LENSES FOR AERIAL PHOTOGRAPHY
=General Considerations.=—The design and selection of lenses for aerial
photography present on the whole no problems not already encountered in
photography of the more familiar sort. Indeed, the lens problem in the
airplane camera is in some particulars more simple than in the ground
camera. For instance, there is no demand for depth of focus—all objects
photographed are well beyond the usually assumed “infinity focus” of
2000 times the lens diameter. Such strictly scientific problems of
design as pertain to aerial photographic lenses are ones of degree
rather than of kind. Larger aperture, greater covering power, smaller
distortion, more exquisite definition—these always will be in demand,
and each progressive improvement will be reflected in advances in the
art of aerial photography. But many lens designs perfected before the
war were admirably suited, without any change at all, for aerial
cameras.
Of the utmost seriousness, however, with the Allies, was the problem of
securing lenses of the desired types in sufficient numbers. The
manufacture of the many varieties of optical glass essential to modern
photographic lenses was almost exclusively a German industry, which had
to be learned and inaugurated in Allied countries since 1914. In
consequence of this entirely practical problem of quantity production
without the glasses for which lens formulæ were at hand, some new lens
designs were produced. Whether any of these possess merits which will
lead them to be preferred over pre-war designs, when the latter can
again be manufactured, remains to be seen.
While the glass problem was still unsolved, aerial cameras had to be
equipped with whatever lenses could be secured by requisition from
pre-war importation and manufacture, and later, with lenses designed to
utilize those glasses whose manufacture had been mastered in the allied
countries. It is important that the historical aspect of this matter be
well understood by the student of aerial photographic methods, for the
use of these odd-lot lenses reacted on the whole design of aerial
cameras and on the methods of aerial photography, particularly in
England and the United States. Almost without exception the available
lenses were of short focus, considered from the aerial photographic
standpoint; that is, they lay between eight and twelve inches. This set
a limit to the size of the airplane camera, quite irrespective of the
demands made by the nature of the photographic problem. Lenses of these
focal lengths produced images which, for the usual heights of flying,
were generally considered too small, and which were, therefore, almost
always subsequently enlarged. Such was the English practice, which was
followed in the training of aerial photographers in America, where
exactly similar conditions held at the start with respect to available
lenses. French glass and lens manufacturers did succeed in supplying
lenses of longer focus (50 centimeters), in numbers sufficient for their
own service, although never with any certainty for their allies. The
French, therefore, almost from the start, built their cameras with
lenses of long focus, and made contact prints from their negatives.
Practices adopted under pressure of an emergency to meet temporary
practical limitations often come to dominate the whole situation. This
is particularly true of aerial photography in the British and American
services. The small apparatus built around the stop-gap short focus
lenses fixed the plane designer's idea of an airplane camera, and the
space it should occupy. This was directly reflected in the designs of
the English planes, and the American planes copied after them. Meanwhile
the American photographic service in France associated itself with the
French service, adopting its methods and apparatus, and using French
planes whose designs were not being followed in American construction.
The task of harmonizing the photographic practice as taught in America,
following English lines, with French practice as followed in the theater
of war, and of adapting planes built on English designs so that they
could carry French apparatus, was a formidable one, not likely to be
soon forgotten by any who had a part in it.
=Photographic Lens Characteristics.=—Whole volumes have been written on
the photographic lens, and on the optical science utilized and indeed
brought into being by its problems. Such works should be consulted by
those who intend to make a serious study of the design of lenses for
aerial use. No more can be attempted, no more indeed is relevant here,
than an outline review of the chief characteristics and errors of
photographic lenses, considering them with special reference to aerial
needs.
