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.
Famous quotes containing the words length and/or shorter:
“Whoever aims publicly at great things and at length perceives secretly that he is too weak to achieve them, has usually also insufficient strength to renounce his aims publicly, and then inevitably becomes a hypocrite.”
—Friedrich Nietzsche (18441900)
“I would rather be old for a shorter time than be old before my time.”
—Michel de Montaigne (15331592)