Focal Length and Digital Cameras

Lenses for 35mm cameras have a different focal
length on a digital camera. The CCDs used in these
cameras are physically smaller than a 35mm
frame. For this reason, lenses used on digital cameras
(with removable lenses) have a longer focal
length. In other words, a 50mm lens will actually
be 80mm on some digital cameras.
If you already own lenses for a traditional film camera
and are shopping for a digital camera that will
work with them, find out what the focal magnifier
is for each camera—it varies depending on the
manufacturer and even the model line. CCD and
CMOS sensors in these cameras vary greatly in size.

Long Focal-Length Lenses

A long focal-length lens provides greater image magnification and a narrower angle
of view than a normal lens. For a 35mm camera, a popular and useful long focal
length is 105mm. For a camera using 2 1/4×2 1/4-inch film, the comparable focal
length is 150mm. For a 4×5-inch view camera, it is about 300mm.
Long lenses are excellent when you cannot or do not want to get close to the subject.
They are also preferred for portraits; most people become self-conscious when a camera
is too close to them, so their expressions are often artificial. A long lens also
avoids the kind of distortion that occurs when shorter lenses used close to a subject
exaggerate the size of whatever is nearest the camera—in a portrait, usually the
nose.
Photographing a person at too close a lens-to-subject distance makes features
nearest the camera appear too large and gives an unnatural-looking dimension
to the head.
There are subtle qualities that can be exploited when you use a long lens. Because a
long lens has less depth of field, objects in the foreground or background can be
photographed out of focus so that the sharply focused subject stands out clearly.
Also, a long lens can be used to create an unusual perspective in which objects seem
to be closer together than they really are
Long lenses do have some disadvantages:
■ Long telephotos are heavier, bulkier, and more expensive than normal focallength
lenses.
■ Because they have relatively shallow depth of field, long lenses must be
focused accurately.
■ Long lenses are difficult to use for handheld shots because they magnify lens
movements as well as subject size.
The shutter speed for a medium-long lens, such as a 105mm lens on a 35mm camera,
should be at least 1/125 second if the camera is handheld. For a 200mm lens,
you will need at least 1/250 second. Otherwise, camera movement might cause blurring.
A tripod or other support is your best protection against blurry photos caused
by camera movement.
A tele-extender or teleconverter contains an optical element
that increases the effective focal length of a
lens. It attaches between the lens and the camera
body and magnifies the image from the lens onto
the film. With these devices, the effective length of
the lens increases, but less light reaches the film. A
converter that doubles the lens, for example, loses
two f-stops of light. Regardless, this tiny addition to
your lens collection is welcome. A tele-extender can
turn a normal telephoto into a super telephoto,
which comes in handy if you’re birding, or in a situation
where you’re extremely far away and can’t
get closer to the subject.

Short Focal-Length Lenses

A short focal-length lens increases the angle of view
and shows more of a scene than a normal lens used
from the same position. A short lens (commonly called a wide-angle lens) is useful
when you are physically prevented (as by the walls of a room) from moving back as
much as would be necessary with a normal lens.
For a 35mm camera, a commonly used short focal length is 28mm. A comparable
lens for a camera using 2 1/4×2 1/4-inch film is 55mm. For a 4×5-inch view camera,
it is 90mm.
Wide-angle lenses have considerable depth of field. A 24mm lens focused on an
object seven feet away and stopped down to f/8 will show everything from four feet
to infinity in sharp focus. Photographers who work in fast-moving situations often
use a moderately wide lens, such as a 35mm lens on a 35mm camera, as their normal
lens. They don’t have to pause to refocus for every shot, because with this type
of lens, so much of a scene is sharp. At the same time, it does not display too much
distortion.
Pictures taken with a wide-angle lens can show both real and apparent distortions.
Genuine aberrations of the lens itself such as curvilinear distortion are inherent in
extremely curved or wide elements made of thick pieces of glass, which are often
used in wide-angle lenses. Although most aberrations can be corrected in a lens of a
moderate angle of view and speed, the wider or faster the lens, the more difficult
and/or expensive that correction becomes.
A wide-angle lens can also show an apparent distortion of perspective, but this is
actually caused by the photographer, not the lens. An object that is close to a lens
(or your eye) appears larger than an object of the same size that is farther away.
Because a wide-angle lens can be focused close to an object, it is easy to get this kind
of exaggerated size relationship. The cure is to learn to see what the camera sees
and either minimize the distortion, or use it intentionally.