The modern photographic lens is, broadly speaking, a development of the
simple convex or converging lens. Its function is the same: to form a
real image of objects placed before it. But the difference in
performance between the simple lens and the modern photographic
objective is enormous. The simple lens forms a clear image only close to
its axis, for light of a single color, and as long as its aperture is
kept quite small as compared to the distance at which the image is
formed. The photographic lens, on the other hand, is called upon to
produce a clear image with light of a wide range of spectral
composition, sharply defined over a flat surface of large area, and it
must do this with an aperture that is large in comparison with the focal
length, whereby the amount of light falling on the image surface shall
be a maximum. This ideal is approximated to a really extraordinary
degree by the scientific combination and arrangement of lens elements
made from special kinds of glass in the best photographic lenses of the
anastigmat type. The result is of necessity a set of compromises,
whereby the outstanding errors are reduced to a size judged permissible
in view of the work the lens is to do. These errors or _aberrations_ are
briefly reviewed below, in order that the reader may readily grasp the
terms in which the performance and tolerances in aerial lenses are
described.
[Illustration:
FIG. 12.—Diagrammatic representation of spherical aberration.]
_Spherical Aberration and Coma._—Suppose we focus on a screen, by means
of a simple convex lens the image of a distant point of light. Suppose
for simplicity that this image is located on the axis of the lens and
that light of only one color is used, such as yellow. It will be found
that the smallest image that can be obtained is not a point, but a small
disc. This is due to the fact that the rays of light passing through the
outer portions of the lens are bent more than those passing through the
lens in the region near the center. This effect is shown in Fig. 12 by
the usual mode of representing it graphically. Here the figures 1, 2, 3,
4, represent distances from the axis of the lens, and the letters A_{1},
A_{2}, A_{3}, A_{4}, the points of convergence of the rays from 1, 2, 3,
4, etc. These distances projected upward on to the produced lens points
form a curve which shows at a glance the extent and direction of the
error due to each part of the lens. This information is of value where
the lens is fitted with an adjustable diafram. With some types of
correction sharper definition may be obtained by reducing the aperture.
With others, however, diaframing impairs definition, by destroying the
balance between under and over correction which averages to make a good
image. In aerial lenses it is not customary to use diaframs, as all the
light possible is desired. Consequently the reduction of spherical
aberration must be accomplished by proper choice of lens elements and
their arrangement.
Off the axis of the lens the image of a point source takes on an
irregular shape, due to oblique spherical aberration or _coma_.
_Chromatic Aberration._—Because of the inherent properties of the glass
of which it is made, a simple collective lens does not behave in the
same way with respect to light of different colors. If one attempts,
with such a lens, to focus upon a screen the image of a distant white
light, it will be found that the blue rays will not focus at the same
point as the red rays, but will come together nearer the lens. Modern
photographic lenses are compounded of two or more kinds of glass in such
a way as to largely eliminate this defect, the presence of which is
detrimental to good definition. Such lenses are called achromatic, and
the property of a lens by virtue of which this defect is eliminated is
called its _chromatic correction_.
Chromatic correction is never perfect, but two colors of the spectrum
can be brought to a focus in the same plane, and to a certain extent the
departure of other colors from this plane can be controlled. Off the
axis of the lens outstanding chromatic aberration results in a
difference in the size of images of different colors, known as _lateral
chromatism_.
Like spherical aberration, chromatic aberration is a contributing factor
to the size of the image of a point source, which determines the
defining power of a lens. It is, however, an error whose effect is to
some extent dependent on the kind of sensitive plate used. Two lenses
may give images of the same size (in so far as it is governed by
chromatic aberration), if a plate of narrow spectral sensitiveness is
used, while giving images of different size on panchromatic plates of
more extended color sensibility. The choice of the region of the
spectrum for which chromatic correction is to be made is thus governed
by the color of the photographically effective light. While in ordinary
photography the blue of the spectrum is most important, in aerial work
where color filters are habitually used with isochromatic plates the
green is most important, and color correction centered about this region
constitutes a real difference of design peculiar to aerial lenses.
Similarly the general use of deep orange or red filters with red
sensitive plates, for heavy mist penetration, would call for a shift of
correction to that part of the spectrum.
