STEREOSCOPIC AERIAL PHOTOGRAPHY
One of the most striking and valuable developments in aerial photography
has been the use of stereoscopic views. Pairs of pictures, taken with a
considerable separation in their points of view and studied later by the
aid of the stereoscope, show an elevation and a solidity which are
entirely wanting in the ordinary flat aerial vista. Often, indeed, these
attributes are essential for detecting and recognizing the nature of
objects seen from above. Stereoscopic aerial photography has been justly
termed “the worst foe of camouflage.”
=Principles of Stereoscopic Vision.=—The ability to see objects in
relief is confined solely to man and to a few of the higher animals in
whom the eyes are placed side by side. When the eyes are so placed they
both see, to a large extent, the same objects in their fields of view.
Owing to the separation of the eyes the actual appearance of all objects
not too far away is different, and it is by the interpretation of these
differences that the brain gets the sensation of relief. Thus in Fig.
149 the two eyes are shown diagrammatically as looking at a cube. The
right eye sees around on the right-hand face of the cube, the left eye
on the left-hand face of the cube. The two aspects which are fused and
interpreted by the brain are shown in the lower diagram.
Stereoscopic views or stereograms, made either by photography or, in the
early days, by careful drawing, consist of pairs of pictures made of the
same object from two different points. For ordinary stereoscopic work
these points are separated by the distance between the eyes,
approximately 65 millimeters or 2¾ inches. These two pictures are then
so viewed that each eye receives its appropriate image from the proper
direction, whereupon the object delineated stands out in relief.
[Illustration:
FIG. 149.—The principle of stereoscopic vision.]
Fusion of the two elements of the stereoscopic picture can take place
without the assistance of any instrument, if the eyes are properly
directed and focussed, but this comes only with practice. Holding the
stereogram well away from the face the eyes are directed to a distant
object above and beyond, in order to diverge the axes. Then without
converging, the eyes are dropped to the picture, which should spring
into relief. It is necessary in moving the eyes from the distant object
to the near stereogram to alter their focus somewhat, depending on how
near the stereogram is held; and the success of the attempt to fuse the
images depends on the observer's ability to maintain the eyes diverged
for a distant object while focussing for a near one. Near-sighted people
(on taking off their glasses) fuse the stereoscopic images quite easily,
since their eyes do not focus on distant objects even when diverged for
them. Transparencies are easier to fuse than paper prints, but in any
case where a stereoscope is not used the separation of image centers
should not be more than that of the eyes.
[Illustration:
FIG. 150.—Common form of prism stereoscope.]
=Stereoscopes.=—The easier and more usual method of fusing the
stereoscopic images is by a _stereoscope_. The simplest form consists
merely of two convex lenses, one for each eye, their centers separated
by a distance somewhat greater than that between the eyes. Their
function is to bring the stereogram to focus, and, by the prismatic
action of the edges of the lenses, to converge the lines of sight which
pass through the centers of the two pictures to a point in space in
front of the observer. The two lenses should be mounted so as to provide
for the adjustment of their separation to fit different eyes and print
spacings. The most common form of stereoscope is that designed by
Holmes, for viewing paper print stereograms (Fig. 150). It has prismatic
lenses of an appropriate angle to converge pictures whose centers are
three inches apart, instead of the lesser distance appropriate to
stereograms intended for fusing without an instrument. No adjustment is
provided for varying the lens separation, but the print can be moved to
and fro for focussing.
[Illustration:
FIG. 151.—Box stereoscope.]
[Illustration:
FIG. 152.—Diagram of mirror stereoscope.]
