THE SHUTTER
=Permissible Exposure in Airplane Photography.=—A definite limitation to
the length of exposure in airplane cameras is set by the motion of the
plane. If we represent the speed of the plane by _S_, the altitude of
the plane by _A_, and the focal length of the lens by _F_, we obtain at
once from the diagram (Fig. 19), that _s_, the rate of movement of the
image on the plate, is given by the relation,
_s_ _F_
——— = ———
_S_ _A_
If we call the permissible movement _d_, then the permissible exposure
time, _t_, is given by the relation—
_d_ _Ad_
_t_ = ——— = ————
_s_ _FS_
As a representative numerical case, expressing all quantities in
centimeters and in centimeters per second, let _F_ = 50, _S_ =
20,000,000/3600 (200 kilometers per hour), and _A_ = 300,000, then
50 × 20,000,000
_s_ = ——————————————— = .9 centimeters
300,000 × 3600
If we take for the permissible undetectable movement, .01 centimeter,
which is, as has been shown, a reasonable figure for lens defining
power, we have, then, that the _longest permissible exposure is .011
second_—in round numbers, one-hundredth.
In flying with a slow plane, or in flying against the wind, the exposure
can sometimes be increased to as much as double this length. Diminishing
_F_ would similarly extend the allowable exposure, but the ratio of _F_
to _A_ approximates to a constant in actual practice; in other words, a
certain resolution and size of image have been found desirable. If
flying is forced higher, a longer focus lens is used; if lower flying is
possible, a lens of shorter focus. This relationship has, of course,
been derived from war-time experience. Probably much of the prospective
peace-time mapping work will impose substantially easier requirements as
to definition and will thus allow longer exposures.
[Illustration:
FIG. 19.—Relative motion of plane and photographic image.]
For low oblique views the longest exposure is much less. Taking 45
degrees as a representative angle for the foreground, and 500 meters as
a representative height, the value of _t_ becomes 1/600.
These figures will illustrate two important points: they show how severe
is the limitation as to exposure, with the consequent heavy demand on
lens and sensitive material speed; and they show how important it is to
secure a shutter with the maximum light-giving power for a specified
length of exposure. This leads to a study of the characteristics as to
efficiency of the two common types of shutter, namely, _shutters at or
between the lens_, and _focal-plane shutters_.
=Characteristics of Shutters Located at the Lens.=—Of the various
shutters located at the lens the most common is the type that is
clumsily but descriptively termed the “between-the-lens” shutter. This
is composed of thin hard rubber or metal leaves or sectors which overlap
and which are pulled open to make the exposure. It may require two
operations, one for setting and one for exposing, or it may, as in some
makes, set and expose by a single motion. Clock escapements, or some
form of frictional resistance, are depended on to control the interval
between opening and closing. This shutter is the one almost universally
employed on small hand cameras and on all lenses up to about two inches
diameter. It gives speeds sometimes marked as high as 1/300 second,
although usually not over 1/100 on actual test.
Between-the-lens shutters have been used to some extent on the shorter
focus (up to 25 centimeter) aerial cameras, notably in the Italian
service. They suffer, however, from two limitations. In the first place
we have not yet solved the mechanical problems met with in trying to
make the shutter of large size (as for 50 centimeter _F_/6 lenses) at
the same time to give high speeds. In the second place the efficiency of
the type is low because a large part of the exposure time is occupied by
the opening and closing of the sectors.
If we define the _efficiency_ of a shutter as the ratio of the amount of
light it transmits during the exposure to the amount of light it would
transmit were it wide open during the whole period, then the efficiency
of the ordinary between-the-lens shutter is of the order of 60 per cent.
This means 1.6 times the motion of the image for the same photographic
action that we should have with a perfect shutter. The accompanying
photographic record (Fig. 20) of the opening and closing process of this
type of shutter clearly illustrates its deficiencies.
[Illustration:
FIG. 20.—Effective lens opening at equal intervals of time: (_a_)
during focal plane shutter exposure; (_b_) during between-the-lens
shutter exposure.]