_Astigmatism and Covering Power._—Suppose the lens forms at some point
off its axis an image of a cross. Suppose one of the elements of the
cross to be on a radius from the center of the field, the other element
parallel to a tangent. The rays forming the images of these two elements
of the cross are subject to somewhat different treatment in their
passage through the lens. The curvature of the lens surfaces is on the
whole greater with respect to the rays from the radial element than to
those from the tangential element. They are therefore refracted more
strongly and come to a focus nearer the lens. The arms of the cross are
consequently not all in focus at once. This error, termed _astigmatism_,
is rather well shown in Fig. 15, where the images of the outlying
concentric circles are sharp in the radial, but blurred in the
tangential direction.
Astigmatism can be largely compensated for, and its character
controlled. The most usual correction brings the two images in focus
together both at the axis, and on a circle at some distance out. This
second locus of coincidence may or may not be in the same plane as the
first, depending on which disposition produces the best average
correction. The mean between the two foci determines the focal plane of
the lens, which is in general somewhat curved. The _covering power_ of a
lens is given by the size of the field which is sufficiently flat and
free from astigmatism for the purpose for which the lens is used. This
is largely determined by the astigmatism, but the other aberrations are
also important.
_Illumination._—The amount of light concentrated by the lens on each
elementary area of the image determines its brightness or illumination.
The ideal image would, of course, be equally bright over its whole area
of good definition, and for lenses of narrow angle this is approximately
true. But when it is desired to cover a wide angle the question of
illumination becomes serious. The relationship between angle from the
axis and illumination is that illumination is proportional to the fourth
power of the cosine of the angle. This relationship is shown in the
following table:
Angle Image brightness
0° 100 per cent.
10° 94.1 per cent.
20° 78.0 per cent.
30° 56.2 per cent.
40° 34.4 per cent.
50° 17.1 per cent.
If the field of view is 60°, which corresponds to an 18 × 24 centimeter
plate with a lens of 25 centimeter focus, the brightness is only 56 per
cent., and the necessary exposure at the edge approximately 1.8 times
that at the center. This effect is shown in Fig. 15. It is very
noticeable if the exposure is so short as to place the outlying areas in
the under-exposure period.
[Illustration:
FIG. 13.—Barrel and pin-cushion distortion.]
_Distortion._—Sometimes a lens is relatively free from all the
aberrations, mentioned above, so that it gives sharp, clear images on
the plate, yet these images may not be exactly similar to the objects
themselves as regards their geometrical proportions; in other words, the
image will show distortion. Lens distortion assumes two typical forms,
illustrated in Fig. 13, which shows the result of photographing a square
net-work with lenses suffering in the one case from “barrel” distortion
and in the other from “pin-cushion” distortion. In the first the corners
are drawn in relative to the sides; in the latter case the sides are
drawn in with respect to the corners. Either sort is a serious matter in
precision photography, such as aerial photographic mapping aspires to
become. It must be reduced to a minimum and its amount must be
accurately known if negatives are to be measured for the precise
location of photographed objects. In general symmetrical lenses give
less distortion than the unsymmetrical (Fig. 14).
[Illustration:
FIG. 14.—Arrangement of elements in two lenses suitable for aerial
work: _a_, Zeiss Tessar; two simple and one cemented components
(unsymmetrical); _b_, Hawkeye Aerial; two positive elements of heavy
barium crown, two negative of barium flint, uncemented
(symmetrical).]
=Lens Testing and Tolerances for Aerial Work.=—Simple and rapid
comparative tests of lenses may be made by photographing a _test chart_,
consisting of a large flat surface on which are drawn various
combinations of geometrical figures—lines, squares, circles,
etc.—calculated to show up any failures of defining power. For testing
aerial lenses the chart should be as large as possible, so that it may
be photographed at a distance great enough for the performance of the
lens to be truly representative of its behavior on an object at infinite
distance. This means in practice a chart of 4 or 5 meters side, to be
photographed at a distance 20 to 30 times the focal length of the lens.
[Illustration:
FIG. 15.—Photograph of a lens testing chart, showing failure in
defining power outside area for which the lens is calculated.]