Another form of stereoscope, one of the first produced, is the mirror
stereoscope (Fig. 152), now used extensively for viewing stereo X-ray
pictures. It consists of two vertical mirrors at right angles to each
other, with their edge of contact between the eyes. The two prints to be
studied are placed to right and left, an arrangement that permits the
use of prints of any size. The convergence point is controlled by the
angle between the mirrors. The Pellin stereoscope (Fig. 153) utilizes
two pairs of mirrors in a way to permit the use of large prints. The
prints are, however, placed side by side on a horizontal viewing table,
which avoids certain difficulties of illumination met with in the
simpler mirror form. The box form of stereoscope (Fig. 151) using either
prisms or simple convex lenses, is particularly adapted for viewing
transparencies, although the insertion of a door at the top provides
illumination for paper prints. The Schweissguth design (Fig. 154) is
intended primarily as an aid to selecting the portions of the prints to
be cut out for mounting. The platform on which the pictures rest is
composed of two long rectangular blocks, on which are plates of glass
raised sufficiently to permit the prints to be slid underneath. The
space between the blocks allows the unused portion of the photograph to
be turned down out of the way. Prints of any size can thus be moved
about until the proper portions for stereo mounting are found. Either
block can be moved in its own plane and also to and from the eye,
whereby two prints of somewhat different scales can be fused.
[Illustration:
FIG. 153.—Pellin double mirror stereoscope.]
=The Taking of Aerial Stereograms.=—The normal separation of the eyes is
altogether too small to give an appearance of relief to objects as far
away as is the ground from a plane at ordinary flying heights. In order
to secure stereoscopic pairs it is therefore necessary to resort to a
method originally employed for photographing distant mountains and
clouds. This is to take the two pictures from points separated by
distances much greater than the interocular separation—by meters instead
of millimeters—corresponding to the positions of the eyes on a veritable
giant. In the airplane this is accomplished by making successive
exposures as the plane flies over the objective, at intervals to be
determined by the speed, the altitude and the amount of relief desired
(Fig. 155).
An all important question which arises immediately is: What separation
of points of view shall we select? If the exposures are too close
together there will be little relief; if too distant the relief will be
so great as to be unnatural, even offensive. Obviously we cannot here
establish a criterion of _natural_ appearance, since the natural
appearance to ordinary human eyes is devoid of relief. We may, however,
define _correct relief_ as that obtained when _the apparent height of
elevated objects is right as compared with their extension or plan_.
[Illustration:
FIG. 154.—Schweissguth stereoscope, used for selecting portions of
prints to be mounted.]
[Illustration:
FIG. 155.—Method of taking stereoscopic pictures.]
In order to secure this condition it is necessary, first, that each
element of the stereoscopic pair be correct in its perspective. This is
fortunately an old photographic problem, already well understood. Its
solution is to view the photograph from a distance exactly equal to the
focal length of the camera lens. Since the normal viewing distance is
not less than 25 centimeters, lenses of this focal length at least are
requisite for correct perspective. Secondly, it is necessary for correct
relief that the two views be taken with a separation equal, on the plane
of the plate, to the separation of the eyes, or 65 millimeters. If _d_
is the interocular distance, a the viewing distance, identical with the
focal length of the lens used, and _A_ the altitude, then _D_, the
distance between exposures, is given by the relation—
_d_ _D_
——— = ———
_a_ _A_
For _a_ = 25 centimeters, _D/A_ = 6.5/25, approximately ¼, or the
interval between exposures must be a quarter the altitude. With a 50
centimeter lens this becomes ⅛, and so on. These figures show the
fallacy of the suggestion sometimes made that we take stereoscopic
pictures by two cameras placed one at the extremity of each wing.
When lenses of more than 25 centimeters focal length are employed, the
stereoscope should be one capable of throwing the convergence point
farther away than the customary 25 centimeters. In the simple lens type
of instrument we can do this by bringing the centers of the lenses
closer together, and by making the focus agree with the convergence
point by adjustment of the distance between lenses and stereogram. If
enlargements are used they should be treated in all respects as
originals made by lenses of the greater foci corresponding to the scale
of the enlargement.
