Light from distant subjects will be in sharp focus in a plane a fixed distance behind the lens. The focal length of a lens is the distance between the lens and the plane of sharp focus. Also, this is where the film would be in order to record a sharply focussed photograph.
Choosing the correct focal length can make or break a photograph. For example, wide-angle lenses (those with short focal lengths such as 20mm), distort near objects, especially at the edge of the photograph. Therefore, you would probably not want to use a wide-angle lens for a close-up portrait of uncle Bob (unless you really want his nose to look unusually large and distorted!). In a landscape photograph, a wide angle lense can exaggerate the size of a foreground object, creating visual impact. Long focal length lens, or telephoto lenses, magnify images at a great distance and help to isolate smaller elements of complex scenes. Very long focal length lenses have to ability to make a scene look "compressed" or all on one plane. In addition, these lenses have very limited depth of field at large aperatures. Using a long lens with a large aperature setting can help isolate a subject by throwing all but the subject out of focus -- eliminating distracting foregrounds or backgrounds.Related Tips:
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