A typical photograph of such a chart is shown in Fig. 15. It reveals at
a glance the more conspicuous lens errors. At the sides and corners the
concentric circles show the lens's astigmatism, by the clear definition
of the lines radial to the center of the field and their blurring in the
tangential direction. The falling off in illumination with increasing
distance from the center is also exhibited; and the blurring of all
detail outside the rectangle for which the lens was calculated shows
that spherical, chromatic, and other aberrations have become
prohibitively large.
But the only complete test of a lens is the quantitative measurement of
errors made on an optical bench. A point source of light, which may at
will be made of any color of the spectrum, is used as the object and its
image formed by the lens in a position where it can be accurately
measured for location, size, and shape by a microscope. A chart giving
the results of such a test is shown in Fig. 16. In the upper left-hand
corner is shown the position of the focus for the different colors of
the spectrum. Below this is recorded the lateral chromatism at 21
degrees, in terms of the difference in focus for a red and a blue ray.
Below this again comes the distortion, or shift of the image from its
proper position, for various angles (plotted at the extreme right) from
the lens axis. To the right of this is the image size, at each angle,
and finally, to the right of the diagram, are plotted the distances of
the two astigmatic foci from the focal plane, together with the mean of
the two foci, which practically determines the shape of the field.
An important point to notice is that these data are uniformly plotted in
terms of a lens of 100 millimeters focal length irrespective of the
actual focal length of the lens measured. Thus this particular chart is
for a 50 centimeter lens but would be plotted on the same scale for a 25
or a 100 centimeter lens. Underlying this practice is the assumption
that all the characteristics of lenses of the same design and aperture
are directly proportional to their focal length. If this were so, then a
50 centimeter lens would give double the size of image that a 25
centimeter does, and so on. As a matter of fact, test shows that the
size of the image does not increase so rapidly as the focal length; so
that while the image size for a 25 centimeter lens would be, say, .05
millimeters per 100 millimeters focal length, it will be only .03 or .04
millimeters per 100 millimeters focal length for a 50 centimeter lens.
The actual size of a point image will therefore be greater, though not
proportionately greater.
[Illustration:
FIG. 16.—Chart recording measurements of lens characteristics.]
The chart presents tests on a good quality lens, and so gives a good
idea of the permissible magnitude of the various errors. In many ways
the most important figure is that for image size, including as it does
the result of all the aberrations. In the example given, this varies
from .075 to .15 mm. actual size. For the same type of lens of 25
centimeters focus this range will be from .05 to .10 mm. Since these are
commonly used focal lengths, a good average figure for image size,
commonly used in aerial photographic calculations, is ⅒ mm. In regard to
astigmatic tolerances, the two astigmatic foci should not be separated
at any point by more than 6 to 7 millimeters, and the mean of these
should not deviate from the true flat field by more than ½ millimeter,
in each case the figures being based on the conventional 100 millimeters
focal length. Distortion should not be over .08 millimeter at 18° or .20
millimeter at 24° from the axis (per 100 millimeters focal length).
=Lens Aperture.=—In the simple lens the aperture is merely the diameter.
In compound lenses the aperture is not the linear opening but the
effective opening of an internal diafram. Photographically, however,
aperture has come to have a more extensive meaning. While in the
telescope the actual diameter of an objective is perhaps the most
important figure, and in the microscope the focal length, in photography
the really important feature is the amount of light or illumination.
This is determined by lens opening and focal length together;
specifically, by the ratio of the lens area to the focal length. The
common system of representing photographic lens aperture is by the ratio
of focal length to lens diameter, the lens being assumed to be circular.
Thus F/5 (often written F.5) indicates that the diameter is one-fifth
the focal length.
Two points are to be constantly borne in mind in connection with this
system of representation. First, all lenses of the same aperture (as so
represented) give the same illumination of the plate (except for
differences due to loss of light by absorption and reflection in the
lens system). This follows simply from the fact that the illumination of
the plate is directly proportional to the square of the lens diameter,
and inversely as the square of the focal length. Secondly, the
illumination of the plate is inversely as the square of the numerical
part of the expression for aperture. That is, lenses of aperture F/4.5
and F/6 give images of relative brightness (6/4.5)^2 = 1.78.
