Perspective and the Normal Lens
People have asked about "normal"
lenses. I found a lot of opinions and a few concrete answers. Here are a few
"expert" answers plus some of my own biased opinions.
We can only see the "correct"
perspective of a print, movie or video if the image is viewed exactly reproduced
to give the same perspective as seen by the camera. That is if the camera shoots
a scene with a certain lens, it must be viewed with an enlargement the same
ratio of image size to distance of viewer as it was photographed. If F=camera
focal length, I=image height on film, O=image height as projected or printed,
then the correct distance to be viewed is D=viewing distance.
(F x O) / I = D
We rarely get a fifteenth row center
seat in a theater, view a 24-inch TV set from 32 inches or look at a photograph
from the same relative distance that it was shot at. Viewing conditions vary.
If a photographer keeps changing his focal lengths, it is very difficult to
view his different images with the "correct" perspective. Early in
photography there were limitations in the design of lens focal lengths. Cameras
only came with "normal" lenses.
Designing wide angle and tele lenses
was difficult. Early B movie cameramen often went out with only 2 lenses, a
35 mm for "long" (wider) shots and a 50 mm for close-ups. If you sat
in the right place in the theater, you saw a "correct perspective"
most of the time. Any extreme angle seemed incorrect to the early audience.
Today we have repeatedly seen images shot by extreme wide angle and tele lenses
and our brains have learned to perceive them all as "normal". Today
the "normal lens" is no longer relevant. Notice how quickly the screen
image from the last available seat in a theater, front row side corner, soon
looks normal to your brain long before you neck gets stiff.
Early in photography it was decided
that a focal length equal to the diagonal of the image formed on the film was
"normal". That gives us a 50-mm lens for 35-mm stills, 75-mm lens
for 2-1/4 square, 150 mm for 4 x 5 etc. That was well and good if the same size
print shot with a normal lens was always viewed at the same distance. But viewing
a wide-angle shot close or a tele shot at a distance can also provide a "correct
Some aspects of vision should be
considered. Our brain "sees" a very wide angle of view, over 120 degrees.
But our brain can only concentrate at one time on a very small image angle of
1.7 degrees on the fovea. Our brain creates a composite image by scanning scenes
with images from the fovea. Concentrate looking at one word and notice the rest
of the page becomes blurred. The rest only becomes in focus when we change our
attention. Our brain creates the images that we "see". See Vision
Cinerama was shot with three cameras
and was very real if seen from a center front seat. Today IMAX is the same.
They provided a very wide angle of view to the audience much like our field
of vision. In this case the audience scans the scene for different areas of
concentration, similar to what we do in real life scenes. In a theater, looking
at TV or a printed picture we view the images within a limited frame and the
image is contained within that frame.
Like much of our "seeing",
a lot of interpretation goes on in our brain to make sense of the images that
we "see". Native people having never seen pictures before are confused
by 2 dimensional images. They haven't learned yet to use the cues of perspective
in a 2 dimensional image. Middle age artists discovered reproducing perspective
only after years of study and experiment.
Some photo writers said that if we
see the same size image in the viewfinder as seen by eye without the camera,
that the lens was "normal". They were ignoring differences in viewfinder
optics that give different sizes of image to the eye. Camera manufacturers adjust
the size of the image to fit a compact economical package on a camera. I personally
don't like the larger images in the viewfinder that some manufacturers brag
about. For composition I find that I must scan a larger image more than to see
it as a whole image than if it is smaller. It's like looking at a picture on
the wall standing too close or sitting in the front row of a theater.
Most of us today use whatever lens
will record what we want to include in the frame or choose a longer lenses to
soften a background or a lens that will make the star look good.
© Copyright 1999-2004 Ron Dexter.
All Rights Reserved.