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the plane perpendicular to the line of sight. The
only way in which the observer can judge of the
relative positions of the objects in the picture in
respect to depth is by means of perspective. If the
perspective be.faulty his interpretation of distances
will be incorrect. Possibly the result will in no
wise diminish his satisfaction with the picture; on
the other hand, it might seriously impair the efiect
the director and cameraman have striven to produce.
This subject is discussed in a paper by A. C.
Hardy and R. W. Conant, present last week at the
meeting of the S. M. P. E. I am introducing the
subject briefly in this paper for many of you who
were not present at the time the paper referred to
was read and in order to add one or two remarks.
In monocular vision the most important indication
of the relative distances from the eye of the
various objects within the field of view is the apparent
size of those objects. Our interpretation of distances
from a consideration of apparent sizes is
based on experience and involves an assumption that
we know something of actual dimensions of the objects
involved. If the actual dimensions of some of
the objects are unusual our judgment of distances
will be incorrect. For example' suppose two automobiles
were visible in an otherwise absolutely blank
field so arranged that no part of the one obscured
any part of the other. We would immediately locate
them in space relative to each other on the basis of
their equality or inequality of apparent size, since
we know from experience that regardless of difierences
in wheel base, the width and height of automobiles
does not difier greatly between different
models. If. however, one of the cars was replaced
by a perfectly proportional miniature our judgment
would be violently upset. The difierence in apparent
size of.the two cars will depend on the distance
o{ the observer from the nearer car as well as upon
their separation, the difierence between them diminishing
as his distance from them increases, so that
his judgment as to the separation of the cars one
from another depends upon some prior judgment as
to his distance from one or the other of them. This
judgment can be formed even in otherwise vacant
field of view on the basis of the apparent size of that
car compared to the extent of his whole field of
view. If this were the first time he had seen an
automobile he would be utterly unable to form any
opinion of its distance because he would have no
idea of its dimensions. If the unknown automobile
appeared in a field containing familiar objects he
would at once be able to form an estimate of its
distance and of its dimensions.
Motion picture sets usually contain familiar objects.
The observer viewing the picture on the
screen estimates his distance from the scene {rom the
appearance of the dominating object within the field
and locates the other objects within the field on this
basis, combined with a consideration of their apparent
relative sizes,
Perspective is a matter of relative apparent sizes.
The expressions, true or faulty perspective, are
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meaningless unless they assume a definite point of
view for the observer. Assuming the lens is free
from distortion, the perspective of a negative will
always be true if viewed from a distance equal to
the distance from the second nodal point of the lens
to the plate or film when the exposure was made'
VieweJ from any other distance, the perspective of
the negative will be faulty. It cannot be said,
there{oie, that perspective depends upon the focal
length of the lens without qualification in the {orm
o{ a specification of the viewing distance. Similarly
the perspective of the projected picture will be true
or false,- according to the relation between the focal
length of the lens used for projection and the viewing
distance of the observer. If the observer stands
atlhe projection machine' true perspective demands
that piojettion be accomplished with a lens whose
focal length is the same as that of the taking lens'
In motion picture practice this is very rarely done,
and as a consequence the perspective of the projected
picture viewed from the projection booth is always
false. This is usually but not always apparent. For
any other distance from the screen the focal length
of the projection lens necessary to give true perspective
can be computed by multiplying the total projection
distance by the focal length of the camera
lens and dividing by the distance from the observer
to the screen. This relation was brought out by
Hardy and Conant, who also point out that for the
usual 2-inch lens used in cinematography the projection
lens should have a focal length of 4 inches
in order that correct perspective be maintained for
an observer seated half way from screen to projector,
leading therefore to best average perspective for
the entire house. The general use of a 4-inch projection
lens would lead to larger screens than are
common. Probably the average projection lens is
nearer 6 inches in the larger and better theatres, so
that better average perspective in the projected picture
would follow the use of a focal length of 3
inches in the camera.
Probably the usual result of false perspective in
the projected picture is a slight deterioration of composition
without the observer realizing that anything
is wrong, but in the case of any well-known object
filling a good share of the field of view some very
unpleasing results follow. To illustrate this I am
using some slides, prepared by Hardy and Conant
and used to illustrate their paper, which demonstrate
how unsatisfactory results can be.
In order to prevent any misunderstanding it remains
to be pointed out that this f alse perspective of
which I have been speaking is not related to the distorted
perspective one sees when he views the picture
on the screen from a seat at the side of the theatre.
One's perception of depth is unaffected by his lateral
displacement with reference to the line of projection,
but one does find the apparent width o{ all
objects in the picture to be afiected thereby while
their heights remain constant. In so {ar as the observer's
perception of depth is concerned only his
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