The focal length of an optical system is a measure of how strongly the system converges or diverges light; it has units of length, and for an idealized thin lens is equal to the distance between the l
Sketch of human eye showing rear focal length f′ and EFL
28 mm lens
50 mm lens
70 mm lens
Depth of field
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The depth of field is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image captured with a camera. See also the closely related depth of focus.
Focal length
…approximately add. Thus, a thin 2.0-dioptre lens placed close to a thin 0.5-dioptre lens yields almost the same focal length as a single 2.5-dioptre lens. Depth of field Dioptre…
A macro photograph showing the defocused effect of a shallow depth of field on a tilted page of text
This photo was taken with an aperture of f/22, creating a mostly in-focus background.
The same scene as above with an aperture of f/1.8. Notice how much blurrier the background appears in this photo.
Aperture = f/1.4. DOF = 0.8 cm