The Telephoto Lens
Thetelephotolens has a focal length that is longerthan that of the image sensor’s diagonal. (They are also called longlenses.) In 35mm
terms, telephoto lenses are those with focal lengths that are longer than 65mm, but for many digital cameras the minimum focal length to be
a telephoto shrinks to 50mm. It all depends on the size of the image sensor. A camera with an image sensor whose diagonal measure is half
that of 35mm film requires only a 300mm lens to get the telephoto power of a 600mm lens on a film camera.
Not surprisingly, the design of a telephoto lens depends on lens elements that are the opposite of those in a wide-angle. They employ
negative, or diverging, lenses that spread the light rays out of the center axis of the lens. The result is that the image that entered the lens
covers a wider area by the time it reaches the image sensor. The sensor records a smaller center area of the total image with the effect that
the area is enlarged in the photograph.
Awide-anglelens is one
that has a focal length
shorter than the diago-
nal size of the film or
image sensor. In the
35mm film world,
any lens with a focal
length of 35mm or
less is considered
wide-angle. Wide-
angle lenses are also
calledshort lenses.
Looking through the viewfinder of a camera
equipped with a wide-angle lens shows
you more than you would normally
see with your peripheral vision
if you simply stared straight
ahead. It includes areas
you would ordinarily
have to turn your
head to see. (The
extreme is a lens
with a focal length of
15mm or less. These
are called fisheye
lenses, so named
because of their bul-
bous shape that takes in a 180° angle of view—
essentially everything that’s in front of the
camera.) The design of a wide-angle
lens emphasizes converging
positiveelements that
squeeze together the
rays of light that pass
through them. By the
time the rays reach
the image sensor,
individual objects in
the scene are small,
but a wider expanse
now fits on the sensor.The Normal Lens
In 35mm photography, a lens with a focal
length of 50–55mm is considered normal.
This is based on a rule of thumb that a nor-
mal lens’s focal length is equal to the diago-
nal distance of the image formed on the
film—or today, on the image sensor. For
cameras with different image proportions,
normal lenses have different focal lengths.
For a camera using 2 1/4'' square film, for
example, the normal lens has a 75mm focal
length. This is important in evaluating digital
cameras, which have no standard image
sensor size. (See Chapter 5, “How
Technology Transforms Cameras and
Lenses.”)
When you look through the viewfinder of
a camera with a normal lens, your subject
looks about the same size, roughly, as it
does to your naked eye. The lens elements
are designed to focus the rays of light
passing through them while still forcing them
off their straightforward paths as little as
possible.
CHAPTER 4 HOW LIGHT PLAYS TRICKS ON PERCEPTION^53
The Wide-Angle
Lens