CHAPTER V

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.

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.

=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.

_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.

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:

_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.

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.

_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.

_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.

_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.

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´).

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.