=Characteristics of the Focal-Plane Shutter.=—Long before the days of
aerial photography the problem of a high-efficiency high-speed shutter
for photographing moving objects on the ground—railway trains or racing
automobiles—had already led to the development of the _focal-plane
shutter_. This is a type peculiarly adapted to the problems of the
airplane camera. It consists essentially of a curtain, running at high
speed close to the photographic plate, the exposure being given by a
narrow rectangular slot.
If the focal-plane shutter is in virtual contact with the sensitive
surface the efficiency, as defined above, is 100 per cent., since the
whole cone of rays from the lens illuminates the plate during the whole
time of exposure. But if the curtain is not carried close to the plate
the efficiency falls off rapidly with distance, especially so for small
apertures of the slot.
[Illustration:
FIG. 21.—Calculation of focal plane shutter efficiency.]
_The efficiency of the focal-plane shutter_ may be calculated as
follows: Let the focal length of the lens be _F_, its diameter be _F/N_,
the width of the slot be _a_, and the distance from plate to curtain _d_
(Fig. 21). Now if the curtain is moving at a uniform speed, the time
taken for the slot to traverse the whole cone of rays, from the instant
it enters till the instant it leaves, will be directly proportional to
_d_ (_F_) _d_
——— (———) + _a_ = ——— + _a_
_F_ (_N_) _N_
If the curtain were in contact with the plate the time taken for the
same amount of light to reach the sensitive surface would be
proportional to _a_. Again defining shutter efficiency as the ratio of
the light transmitted to what would have been transmitted were the
shutter fully open for the total time of exposure, the efficiency, _E_,
is given at once by the expression—
_a_
_E_ = ———————————
_d/N_ + _a_
As an example let the lens aperture be _F_/6, so that _N_ = 6; let _d_ =
1, and _a_ = 1, then _E_ = 6/7. In the French deMaria cameras, where _d_
= 4 centimeters, _E_ = 60 per cent. for the aperture assumed, which is
representative. Fig. 22 exhibits diagrammatically the chief
characteristics of the focal plane shutter.
[Illustration:
FIG. 22.—Characteristics of focal plane shutter.]
In view of the necessity for _some_ distance between shutter and plate
it is obviously important to keep _a_ as large as possible, depending
for the requisite shutter speed on the velocity of the curtain. Large
aperture and high curtain speed are also found to be desirable when we
consider the distortion produced by the focal-plane shutter.
=Distortions Produced by the Focal-plane Shutter.=—While the time of
exposure of any point on the plate can, with the focal-plane shutter,
easily be made 1/100 second or less, the whole period during which the
shutter is moving is much greater than this. For instance, a 1
centimeter opening which gives 1/100 second exposure takes ⅒ second to
move across a 10 centimeter plate, or nearly ⅕ second for an 18
centimeter plate. With a moving airplane this means that the point of
view at the end of the exposure has moved forward compared to that at
the beginning, by the amount of motion of the plane in the interval. If
the shutter moves in the direction of motion of the plane the image will
be magnified; if in the opposite direction, it will be compressed along
the axis of motion. The amount of this distortion is calculated as
follows:
Let the velocity of the plane be _V_, and that of the shutter be _v_.
Let the focal length of the camera be _F_, and the altitude _A_. If the
camera were stationary, a plate of length _l_ would receive on its
surface an image corresponding to a distance _A/F_ × _l_ on the ground.
Due to the motion of the shutter the end of the exposure occurs at a
time _l/v_ after the start. In this time the plane has moved a distance
_V_ × _l/v_; hence the point photographed at the end of the shutter
travel is _Vl/v_ within or beyond the original space covered by the
plate, depending on the direction of motion of the curtain. The
distortion, _D_, is given by the ratio of this distance to the length
corresponding to the normal stationary field of view:
_V/v_ × _l_ _VF_
_D_ = ——————————— = ————
_A/F_ × _l_ _vA_
When _V_ = 200 kilometers per hour, _v_ = 100 centimeters per second,
_F_ = 50 centimeters, _A_ = 3000 meters, we have—
20,000,000 × 50 1
_D_ = ———————————————————— = approximately ———
3600 × 100 × 300,000 100
Or if the actual distance error on the ground is desired,
_Vl_
———— = 10.8 meters
_v_
As a percentage error this one per cent. is small compared with other
uncertainties, such as film shrinkage or the error of level of the
camera. As an absolute error in surveying, thirty feet is, of course,
excessive.