What lens aperture, and therefore what image brightness, is feasible, is
determined chiefly by the angular field that must be covered with any
given excellence of definition. The largest aperture ordinarily used for
work requiring good definition and flat field free from distortion is
F/4.5. Anastigmatic lenses of this aperture cover an angle of 16° to 18°
from the axis satisfactorily, which corresponds to an 18 × 24 centimeter
plate with a lens of 50 centimeters focus. Lenses with aperture as large
as F/3.5 were used to some extent in German hand cameras of 25
centimeters focal length, with plates of 9 × 12 centimeters. English and
American lenses of this latter focal length were commonly of aperture
F/4.5, designed to cover a 4 × 5 inch plate.
As a general rule the greater the focal length the smaller the
aperture—a relationship primarily due to the difficulty of securing
optical glass in large pieces. Thus while 50 centimeter lenses of
aperture F/4.5 are reasonably easy to manufacture, the practicable
aperture for quantity production is F/6, and for 120 centimeter lenses,
F/10. This means that a very great sacrifice of illumination must be
faced to secure these greater focal lengths. As is to be expected from
the state of the optical glass industry, the German lenses were of
generally larger aperture for the same focal lengths than were those of
the Allies. Besides the F/3.5 lenses already mentioned, their 50
centimeter lenses were commonly of aperture F/4.8, their 120 centimeter
lenses of aperture F/7, or of about double the illuminating power of the
French lenses of the same size.
Demands for large covering power also result in smaller aperture. The 26
centimeter lenses used on French hand cameras utilizing 13 × 18
centimeter plates were commonly of aperture F/6 or F/5.6. The lens of
largest covering power decided on for use in the American service was of
12 inch focus, to be used with an 18 × 24 centimeter plate (extreme
angle 26°); the largest satisfactory aperture for this lens is F/5.6.
Ordinarily the question of aperture is closely connected with that of
diaframs, whereby the lens aperture may be reduced at will. Diaframs
have been very little used in aerial photography. All the aperture that
can be obtained and more is needed to secure adequate photographic
action with the short exposures required under the conditions of rapid
motion and vibration peculiar to the airplane. Any excess of light, over
the minimum necessary to secure proper photographic action, is far
better offset by increase of shutter speed or by introduction of a color
filter. For this reason American aerial lenses were made without
diaframs. In the German cameras, however, adjustable diaframs are
provided (Fig. 43), controlled from the top of the camera by a rack and
pinion. In the camera most used in the Italian service an adjustable
diafram is provided, but this is occasioned by the employment of a
between-the-lens shutter of fixed speed, so that the only way exposure
can be regulated is by aperture variation, a method which has little to
recommend it.
=The Question of Focal Length.=—In aerial photography the lens is
invariably used at fixed, infinity, focus. Under these conditions the
simple relationship holds that the size of the image is directly
proportional to the focal length and inversely proportional to the
altitude. If any chosen scale is desired for the picture the choice of
focal length is determined by the height at which it is necessary to
fly. This at least would be the case were there no limitation to the
practicable focal length—which means camera size—and were one limited to
the original size of the picture as taken; that is, were the process of
enlargement not available. But the possibility of using the enlarging
process brings in other questions: Is the defining power of a short
focus lens as good in proportion to its focal length as that of a long
focus lens? If so a perfect enlargement from a negative made by a short
focus lens would be identical with a contact print from a negative made
with a lens of longer focus. Is defining power lost in the enlarging
process with its necessary employment of a lens which has its own errors
of definition and which must be accurately focussed?
