PERSPECTIVE AND DEPTH

One of the paradoxes of vision is that while
the image projected onto the retina obeys
the laws of optics and shows distant objects
smaller than nearer ones, the brain, given
sufficient clues, knows their proper size. And, in
one view, the brain accepts both realities—distant
objects that are small and full-scale at the same
time. The same thing happens with linear
perspective. The parallel sides of a road stretching
away from us converge optically but at the same
time are perceived as straight and parallel. The
explanation for this is known as “constancy
scaling” or “scale constancy,” a little-understood
perceptual mechanism that allows the mind to
resolve the inconsistencies of depth. Its impact on
photography is that the recorded image is purely
optical, so that distant objects appear only small,
and parallel lines do converge. As in painting,
photography has to pursue various strategies to
enhance or reduce the sense of depth, and images
work within their own frame of reference, not
that of normal perception.
Photography’s constant relationship with
real scenes makes the sense of depth in a picture
always important, and this in turn influences
the realism of the photograph. In its broadest
sense, perspective is the appearance of objects in
space, and their relationships to each other and
the viewer. More usually, in photography it is
used to describe the intensity of the impression
of depth. The various types of perspective and
other depth controls will be described in a
moment, but before this we ought to consider
how to use them, and why. Given the ability
to make a difference to the perspective, under
what conditions will it help the photograph to
enhance, or to diminish, the sense of depth?
A heightened sense of depth through strong
perspective tends to improve the viewer’s sense
of being there in front of a real scene. It makes
more of the representational qualities of the
subject, and less of the graphic structure.
The following types of perspective contain
the main variables that affect our sense of depth
in a photograph. Which ones dominate depends
on the situation, as does the influence that the
photographer has over them.
LINEAR PERSPECTIVE
In two-dimensional imagery, this is, overall, the
most prominent type of perspective effect. Linear
perspective is characterized by converging lines.
These lines are, in most scenes, actually parallel,
like the edges of a road and the top and bottom
of a wall, but if they recede from the camera,
they appear to converge toward one or more
vanishing points. If they continue in the image
for a sufficient distance, they do actually meet at
a real point. If the camera is level, and the view is
a landscape, the horizontal lines will converge on
the horizon. If the camera is pointed upward, the
vertical lines, such as the sides of a building, will
converge toward some unspecified part of the sky;
visually, this is more difficult for most people to
accept as a normal image.
In the process of convergence, all or most
of the lines become diagonal, and this, as we’ll
see on pages 76-77, induces visual tension and
a sense of movement. The movement itself adds
to the perception of depth, along lines that
carry the eye into and out of the scene. By
association, therefore, diagonal lines of all kinds
contain a suggestion of depth, and this includes
shadows which, if seen obliquely, can appear as
lines. So a direct sun, particularly if low in the
sky, will enhance perspective if the shadows it
casts fall diagonally. Viewpoint determines the
degree of convergence, and the more acute the
angle of view to the surface, the greater this is—
at least until the camera is close to ground level,
at which point the convergence becomes extreme
enough to disappear.
The focal length of lens is another important
factor in linear perspective. Of two lenses aimed
appropriate place in the scene, it helps to establish
perspective. Also associated with diminishing
perspective are placement (things in the lower
part of the picture are, through familiarity,
assumed to be in the foreground) and overlap
(if the outline of one object overlaps another,
it is assumed to be the one in front).
directly towards the vanishing point of a scene,
the wide-angle lens will show more of the
diagonals in the foreground, and these will tend
to dominate the structure of the image more.
Hence, wide-angle lenses have a propensity to
enhance linear perspective, while telephoto lenses
tend to flatten it.
DIMINISHING PERSPECTIVE
This is related to linear perspective, and is in
fact a form of it. Imagine a row of identical
trees lining a road. A view along the road would
produce the familiar convergence in the line
of trees, but individually they will appear to
be successively smaller. This is diminishing
perspective, and works most effectively with
identical or similar objects at different distances.
For similar reasons, anything of recognizable
size will give a standard of scale; in the
appropriate place in the scene, it helps to establish
perspective. Also associated with diminishing
perspective are placement (things in the lower
part of the picture are, through familiarity,
assumed to be in the foreground) and overlap
(if the outline of one object overlaps another,
it is assumed to be the one in front).
AERIAL PERSPECTIVE
Atmospheric haze acts as a filter, reducing the
contrast in distant parts of a scene and lightening
their tone. Our familiarity with this effect (pale
horizons, for example), enables our eyes to use
it as a clue to depth. Hazy, misty scenes appear
deeper than they really are because of their strong
aerial perspective. It can be enhanced by using
backlighting, as in the example below, and by
not using filters (such as those designed to cut
ultraviolet radiation) that reduce haze. Telephoto
lenses tend to show more aerial perspective than
wide-angle lenses if used on different subjects,
because they show less of nearby things that have
little haze between them and the camera. Favoring
the blue channel when using channel mixing to
convert an RGB digital image to black and white
also accentuates the effect.
TONAL PERSPECTIVE
Apart from the lightening effect that haze has on
distant things, light tones appear to advance and
dark tones recede. So, a light object against a dark
background will normally stand forward, with a
strong sense of depth. This can be controlled by
placing subjects carefully, or by lighting. Doing
the reverse, as we saw on pages 46-47, creates a
figure-ground ambiguity.
COLOR PERSPECTIVE
Warm colors tend to advance perceptually and
cool colors recede. Other factors apart, therefore,
a red or orange subject against a green or blue
background will have a sense of depth for purely
optical reasons. Again, appropriate positioning
can be used as a control. The more intense the
colors, the stronger the effect, but if there is a
difference in intensity, it should be in favour of
the foreground.