The distortion is diminished for any specified shutter speed by making
the speed of travel of the curtain as large as possible and by
correspondingly increasing the aperture. In connection with film
cameras, another solution which has been suggested is to move the film
continuously during the exposure in the direction of the plane's motion.
The requisite speed of the film _v'_ to eliminate distortion is given by
the relation:
_v'_ _F_
———— = ———
_V_ _A_
For the values of _V_, _F_, and _A_ used above, _v'_ = .92 centimeters
per second. This speed is clearly that which holds the image stationary
on the film—a fact which suggests another object for such movement,
namely, to permit of longer exposures.
The effect of focal plane distortion may be averaged out in the making
of strip maps, if the shutter is constructed so as to move in opposite
directions on successive exposures. The first picture will be magnified,
the second compressed, and so on, but a strip formed of accurately
juxtaposed pictures will be substantially accurate in over-all length.
Such a shutter is embodied in one of the German film cameras (Fig. 61).
Distortion of the kind above discussed is absent with between-the-lens
shutters, which may conceivably be improved in efficiency and in
feasible size. If so they would merit serious consideration for aerial
mapping.
=Methods and Apparatus for Testing Shutter Performance.=—With a
focal-plane shutter the desirable qualities in performance are three in
number: (1) _Adequate speed range_, which may be taken as from 1/50 to
1/500 second for aerial work, (2) _good efficiency_, which has already
been treated, and (3) _uniformity of speed_ during its travel across the
plate. Before the advent of aerial photography little attention was paid
to speed uniformity, differences of 50 per cent. in initial and final
speed being common in focal-plane shutters, and but little noticed in
ordinary landscape work because of the natural variation of brightness
from sky to ground. In the making of aerial mosaic maps the
non-uniformity of density across the plate results in a most offensive
series of abrupt changes of tone at the junction points of the
successive prints (Fig. 140), an effect which must be minimized by
manipulation of the printing light.
Instruments for testing the speed and uniformity of action of
focal-plane shutters are an essential part of any laboratory for
developing or testing photographic apparatus and some simple device for
setting and checking shutter speed should be available in the field.
Every such speed tester must contain some form of time counting
element—pendulum, tuning fork or clock-work. Elaborate shutter testers,
suitable for determining all the characteristics of all types of
shutter, have been developed and used in certain of the photographic
research laboratories. For the study and setting of focal-plane shutters
(whose efficiency need not be measured, as it can be simply calculated
from linear dimensions), the following simple kinds of apparatus are
adequate:
[Illustration:
FIG. 23.—Apparatus for testing focal plane shutter speed throughout
the travel of the curtain.]
_Clock dial type of shutter tester._ This consists essentially of a
black clock dial carrying a white pointer which makes its complete
revolution in one second or less. If this dial is photographed by the
camera under test, the width of the sector traced during the exposure by
the moving pointer shows the time interval. If the dial is photographed
at several points on the plate—beginning, middle and end of the shutter
travel—the complete characteristics of the shutter can be determined.
_Interrupted light type of shutter tester._ For the study of uniformity
of shutter action alone the apparatus shown in Fig. 23 may be employed.
_A_ is a high intensity light source, such as an arc or a gas filled
tungsten lamp. _L_ is a convex lens, focussing an image of the light
source on a small aperture in the screen _E_. _D_ is a sector disc
which, driven by the motor _M_, interrupts the transmitted light with a
frequency determined by the number of openings of the sector and by the
speed of rotation, which must be measured by a tachometer. The light
diverging from the aperture in _E_ falls upon the shutter _S_, which for
this test is reduced to a narrow slit of one millimeter or less. Passing
through the shutter opening the light falls upon the photographic plate
_P_. The principle is simple: If the light is uninterrupted, the plate
_P_ is exposed at all points; due to the interruptions, a series of
parallel lines of photographic action result, and their distance apart
gives a measure of the speed of the shutter at any chosen point in its
travel. A performance curve of the French Klopcic shutter is shown in
Fig. 24. The variation in speed lies over a range of two to one. So
serious is this defect in these shutters that diaframs are sometimes
inserted in the French cameras to cut off part of the light from the
lens on the most exposed end of the plate. This expedient produces
uniformity of photographic action, but does not overcome the movement of
the image, which is one of the chief faults of excessive exposure.