Certain factors which enter into comparisons of this sort in other lines
of work, such as astronomical photography, play little part here. These
are, first, the optical resolving power of the lens, which is
conditioned by the phenomena of diffraction, and is directly as the
diameter; and, second, the size of the grain of the plate emulsion. The
first of these does not enter directly, because the size of a point
image on the axis of the lens, due merely to diffraction, is very much
less than that given by any photographic lens which has been calculated
to give definition over a large field, instead of the minute field of
the telescope. Yet it may contribute toward somewhat better definition
with a long focus lens because of the actually larger diameter of such
lenses. The second factor is not important, because, as will be seen
later, the resolving power of the plates suitable for aerial photography
is considerably greater than that of the lens. The emulsion grain is in
fact only a quarter or a fifth the size of the image as given by a 25
centimeter lens, and enlargements of more than two or three times are
rarely wanted.
A series of experiments was made for the U. S. Air Service to test out
these questions, using a number of representative lenses of all focal
lengths, both at their working apertures and at identical apertures for
all. With regard to lens defining power, as shown by the size of a point
image, the answer has already been reported in a previous section.
Lenses of long focus give a relatively smaller image than lenses of the
same design of short focus. In regard to the whole process of making a
small negative and enlarging it, the loss of definition is quite marked,
as compared to the pictures of the same scale made by contact printing
from negatives taken with longer focus lenses.
This answer is clear-cut only for lenses calculated to give the same
angular field. Thus a 10 inch lens covering a 4 × 5 inch plate has about
the same angle as a 50 centimeter lens for an 18 × 24 centimeter plate.
When, however, it comes to the longer foci, such as 120 centimeters, the
practical limitation to plate size (18 × 24 cm.) has been passed, and
the angular field is less than half that of the 50 centimeter lens. The
120 centimeter lens need only be designed for this small angle, with
consequent greater opportunities for reduction of spherical aberration.
It is therefore an open question whether a 50 centimeter lens designed
to cover a plate of linear dimensions 50/120 times that used with the
regular 50 centimeter lens could not be produced of such quality that it
would yield enlargements equal to contacts from a 120 centimeter lens.
If so, lenses of larger aperture could be used, and a considerable
saving in space requirements effected.
Focal lengths during the Great War were decided by the nature of the
military detail which was to be revealed and by the altitudes to which
flying was restricted in military operations. In the first three years
of the war the development of defences against aircraft forced planes to
mount steadily higher, so that the original three or four thousand feet
were pushed to 15,000, 18,000, and even higher. Lenses of long focus
were in demand, leading ultimately to the use of some of as much as 120
centimeters (Fig. 41). In the last months of the war the resumption of
open fighting made minute recording of trench details of less weight,
while the preponderance of allied air strength permitted lower flying.
In consequence, lenses of shorter focus and wider angle came to the
fore, suitable for quick reconnaissance of the main features of new
country. At the close of the war the following focal lengths were
standard in the U. S. Air Service, and may be considered as well-suited
for military needs. Peace may develop quite different requirements.
Focal length Aperture Plate size
10 inch F/4.5 4 × 5 inch
26 cm. F/6 13 × 18 cm.
12 inch F/5.6 18 × 24 cm.
20 inch F/6.3 to F/4.5 18 × 24 cm.
48 inch F/10 to F/8 18 × 24 cm.
The question of the use of _telephoto lenses_ in place of lenses of long
focus is frequently raised. Lenses of this type combine a diverging
(concave) element with the normal converging system, whereby the effect
of a long focus is secured without an equivalent lens-to-plate distance.
This reduction in “back focus” may be from a quarter to a half. Were it
possible to obtain the same definition with telephoto lenses as with
lenses of the same equivalent focus, they would indeed be eminently
suitable for aerial work because of their economy of length. But
experience thus far has shown that the performance of telephoto lenses,
as to definition and freedom from distortion, is distinctly inferior, so
that it is best to hold to the long focus lens of the ordinary type.
=Lenses Suitable for Aerial Photography.=—Among the very large number of
modern anastigmat lenses many were found suitable for airplane cameras
and were used extensively in the war. A partial list follows: The Cooke
Aviar, The Carl Zeiss Tessar, the Goerz Dogmar, the Hawkeye Aerial, the
Bausch and Lomb Series Ic and IIb Tessars, the Aldis Triplet, the
Berthiot Olor.
