The Physics
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Focal Length of a Camera Lens

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Giancoli, Douglas C. Physics Principles with Applications. Englewood Cliffs, New Jersey: Prentice Hall, 1980. "Camera lenses are categorized into normal, telephoto, and wide angle, according to focal length and film size. A normal lens is one that covers the film with a field of view that corresponds approximately to that of normal vision. A normal lens for 35-mm film has a focal length in the vicinity of 50 mm. A telephoto lens, as its name implies, acts like a telescope to magnify images… Thus a 200-mm telephoto lens for use with a 35-mm camera gives a 4 × magnification over the normal 50-mm lens." 50 mm
"Camera." The Encyclopedia Americana. International Edition. Volume 5. Danbury, CT: Grolier, 2001. 266. "standard lens which produces a 24 × 36 mm (1 × 1.5 inch) negative — has a 50 mm focal length. The normal lens for a camera using 4 × 5 inch (100 × 125 mm) film is 135 mm…. Any lens with a focal length that is shorter than is normal for a given film size is considered a wide angle lens. Thus, a 35 mm lens for a 35 mm camera would be a wide-angle as would a 90 mm lens for a 4 × 5 inch camera…. Zoom lenses usually offer a range of focal lengths of about two or three to one. Typical ranges are 36 to 85 mm (1.5 to 3.33 inches)" 50 mm
Morton, Timothy Dwight. "What is focal length?" Photographic Society of America Journal. Volume 65 Issue 10, October 1999, 20-21. [see below] 20–800 mm
Langsford, Michael. The Single Lens Reflex Handbook: All about you single lens reflex camera and how to use it: a complete basic guide. New York: Knopf. 1980 "On a single lens reflex (SLR) camera… Standard lenses are 50/55 mm, and fastest. Wide angles range from 18 mm to 35 mm, telephotos from 80 mm to 500 mm, or more." 50–55 mm
18–35 mm
(wide angle)
80–500 mm
Alesse, Craig. Basic 35 mm Photo Guide for beginning photographers. New York: Amherst Media, 1989. "The most commonly purchased lens is a normal lens. Its focal length is about 50 mm. A normal lens is popular because it has the same perspective as the human eye." 50 mm
Lens Type Film Format
35mm 6x4cm 6x6cm 6x7cm 5x4in
Ultra-wide 20mm 35mm 40mm 45mm 65mm
Wide-angle 28mm 45mm 50mm 55mm 90mm
Standard 50mm 75mm 80mm 90mm 150mm
Short-telephoto 90mm 140mm 150mm 180mm 270mm
Medium-telephoto 200mm 300mm 350mm 420mm 520mm
Long-telephoto 300mm 450mm 500mm 600mm 800mm

The focal length of a camera lens is the distance between the center of the lens and the film when an in-focus image is formed of an object very far away. The focal length of a camera lens is displayed on the barrel of the lens along with the measurement of the largest aperture and the maker. The focal length of a lens establishes the field of view of the camera. The shorter the focal length is, the larger the field of view. The magnification factor of the picture of an object and the object's actual size can be found by dividing the focal length of the camera lens used by the focal length of a standard lens.

There are three general types of lenses for a camera: normal or standard, wide angle, and telephoto. The focal length of a normal lens for a 35 mm SLR camera is approximately 50 mm. A standard or normal lens produces a picture with a perspective similar to the human eye. The focal length of a wide angle lens is any measurement less than 50 mm, but is typically 28 mm. A wide angle lens makes things appear smaller and distorts the view if the object is too close to the camera. The focal length of a telephoto lens ranges from 60 to 1000 mm. A telephoto lens magnifies the subject while at the same time narrowing the field of vision. These lenses create an image that looks flatter than that produced by a standard lens.

Miriam Janove -- 2002