[Illustration:
FIG. 24.—Performance of Klopcic shutter.]
[Illustration:
FIG. 25.—Optical system of shutter tester for Air Service, U. S. Army.]
A more complete apparatus, adapted both to absolute speed determinations
and to the study of uniformity of action, is that worked out and used in
the United States Air Service (Fig. 25). At _A_ is a high intensity
light source, an image of which is focussed by the lens _L__{1} upon a
slit _E_, in front of which stands a tuning fork _T_, of period 1024 or
2048 per second. The light diverging from the slit is received by a
second lens, _L__{2} which is arranged either to focus the slit image
upon the shutter curtain or to render the rays parallel, so that an
entire camera may be inserted. In the latter case the camera lens
_L__{3} serves to focus the slit image on the curtain _C_. After passing
through the curtain aperture the light is focussed by the lens _L__{4}
on the rotatable drum _D_, which carries a strip of sensitive film.
The operation of testing a shutter consists in focussing the slit image
on the portion of the shutter whose performance is required, striking
the tuning fork to set it vibrating, rotating the drum rapidly and
setting off the shutter. There is thus obtained on the sensitive film an
exposed strip resembling in appearance the edge of a saw, the number of
teeth showing the time interval in vibrations of the tuning fork. Three
exposures usually give all the points necessary for a practical
knowledge of the shutter's uniformity of action. A point of some
importance, learned from numerous shutter tests, is that a focal-plane
shutter should be tested in the position in which it is to be used.
Aerial camera shutters should be tested in the horizontal position.
=Types of Focal-plane Shutters.=—A variety of means have been utilized
for securing the necessary variation in speed in focal-plane shutters.
Their success is to be measured by the actual speed range and by the
uniformity of speed attained. In aerial cameras at present in use we
find _variable tension_ of the curtain spring, the aperture being fixed;
_variable opening_ with fixed tension; _multiple curtain openings_ with
fixed spring tension; and combinations of two or all of these methods of
speed control. The problem of covering the aperture during the operation
of winding up or setting the shutter has led to further elaborations of
shutter mechanism. These take the form of _lens or shutter flaps_,
_auxiliary curtains_, and shutters of the _self-capping_ type. Shutters
embodying all these features are briefly described below.
=Representative Shutters.=—The Folmer variable tension shutter is used
on the United States Air Service hand-held and hand-operated plate
camera and on some of the film cameras. It consists of a fixed aperture
curtain wound on a curtain roller in which the spring can be set to
various tensions, numbered 1 to 10. The range of speeds attainable is at
best about three to one, or from 1/100 to 1/300 second, considerably
shorter than the range indicated as desirable. Its uniformity of travel
is variable with the tension, as shown by representative performance
curves in Fig. 30. Lacking any self-capping feature the shutter is
provided either with an auxiliary curtain, or in the hand-held camera
with flaps in front of the lens, opened by the exposing lever before the
curtain is released (Fig. 39). This shutter is made a removable unit in
the 18 × 24 centimeter hand-operated camera, but is built into the
hand-held and film cameras.
[Illustration:
FIG. 26.—Removable four-slit shutter of German (Ica) camera, showing
flaps.]
_The Ica shutter_ used on the standard German aerial cameras is a good
example of the multiple slit curtain (Fig. 26). Four fixed aperture
slits are provided, with a single tension, the openings roughly in the
ratio 1, ½, ¼, ⅛, which when the spring tension is properly adjusted
give exposures of 1/90, 1/180, 1/375, 1/750 second. To pass from one
exposure time to another the setting milled head is wound up to
successively higher steps or else exposed one or more times without
resetting, depending on the direction it is desired to go. Capping
during setting, or during exposure, in order to change the opening, is
provided for by a pair of flaps on the shutter unit, which open into the
camera body. The mechanical work on these shutters is of excellent
quality, the curtain running with exceptional smoothness. Provision is
made for adjusting the tension until the marked speeds are attained;
this is presumably done in a repair laboratory to which the shutter only
need be sent, as it is a removable unit. Tests made on one of these
shutters wound to its highest tension are shown in Fig. 30. The marked
speeds are not attained, and there is considerable lack of uniformity
from start to finish of the travel.