=The Question of Plate Size and Shape.=—Plate size is determined by a
number of considerations, scientific and practical. If the type of lens
is fixed by requirements as to definition, then the dimensions of the
plate are limited by the covering power. From the standpoint of economy
of flights and of ease of recognizing the locality represented in a
negative, by its inclusion of known points, lenses of as wide angle as
possible should be used. If the focus is long, this means large plates,
which are bulky and heavy. If the finest rendering of detail is not
required a smaller scale may be employed, utilizing short focus lenses
and correspondingly smaller plates. Thus a six inch focus lens on a 4 ×
5 inch plate would be as good from the standpoint of angular field as a
12 inch on an 8 × 10 inch plate. This is apt to be the condition with
respect to most peace-time aerial photography, which may be expected to
free itself quickly from the huge plates and cameras of war origin.
For work in which great freedom from distortion of any sort is
imperative, small plates will be necessary, for two reasons. One is that
the characteristic lens distortions are largely confined to the outlying
portions of the field. The other is that a wide angle of view inevitably
means that all objects of any elevation at the edge of the picture are
shown partly in face as well as in plan, which prevents satisfactory
joining of successive views (Fig. 128). In making a mosaic map of a
city, if a wide angle lens is employed with large plates, the buildings
lying along the junctions of the prints can be matched up only for one
level. If this is the ground level, as it would be to keep the scale of
the map correct, the roofs will have to be sacrificed. In extreme cases
a house at the edge of a junction may even show merely as a front and
rear, with no roof, while in any case the abrupt change at these edges
from seeing one side of all objects to seeing the opposite side is not
pleasing.
The table in a preceding section gives the relation of plate size to
focal length found best on the whole for military needs. Deviations from
these proportions in both directions are met with. In the English
service the LB camera, which uses 4 × 5 inch plates, is equipped with
lenses of various focal lengths, up to 20 inches. The German practice,
as well as the Italian, was almost uniform use of 13 × 18 centimeter
plates for all focal lengths. Toward the end of the war, however, some
German cameras of 50 centimeter focal length were in use employing
plates 24 × 30 centimeters.
It will be recognized that these plate sizes are chosen from those in
common use before the war. A similar observation holds with even greater
force on the question of plate _shape_. Current plate shapes have been
chosen chiefly with reference to securing pleasing or artistic effects
with the common types of pictures taken on the ground. These shapes are
not necessarily the best for aerial photography. Indeed the whole
question of plate shape should be taken up from the beginning, with
direct reference to the problems of aerial photography and photographic
apparatus.
A few illustrations will make this clear, taking Fig. 17 as a basis. If
it is desired to do spotting (the photography of single objectives), the
best plate shape would be circular, for that shape utilizes the entire
covering area of the lens. If it is desired to make successive
overlapping pictures, either for mapping, or for the production of
stereoscopic pairs, a rectangular shape is indicated. If the process of
plate changing is difficult or slow, it is advisable, in order to give
maximum time for this operation, to have the long side of the rectangle
parallel to the line of flight (indicated by the arrow). If economy of
flights is a consideration, as in making a mosaic map of a large area,
it is advantageous to have as wide a plate as the covering power of the
lens will permit. Reference to Fig. 17 shows that this means a plate of
small dimensions in the direction of flight. If the changing of plates
or film is quick and easy, the maximum use of the lens's covering power
is made by such a rectangle whose long side approximates the dimensions
of the lens field diameter. This is in fact the choice made in the
German film mapping camera (Figs. 61 and 63), whose picture is 6 × 24
centimeters. An objection to this from the pictorial side, lies in the
many junction lines cutting up the mosaic. Another objection, if the
plane does not hold a steady course, is the failure to make overlaps on
a turn. (Fig. 62.) Here as everywhere the problem is to decide on the
most practical compromise between all requirements.
[Illustration:
FIG. 17.—Possible choices of plate shape.]
