Optical Properties related to Focal
Length
-
Image magnification refers to the size of
the image that a lens transmits to camera sensor or film, relative to
the actual size of the subject. Focal length and image magnification
are directly related:
- The longer the focal length, the greater the magnification. That is,
if the camera-to-subject distance is kept constant, doubling the focal
length doubles the image magnification.
-
Angle of view is also related to both
image size and focal length. Angle of view establishes the area of the
scene before the camera that the lens fits within the frame of the
negative. Focal length and angle of view are inversely related:
- The shorter the length, the greater the angle of view. Thus, if
camera-to-subject distance is kept constant, doubling the focal length
halves the angle of view, and vice versa.
-
Image perspective is another optical
property that is related to lens focal length, but in a more complex
way that can lead to misunderstandings unless clearly explained.
Perspective refers to the apparent relative size and spacing of objects
at different distances from a viewer. Various focal lengths of lenses
are often thought of as having their own characteristic perspective
effects upon the image: in the "telephoto effect," for instance, there
is a foreshortening of the scene-objects at varying distances will
appear abnormally compressed together, when compared to how the eye
normally views them. The opposite occurs in the "wide-angle
effect"-distant and nearby objects will appear to be separated to an
abnormal degree. Another way of looking at it is that in telephoto
images identical objects at various distances will appear not greatly
different in size, but the same objects at the same varying distances
will often appear substantially different in size when a lens of
shorter focal length is used.
These deviations from normal image perspective are not, however, an
inherent function of the lens. They are instead a function of the use
of the various lenses, being actually related to camera-to-subject
distance: wide-angle lenses are ordinarily used relatively close to a
subject, whereas telephoto lenses are ordinarily used at relatively
great distances. As the distance between lens and subject is varied,
image magnification varies; nearer subjects thus appear more enlarged
than more distant ones. Since image magnification is in direct
proportion to the lens focal length, long-focal-length, normal, and
wide-angle lenses would have to be at different distances from a given
subject to produce images of the same size. You can prove that the
varying perspective effect is not due to focal length by making two
pictures of the same subject at the same lens-to-subject distance. The
first photograph is made with a short-focal-length lens, the second one
with a long-focal-length lens. Compare the end result and enlarge that
portion of the short-lens image that coincides with the other, until it
matches the magnification of the long-lens image. The image perspective
will be similar.
|