When all the conditions are covered, the appearance presented in the
stereoscope is that of a _model_ of the original object at a distance
_a_, and _a/A_ times natural size. If pictures are made at exposure
intervals less than those indicated for correct relief, they show
insufficient relief. This does not, however, give an unnatural effect,
because anything between no relief and “correct” relief appears natural
with large objects which are not ordinarily seen in relief by eyes not
Brobdignagian. Conversely, stereograms made with too large exposure
intervals show exaggerated relief. Yet this is often no objection. It is
indeed rather an advantage if we wish to bring objects of interest to
notice. Consequently, so long as the exaggeration of relief is not
offensive, the permissible limits of exposure interval are pretty large.
Actually, the eye tolerates such great deviations from strictly normal
conditions that satisfactory stereoscopic effects are obtained for
pictures viewed at very different distances from the focal length of the
taking lens, and with the axes of the eyes parallel or even diverging,
although there is some strain whenever focus and convergence points
differ. On the whole, therefore, it may be said that the conditions
above laid down for correct relief are only a normal, to be approximated
as nearly as is practicable.
Having established the correct relation of taking points for stereos the
next problem is how to determine these when in the plane. The simplest
way is by means of a _stereoscopic sight_. This consists essentially of
two lines of sight (fixed by beads, crosses, or other objects), inclined
toward each other at the angle determined by the ratio of the ocular
separation to the focal length of the lens. If the back sight is made a
single bead or cross, the rest of the stereo sight will consist of two
beads or crosses, separated from each other by the ocular distance of 65
millimeters, and distant from the back sight by the focal length of the
lens (Fig. 157). The first picture is taken when the object is in line
with the forward pointing line of sight, the second when it lies along
the backward pointing one. Like other sights, the stereoscopic sight may
be attached either to the camera, or if this is fixed in position, to
any convenient part of the plane. A very simple sight for vertical
stereoscopic photography consists of an inverted V painted on the side
of the fuselage, so that the eye can be placed at the vertex and sighted
along either leg.
The common method of determining the space between exposures is by the
_time_ interval. If _V_ is the speed of the plane, and _t_ the desired
time interval, we have, from the last equation—
_D_ _dA_
_t_ = ——— = ————
_V_ _aV_
If _A_ = 2000 meters, _d_ = 65 millimeters, and _a_ = 25 centimeters,
and if the plane is traveling 200 kilometers per hour, the time interval
must be—
.065 × 2000 × 3600
—————————————————— = 9.4 seconds
.25 × 200,000
At 1000 meters altitude the interval will be half this, and so on in
proportion. If the pictures are taken with a 50 centimeter focus camera,
and are hence to be viewed at 50 centimeters convergence distance
instead of at 25, the time will again be halved. These relations are
clearly shown in the diagram (Fig. 156). Here the left-hand portion
shows how to find the stereoscopic base line at each altitude for each
focal length; while the right-hand portion shows how to translate this
into time interval for any plane velocity. The Burchall slide rule (Fig.
130) shows another way to arrange these data in form for rapid
calculation.
[Illustration:
FIG. 156.—Chart for calculating intervals between exposures for
stereoscopic pictures.]
Plates used for stereoscopic negatives should be at least twice as long
as the ocular separation, if correct relief is desired, and the full
size of the stereoscope field is to be utilized. This relation follows
at once if we consider that we wish to cut from each negative a
rectangle 65 millimeters wide, and that the image of the target has
shifted 65 millimeters between exposures. If the plate is larger than
this there is opportunity to select the view, or to pick several. If the
plate is smaller the elements of the stereogram must be narrow strips.
This, however, holds only for contact prints.
The ordinary English practice in making stereo negatives is to take
successive pictures with an overlap of 60 to 75 per cent. This practice
is probably dictated by the 4 × 5 inch plate, since 60 per cent. overlap
on 4 inches means a separation of just over an inch and a half instead
of 2¾, but it leaves 2½ inches of picture common to the two negatives.
With ¾ overlap the common portion is 3 inches, which permits of cutting
2¾ inch prints, and allows some latitude for irregular motion of the
plane or for chance error in calculation of intervals. Data on the basis
of ¾ overlaps for a 4-inch plate are shown in connection with Fig. 155
which shows in diagrammatic form the variation of exposure interval with
height, together with other points of interest.
