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Exploring lens control in photography, including focal distance, focal length (zoom), depth of field, and lens speed

• The focal distance is how far the subject is from the optical center of the lens. The focal distance is controlled by adjusting the distance between the optical center of the lens and the film. Some older cameras moved the film, but most modern camera move an element of the lens.
• The focal length, aka f, of a lens determines the magnification, i.e. the zoom or the size of the image on the film. The focal length is the distance between the optical center of the lens and the focal point (the point where light is focused when the focal point is infinite).
• The depth of field is the range in which object will appear focused. This is a matter of both the focal distance and focal length.
  • Short focal lengths have large depth of field, whereas long focal lengths have small depth of field.
  • Long focal distances have large depth of field, whereas short focal distances have small depth of field.
• The lens speed, aka ƒ, f-number, or f-stop. A ratio of the focal length to the fixed diameter, i.e. ƒ = f/D.

Page Modified: (Hand noted: 2007-10-27 02:53:41Z) (Auto noted: 2010-12-24 22:59:06Z)