Focal Length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.

In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with a wider angle of view. On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection.

Read more about Focal Length:  Thin Lens Approximation, General Optical Systems, In Photography

Famous quotes containing the word length:

    And my spirit is grown to a lordly great compass within,
    That the length and the breadth and the sweep of the marshes of
    Glynn
    Will work me no fear like the fear they have wrought me of yore
    When length was failure, and when breadth was but bitterness sore,
    And when terror and shrinking and dreary unnamable pain
    Drew over me out of the merciless miles of the plain,—
    Oh, now, unafraid, I am fain to face
    The vast sweet visage of space.
    Sidney Lanier (1842–1881)