=Elevation Possible to Detect in Stereoscopic Views.=—Can the actual
difference in elevation be discovered by the use of stereoscopic views?
An approximate idea may be obtained from the following considerations:
Suppose we have two small point-like objects, one above the other, such
as a street lamp globe and the base of the lamp pillar. In a view taken
from directly overhead these will be superposed, and so will not be
capable of separation. But, as the point of view is shifted sideways,
the two objects separate, until a point is reached where they can just
be distinguished as double. When this condition holds for either picture
of the stereoscopic pair it will be possible to obtain stereoscopic
relief.
Now the separation which can just be distinguished is commonly assumed
to be one minute of arc. This angle corresponds to about 1/3400 the
distance from the eye to the object. If the object is assumed at a
distance _a_ from the face, and on a line with one of the eyes, which
are separated by the distance _d_, then (all angles being small) the
object must be of height _a/d_ times the horizontal distance which
corresponds to one minute. For 25 centimeters' viewing-distance this
quantity is about 4, so that the least perceptible elevation is 4/3400
or about 1/900. The stereogram having been made under conditions
giving correct relief, this fraction is also the fraction of the
altitude of the plane when the photograph was taken which may be
detected. An object as high as a man (6 feet) should be visible as a
projection in a stereoscopic view taken at 6 × 900 = 5400 feet. This
relation—1/900—holds (irrespective of the focal length of the lens),
as long as the conditions for correct relief are maintained.
=Stereoscopic Aerial Cameras.=—Cameras for aerial stereoscopic
photography need in no way differ in construction from those made for
mapping or spotting, provided only they permit exposures to be made at
short enough intervals. The addition of special sights, as already
discussed, constitutes the only real difference between single view and
stereoscopic aerial cameras. But even without such sights ordinary
aerial cameras are applicable to stereo work by the usual procedure of
determining the exposure spacing by time.
One scheme employed for taking low stereos, where the interval is only
two or three seconds, is to mount two cameras in the plane, exposing
them one after the other at the correct interval. Another method which
has been tried with success is the use of a double focal-plane shutter
in a single lens camera (Fig. 157). The two shutters are side by side,
with their slots parallel to the line of flight. To take a stereo
negative we expose first the shutter nearer the tail of the plane, and
then the other, after an interval which can be calculated from the speed
and altitude, or, better, determined by a stereoscopic sight. The two
views are thus obtained on a single plate. Prints from these negatives
are transposed right and left, and, if the prints are viewed in an
ordinary stereoscope, have to be cut apart and transposed for mounting,
or else this may be done to the negatives.
[Illustration:
FIG. 157.—Aerial hand camera fitted with two complimentary shutter
slits and double sight, for stereoscopic photography.]
In this connection attention may be drawn to an alternative method of
viewing stereograms, which may be used on transposed prints—a method
which needs no instrument, and so has sufficient advantage to even
warrant mounting ordinary stereoscopic pairs in the transposed position
for observation. This method consists in crossing the optic axes, in the
fashion illustrated in Fig. 158. A finger is held in front of the face
in such a position that the left stereogram element and the finger are
seen in line by the right eye; the right element and the finger by the
left eye. The proper position is found by alternately closing each eye,
and advancing or retracting the finger. Then both eyes are opened and
converged on the finger tip, which is thereupon dropped, leaving the
picture standing out in relief. An opportunity to try this method is
afforded by Fig. 159.
[Illustration:
FIG. 158.—Method of fusing transposed stereoscopic images by crossing
the optic axes.]
=Stereo Obliques.=—The theory of making oblique stereo pictures is
identical with that of other stereos. The only problem peculiar to
obliques is that of making the exposures at short enough intervals
apart. This problem is due largely to the fact that oblique views are
ordinarily taken from low altitudes, for the purpose of “spotting”
particular objects, rather than for mapping the gross features of an
extended area. The same problem of how to secure a short exposure
interval is met with when we attempt to take vertical stereos from a low
altitude, but as already discussed, it is much preferable from the
pictorial standpoint that pictures of definite small objectives be made
obliquely.
