Focal Length of Camera Lenses

Focal Length of Camera Lenses

The lens is the camera's primary image-forming device. Light rays reflected from most subjects travel randomly in different directions. Thus, light passing through the open shutter of a camera with its lens removed would strike the film without forming an image. A lens forms an image by gathering light rays and focusing them on the camera sensor or film located the proper distance behind it. All lenses have certain physical or optical characteristics that are of practical concern to photographers. The most important characteristic is focal length.

Nearly all camera lenses are made up of more than one element, or piece of glass or air space between two pieces of glass. Different types of glass having various shapes and optical qualities are played off one another to produce lenses with various desirable characteristics. Nevertheless, for ease of definition, we will first assume that the lens consists of one element (which, in fact, is all many early lenses had). For a thin, single-element lens, then, of symmetrical configuration, the focal length is the distance between the center of the lens and the image projected by that lens when the lens is focused at infinity (or, practically speaking, on a very distant object).

Because the point from which the focal length is measured (the rear nodal point) can be placed at any of a variety of locations along the optical axis, and because we laymen can become easily lost in a sea of technical terms, we will refer, for the sake of convenience rather than accuracy, to the rear nodal point as the "optical center" of the lens.

Lenses are divided into three basic types, depending - the relation between the focal length and the film size (the characterization of lenses started since the era of film photography, thus the relation is all base on film size):

  1. Normal.

  2. Those of shorter than normal focal length, which are called wide-angle.

  3. Those of longer than normal focal length, which are called telephoto or long-focus, depending on their optical design.

For any given film size a normal lens has a focal length more or less equal to the length of a diagonal line across the usable film format. For example, the diagonal across a 24 x 36 mm frame of 35 mm film is approximately 43mm. In practice there is quite a bit of leeway, and lenses ranging from about 40 to 58 mm in focal length are generally considered normal; most normal lenses are about 50 mm, however.

Lenses of normal focal length have angles of view that approximate that of the useful vision in the human eye. Although the eye can form an image close to 180 degrees in angular acceptance, only the center of that image gives clear viewing; the peripheral areas generally serve only to detect motion. What we see most clearly, and what we actually pay attention to most closely, is what falls within an acceptance angle of about 45 to 55 degrees.

Wide-angle lenses have focal lengths shorter than normal - for example, in the 35 mm format, focal lengths of 35 mm or shorter are wide-angle; long-focal-length and telephoto lenses, on the other hand, have focal lengths longer than normal. Long-focal length lenses begin for the 35 mm format at about 70 mm. A lens of conventional, approximately symmetrical construction is called a long-focus lens. A true telephoto lens is one whose design places the optical center of the lens forward of the physical center of the lens; in many cases it is forward of the entire lens. The result is a shorter, more compact lens mount.

A similar situation holds with respect to many wide-angle lenses, especially those designed for single-lens reflex cameras. Such lenses are in fact retro-focus wide-angle lenses: their optical center is behind the physical center of the lens. This design is necessary to leave room behind the lenses for the moving mirror.

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