_L camera variable-aperture shutter._ The shutter of the L type camera
(Fig. 27) is representative of one of the most primitive methods of
varying aperture. The two jaws of the slit are held together by a long
cord passing completely around the aperture, fastened permanently at one
end and attached at its other end by a sliding clasp or saddle. As this
saddle is forced in one direction the slit is closed, in the opposite
direction the cord becomes slack, and after the shutter is released once
or twice the slit assumes a wider opening. A chronic trouble is the
breaking of the cords. Its opening can be changed only after the plate
magazine is removed.
[Illustration:
FIG. 27.—“L” type camera showing open negative magazines and shutter
mechanism.]
_U. S. Air Service variable-aperture shutter._ This shutter is
incorporated in the American deRam and in other late American cameras
(Fig. 28). Its characteristic feature is the introduction of an idler,
whose distance from the main curtain roller can be varied. Tapes whereby
the following curtain is attached to the spring roller pass over this
idler, and by changing its position the aperture or distance between the
two curtain elements is altered over a large range. Tests of this
shutter are shown in Fig. 30. A speed of 1/50 second is provided for by
a slit width of five centimeters, and the highest speed is fixed only by
the practical limit of approach of the jaws. Experiment shows great
uniformity of rate of travel to be attainable by combining careful
choice of spring length and tension with good workmanship in the
mechanical features. Variable-aperture fixed-tension shutters have a
definite advantage over the variable-tension type in that they can
utilize for all speeds that tension which gives uniform action. The
capping feature of this shutter is provided in the American deRam by
flaps, in the automatic film camera by an auxiliary curtain. The shutter
is removable in the deRam, but built into the other camera.
[Illustration:
FIG. 28.—Variable aperture curtain developed in U. S. Air Service, and
used in American deRam, and “K” type automatic film cameras.]
_The Klopcic_ variable-tension, variable-aperture, self-capping shutter
is an example of an attempt to meet all shutter requirements with an
entirely self-contained mechanism. It is shown diagrammatically in Fig.
29. Tapes _G__{1}, _G__{2} are used to connect the following curtain _B_
directly to the spring roller _T_, at a fixed distance, while the
leading curtain, _A_, may be slid along the tapes by small friction
buckles, _C__{1}, _C__{2}, auxiliary springs _R__{1}, _R__{2} serving to
keep it taut in any position. When the shutter is being set the buckles
are arrested against stops while the winding-up continues for what is to
be the following half of the curtain in exposing. When released the
curtain moves across with an aperture fixed by the point of setting of
the buckle stops. At the end of the travel the buckles are arrested by
other stops, while the following portion of the curtain continues its
travel to the end. On re-winding, therefore, the aperture is closed.
Variable tension as well as variable aperture is provided, although
little used. In the French cameras a lens flap is also inserted behind
the lens, but this is not needed if the self-capping feature functions
properly. On the hand cameras this flap is said to be necessary in order
to prevent a curious kind of accident: if the camera is held on the
knee, pointing upward, an image of the sun may be formed on the curtain
and burn a hole through it.
[Illustration:
FIG. 29.—Mechanism of Klopcic variable aperture self-capping shutter.]
The performance of the French shutter in respect to uniformity has
already been shown in Fig. 24. It leaves very much to be desired.
Besides non-uniformity of action during its travel it exhibits another
common defect of variable-tension shutters, namely, the curtain must be
released several times after a change of tension before the new speed is
established (Fig. 30, tensions 5 and 5´).
[Illustration:
FIG. 30.—Performances of various shutters used on aerial cameras.
Speeds expressed in reciprocals of fractional parts of one second.]
The French shutter as made for the deMaria cameras is a removable unit.
The small size (13 × 18 cm.) sets by the straight pull of a projecting
pin, the larger (18 × 24 cm.) by winding up a milled head. The former is
the more convenient motion for an aerial camera. Care must be taken with
either type that the motion of setting is not stopped when the first
resistance is encountered; this occurs when the tape buckles strike
their stop and the slit begins to open.