Another reason for taking stereo obliques from points but little
separated is of some interest in connection with the discussion above
given of “correct” and “natural” relief. When the relief is “correct”
the object appears, as already stated, to be a small model in its true
proportions, standing at the convergence distance. When the eyes are
converged to a small object 25 to 50 centimeters away all objects beyond
are hopelessly transposed and confused. This does not happen when we
look at large distant objects, since their background is at a distance
effectively but little beyond them. As a result, when a stereo oblique
is made in “correct” relief of such an object as the Washington monument
with buildings beyond, the confusion of the background presents an
appearance entirely contrary to our visual experience with objects as
large as the neighboring buildings are known to be. This effect may be
avoided by choosing a uniform background such as grass, or by taking the
pictures very much closer together, at the expense of “correct” but at a
gain in “natural” relief.
Stereo obliques can of course only be made with any facility by
laterally pointing cameras. From the calculations already given it
appears that a “correct” stereo oblique of an object 500 meters away
will mean exposures only two or three seconds apart, too short an
interval for any of the ordinary plate-changing and shutter-setting
mechanisms; and the case is even worse should less relief be desired.
One solution of this problem has been the use, already mentioned, of two
cameras mounted together, either side by side or one over the other,
with separate shutter releases. Both releases may be controlled by the
observer, using a sight, or else pilot and observer may work in harmony
as has been recommended in the English service, where the pilot releases
one shutter and the observer counts time from the instant he sees the
first shutter unwind and releases the second.
[Illustration:
FIG. 159.—Oblique stereogram made with stereoscopic aerial camera
(Fig. 157). To be viewed by crossing the optic axes (Fig. 158).]
A very satisfactory apparatus for the taking of stereo obliques consists
of a 10-inch focus hand-held camera (Fig. 157), provided with a
two-aperture focal-plane shutter. The right-hand half of one curtain
aperture is blocked out, the left-hand half of the other. The first
pressure on the exposing lever exposes one-half of the plate, the second
the other. A stereoscopic sight of the type already described is placed
on the bottom. To make an oblique stereo negative the camera is held
rigidly by resting the elbows on the top of the fuselage and the first
exposure is made when the object comes in line with the rear sight and
the leading front sight. The eye is then moved so as to look along the
line of the rear sight and the following front sight, and when the
object is again in alinement the second pressure is given the exposing
lever. Fig. 159 shows a stereo oblique made by this camera. The elements
are transposed right and left, and the stereogram may be viewed by
crossing the optic axes as shown in Fig. 158, or the two pictures may be
cut apart and remounted.
=The Mounting of Aerial Stereograms.=—The first step in making the
printed stereogram is to select two pictures taken on the same scale,
but from slightly different positions. These may be two chosen from a
collection made for other purposes, or else a pair taken at distances
calculated to fit them for stereoscopic use. The next step is to mark
the center of each picture, either with easily removed chalk or with a
pin point. They are then superposed, and afterward carefully moved apart
by a motion parallel to the line joining their centers when superposed.
The final step before mounting is to mark out and cut the two elements,
their bases being parallel to the line of centers, their horizontal
length the distance between the optic axes of the stereoscope (or as
near this as the size of the prints will permit). They are then mounted
on a card, with their centers separated by approximately 65 millimeters.
The right-hand view is the one showing more of the right-hand side of
objects, and vice versa. This process of arranging, cutting, and
mounting is shown clearly in Fig. 160. In this case the stereoscopic
elements lie symmetrically about the line joining the centers of the
original prints. This is not necessary, as they may be selected from
above or below this line so long as their bases are parallel to it. A
simplification of this method consists in superposing the two prints,
laying over them a square of glass of the size to which they are to be
cut, then turning it so that a side is parallel to the line of centers,
and cutting around it through both prints with a sharp knife. The
principle and results are of course the same with both methods.
