How to use Long-Focal-Length Lenses
Probably the most immediately noticeable feature of long-focal-length
lenses is their power of image magnification: the longer the focal
length, the greater the magnification from a given subject-to-camera
distance. This feature comes in handy in a variety of circumstances.
Apparent Near Approach
From a given camera position, using a
long-focal-length lens makes a distant subject appear closer. This
apparent near approach is useful in bird and other wildlife
photography, or in the enlarged-image photography of distant people or
land features. A telephoto or long-focus lens is not the equivalent of
a true close approach because atmospheric interference often makes
itself evident, as do other factors that will be mentioned in following
paragraphs, but it is a substitute often imposed by necessity.
Related to the foregoing, but different
because atmospheric interference should have no significant effect, is
the making of "distant" close-ups of small subjects, taking advantage
of the long working distance of long-focal-length lenses. Thus, it is
easier to get good close-seeming photos from the sidelines of sporting
events or hazardous occurrences. It is also easier to get good
photographs of such timid creatures as butterflies, dragonflies,
snakes, and frogs; or of such dangerous animals as poisonous reptiles
and small fierce mammals.
The use of long-focal-length lenses for
perspective adjustment is more subtle, and requires understanding of
the optical circumstances. First, at a given f-stop there will be less
depth of field when image magnification is increased. But if the image
magnification on the film and the f-number of any two lenses are equal,
the depth of field will be the same in any given situation.
Second, and more important, keeping image magnification unchanged
requires different camera-to-subject distances for lenses with
different focal lengths. Therefore, there are relative size differences
between foreground and background subjects. The longer the focal length
of the lens, the less the scale difference between near and far
subjects, because the working distance has been increased.
This perspective effect-changes in proportional differences between the
foreground and background image magnifications effected by changing
focal lengths-can be put to various uses. Say, for instance, you want
to photograph a building with a sculpture in front of it.
You might want to emphasize the building, or the sculpture, and your
choice determines your method.
Using a lens of normal focal length, you could make two pictures, each
emphasizing one feature-one image dominated by the building, the other
by the sculpture. But in the latter image, only a portion of the
building would appear in the background. If you wanted to show the
whole of both in both pictures, and also wanted the sculpture to have a
different prominence in each, you would need only to use different
lenses. If the image of the building were kept the same size in both
pictures. the picture made with a short-focal-length lens (a wide-angle
lens, say) would have a larger image of the sculpture than would a
picture made with a long-focal-length or telephoto lens. The greater
the difference in the two focal lengths, the greater the difference in
the size of the sculpture in the two resulting pictures, if the image
magnification of the building is kept constant.
As you may recall, the normal lens is
supposed to approximate human vision. It would seem to be the perfect
lens, then, for taking portraits, for the photographic image of a
person will then duplicate our own vision of the person.
For film photography however, this reasonable assumption is not in fact
correct for any film size smaller than 5 x 7 inches. To preserve the
kind of perspective that is familiar to most of us (because of our
culture's normal social approach distance), and that will therefore
avoid undue apparent distortion of the sitter's features, a certain
distance must be kept between the camera and the subject. This is
especially true when the 35 mm camera is being used for a
head-and-shoulders portrait. Filling the viewfinder frame with this
part of the anatomy when using the standard 50 mm lens will bring you
too close for good perspective, and so close that the sitter will feel
a sense of encroachment. A much-preferred focal length for this use
would be a 105 mm lens. It doubles the camera-to-subject distance, and
makes everyone feel more comfortable.
The larger the film format, the longer the focal length of the normal
lens, and so the greater the working distance. Thus, distortion and
social distance are both less a problem. With a 21/4 x 21/4 inch
twin-lens reflex camera, the lens has a normal focal length of about 85
mm: usually no lens change is possible. Therefore, it would be best to
stand back a little and compose the head and shoulders in about a 1 x
11/2 inch area of the viewing screen, to avoid close-approach
distortion. With a 4 x 5 inch (9 x 12 cm) format, a lens of about 180
mm works out well for portraits. Beyond that film size, normal lenses
are quite acceptable for head-and-shoulders portraits.