=Focussing.=—The process of focussing aerial cameras was at first deemed
a mystery, though undeservedly so. A belief was long current that
“ground” focus and “air” focus differ. In other words, that a camera
focussed upon a distant object on the ground would not be in focus for
an object the same distance below the camera when in the plane. Belief
in this mysterious difference went so far that certain instruction books
describe in detail the process of focussing a camera by trial exposures
from the air.
Careful laboratory tests performed for the U. S. Air Service showed that
neither low temperature nor low pressure, such as would be met at high
altitudes, alter the focus of any ordinary lens by a significant amount,
and that the possible contraction of the camera body was of negligible
effect on the focus (not more than 1/200 per cent. per degree centigrade
with a metal camera). In complete harmony with these tests has been the
experience that if the ground focussing is done carefully, by accurate
means, then the air focus is correct. The whole matter thus becomes one
of precision focussing.
The best method, applicable if the air is steady, is to focus by
_parallax_. The ground glass focussing screen is marked in the center
with a pencilled cross. Over this is mounted, with Canada balsam, a thin
microscope cover-glass. The camera is directed on an object a mile or
more away, and the image formed by the lens is examined by a magnifying
glass through the virtual hole formed by the affixed cover-glass. With
the pencil line in focus the head is moved from side to side. If the
image and pencil mark coincide they will move together as the head is
moved. If the image moves away from the pencil mark and in the _same_
direction as the eye moves, the image is too near the lens. If the image
moves away in the _opposite_ direction to the motion of the eye, it is
too far from the lens. In either case the focus is to be corrected
accordingly.
In place of a distant object, which may waver with the motion of the
air, we may use an image placed at infinity by optical means. The
_collimator_, an instrument for doing this, consists of a test object
(lines, circles, etc.) placed accurately at the focus of a telescope
objective. The camera lens is placed against this and focussed by
parallax, as with a distant object. Collimators are employed in camera
factories, and should be part of the equipment of base laboratories
where repairing and overhauling of cameras is done.
=Lens Mounts.=—All that is required for the mounting of an aerial camera
lens is a rigid platform, with provision for enough motion of the lens
to adjust its focus accurately. As already explained, the lens works at
fixed, infinity, focus, and therefore needs no adjustment during use. It
must be held far more rigidly than would be possible by the bellows,
which is an almost invariable adjunct of focussing cameras. The use of
ordinary types of hand cameras on a plane is rarely successful just
because of the bellows, which is strained and rattled by the rush of
wind.
The lens mountings thus far used have been simple affairs. In the French
cameras the lens is merely screwed into a flange which in turn is
fastened by screws to a platform in the camera body. Adjustment for
focussing is not provided; instead, the flange is raised on thin metal
rings or washers, cut of such thickness by trial as to bring the lens to
focus, once and for all.
The U. S. Air Service method of mounting is to provide the lens barrel
with a long thread, which screws into a flange that in turn is mounted
on a platform in the camera cone, by means of thumb-screws. The lens is
focussed by screwing in and out, and then clamped by a screw through the
side, bearing on the thread. The whole mount may be quickly removed by
loosening the thumb-screws, and once focussed in one cone, can be
transferred to another similar, machine-made cone without change of
focus. Fig. 18 shows a 20 inch lens mounted in this manner. The
photograph shows as well the ring on the front of the lens by means of
which circular color filters may be held in place. This ring screws down
on the filter, and the catch is dropped into the nearest vertical groove
to the tight position.
[Illustration:
FIG. 18.—50 centimeter F/6 lens in U. S. standard mount, showing color
filter retaining ring and catch.]
A somewhat different and better method of tightening the lens in the
flange, when focussed, has been adopted in the English lens mount, which
is in general similar to the American. The threaded part of the flange
is split by a slot cut parallel to the flange base, and a screw is run
into the flange from the front, through the split portion. By tightening
this screw, which is always accessible, the split part of the flange is
squeezed together, thus rigidly holding the lens barrel.