[Illustration:
FIG. 160.—Method of arranging stereoscopic prints for cutting.]
If large numbers of stereoscopic prints are required it is necessary,
for economy of time, either to photograph a finished stereogram and make
prints from this copy negative, or to set up special printing machines.
Under the general discussion of printing devices a stereoscopic printer
is described (the Richard) in which the two negatives are placed so that
stereo prints can be got by two successive printings on one sheet of
paper.
=Uses of Stereoscopic Aerial Views.=—Attention has already been called
to the characteristic flatness of the aerial view. Neither the picture
on the retina nor that on the photographic plate affords any adequate
idea of hills and hollows. Unless shadows are well defined, small local
elevations and depressions cannot be distinguished from mere difference
in color or marking. Even in the presence of shadows it is often only by
close study that differences of contour are noticeable. But with
stereoscopic views these features stand out in a striking manner. Taking
our illustrations from military sources, we may note the use of
stereoscopic pictures to detect undulations of ground in front of
trenches (Fig. 161). They reveal the hillocks, pits, small quarries,
streams flowing behind high banks, and other features which make the
attack hard or easy. Commanding positions are shown, the boundaries of
areas exposed to machine-gun fire, and the defilades where the attackers
may pause to reform. Concrete “pill boxes” are located in the midst of
shell holes of the same size and outline, and can be differentiated from
them.
Railway or road embankments and cuts can be detected and studied to
extraordinary advantage in stereoscopic pictures. Thus what appears to
be a mine crater on a level road, easily driven around, may be a gap
blown in an embankment, a serious obstacle indeed. Bridges, observation
towers and other elevated structures jump into view in the stereoscope
when often they have entirely eluded notice in the ordinary flat
picture. Once presented in relief, camouflaged buildings or gun
emplacements, however carefully painted, are ridiculously easy to pick
out.
[Illustration:
FIG. 161.—Typical stereogram of military detail. Fuse by looking at a
distant object over the top of the page, and quickly dropping the
eyes to the print.]
Practical peace-time applications of stereoscopic views can easily be
foreseen following the lines of war experience. Such, for instance,
would be the study of proposed railway or canal routes. A series of
stereograms would obviate the necessity of contour surveys, at least
until the exact route was picked and construction work ready to start.
Apart from their utilitarian side, however, stereoscopic views have very
great pictorial merit. Stereoscopic pictures of cathedrals, public and
other large buildings, have often great beauty, and afford opportunities
for the study of form given by no other kind of representation, short of
expensive scale models. They may very well lead in the near future to a
revival of the popularity of the stereoscope.
=Impression of Relief Produced by Motion.=—An appearance of solidity can
be obtained in _moving pictures_ by the simple expedient of slowly
moving the camera laterally as the pictures are taken. As an
illustration, if the moving picture camera is carried on a boat while
structures on the shore are photographed, when these are projected on
the screen they appear in relief, due to the relative motion of
foreground and background. As relief of this sort is not dependent on
the use of the two eyes, it demands no special viewing apparatus. This
idea has been utilized to a limited extent in ordinary moving picture
photography by introducing a slow to-and-fro motion of the camera, but
this can hardly be considered satisfactory, since this motion is so
obviously unnatural.
In moving pictures made from the airplane the normal rapid motion of the
point of view is ideal for the production of the impression of relief in
the manner just described. For instance, in moving pictures of a city
made from a low flying plane, the skyscrapers and spires as they sweep
past stand forth from their more slowly moving background in bold and
satisfying solidity. In fact, such pictures probably constitute the most
satisfactory solution yet found of the vexing problem of “stereoscopic”
projection. No better medium can be imagined for the travel lecturer to
introduce his audience to a foreign city than to throw upon his screen a
film made in a plane approaching from afar and then circling the
architectural landmarks at low altitudes.