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:

    Punishment followed on a grand scale. For ten days, an unconscionable length of time, my father blessed the palms of his child’s outstretched, four-year-old hands with a sharp switch. Seven strokes a day on each hand; that makes one hundred forty strokes and then some. This put an end to the child’s innocence.
    Christoph Meckel (20th century)