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):
Those of shorter than normal focal
length, which are called wide-angle.
Those of longer than normal focal length,
which are called telephoto or long-focus, depending on their optical
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.