Page:EB1911 - Volume 21.djvu/542

Rh tensions of the springs. Thus the new Goerz-Anschutz shutter has ten tensions and nine curtain aperture providing for ninety different speeds or exposures, ranging from to  of a second, besides autobulb exposures from to 5 seconds and time exposures (fig. 64).

. 64.—Goerz-Anschutz Focal Plane Shutter.

Most of these shutters are now provided with a self-capping device for protecting the sensitive plate during the setting of the shutter. As the slit moves progressively over the plate, if it is too narrow or moving too slowly, it may cause distortion of the images of quickly moving objects, especially if near file camera, but with due care in regulating the width of the slit and the duration of exposure this is practically not often perceptible, especially if the slit is arranged to move in the same direction as the object.

The theory of these shutters is discussed by Coventry (op. cit. p. 69), more fully by Fabre (T. E. P. Suppl., C. p. 128), and their practical use in Focal Plane Photography (“Photo-Miniature Series,” No. 77, 1907).

7. Moving Blade Central Shutters.—These shutters, in which two thin metal or ebonite plates or opaque curtains with round or rectangular apertures, or in other cases two curved blades, pass very quickly over each other in opposite directions, are largely used in many patterns fitted between the lenses of a combination or attached to them in front or behind. Formed of two single lateral shutters opening and closing in the centre of the lens aperture, they become central, the exposure taking place during the short period in which the openings are passing each other or the curved blades opening out and closing again. To obtain the greatest efficiency the size of the openings should correspond with the full aperture of the lens. If each plate moves as fast as a drop shutter the combination gives double the speed, corresponding to half the exposure. The sensitive plate will be most evenly and strongly illuminated when the leaves of the shutter work inside the lens near the diaphragm, as in Bausch and Lomb’s “Unicum” and other similar between-lens shutters, in general use (fig. 65). This the shutter, but necessitates the fitting of the lens to with adapters it is possible to fit other lenses.

. 65.— Bausch and Lomb’s “Umcum” Shutter. . 66.—Lancaster’s “See-Saw” Shutter. Some forms are, however, suitable for use in front of the lens, such as the “Constant” and Lancaster’s “See-Saw” (fig. 66), while those of the double roller-blind type can be used either in front of or behind the lens, though this position is not a favourable one. In these the rectangular form of aperture is the best, circular apertures cutting off a good deal of light, as in the case of drop shutters W. B. Covent (op. cit. p. 60) has discussed the action of the double roller-blind shutter as typical of the central class of shutters, and shows that while, under similar conditions, with the lateral shutter the effective exposure is constant and the actual exposure variable at all apertures, it is the reverse with the central shutters, and it will not be so easy to calculate exposures with different sized stops. A central shutter, acting as a diaphragm of variable aperture, gives a more efficient exposure than a lateral shutter of the same dimensions, as long as the opening is greater than the lens aperture, the coefficient of illumination of the lens varying as long as the shutter opening is smaller than that of the diaphragm used it is desirable, therefore, to increase the speed and use as large an aperture as possible, so that the diaphragm used may be entirely uncovered during the greater part of the exposure.

8. Iris Shutters.—These are a further development of the double curved blade central shutters, and constructed on the principle of the “Iris” diaphragm, with several leaves opening out from the centre of the lens and closing again. They are usually fitted between the lenses of double objectives, and can be made very light and compact. Theoretically this central position of the shutter is the best, and the “Iris” is the best form for ensuring the most equal distribution of light over the plate, provided, as before, that the opening is equal to the full aperture of the lens.

67.—Goerz’s “Sector” They are made so that the periods of opening and closing may be as short as possible compared with that of full opening. They require great care in construction and fitting to the lens, and so are expensive. They can, however, be used with convertible sets of lenses of different foci fitting the same mount. Several forms are made by British and foreign makers, with three, four or more leaves. Goerz’s "Sector" shutter (fig. 67) may be taken as a type. Georgen’s “Central” shutter is very light and smooth in working, and can be used in front of a lens for telephoto work. Further details regarding the different forms of shutters, theoretical and practical, will be found in the works by Abney, Coventry, Eder, Fabre and Londe.

Exposure Meters.

When gelatin dry plates came into general use, and were made of many different degrees of sensitiveness, the want of a guide to the (proper exposure or the various makes of plates under different conditions of lighting began to be felt, and several methods were devised for meeting it. Some of them depend solely upon data derived from observations of the action of the principal factors affecting the result, namely: (1) the speed of the plate; (2) the actinic power of the sun’s light for the time of year in a given latitude and its position at the particular time of day; (3) the effective diaphragm aperture of the lens; (4) the nature of the subject and its illumination as affected by local and atmospheric conditions. With others these data are supplemented by, and practically based upon, actinometric observations of the action of the light upon sensitive paper exposed near the camera or the subject at the time. Both methods are in many cases of undoubted use, but the information given by instruments of this kind can only be considered as approximate, and much is left to the judgment of the operator, whose surest guide will be an intelligent study of the principles on which these instruments are based, together with Carefully-recorded observations of the combined working of his lenses, shutters, plates and methods of development under the varying conditions of practical work. Before using any of these instruments it is necessary to know approximately the relative sensitiveness or “speed” of the plate in use. In the early days of gelatin dry plates their rapidities were stated as so many times those of wet plates, or (as they are still) “ordinary,” “instantaneous,” “rapid” or “extra-rapid,” terms which, though suitable for one make of plate, may not be so for others. This was improved upon by the adoption, in 1878, of Leon Warnerke’s “Sensitometer,” which was in use as a standard for some years. It consisted of a transparent scale of 25 squares of different intensities, marked with opa ue numbers and arranged so that each third number indicated a douliled rapidity. This was placed in a frame in front of the sensitive late, and exposed for thirty seconds to the constant light emitted by a phosphorescent tablet, supplied with the instrument, which was previously excited by burning one inch of magnesium ribbon in front of it. The exposed plate was then developed and fixed, and the highest number visible indicated the rapidity of the plate. In 1890 F. Hurter and V. C. Driffield introduced an entirely new system of calculating the sensitiveness of plates of different rapidities. They make a series of exposures in seconds on different parts of the plate in geometrical progression with a standard candle at one metre distance. After development for a certain fixed period with a standard develo er, fixing, washing and drying, the “densities” or logarithms of the opacities of the different parts are measured by a special photometer and lotted on a skeleton diagram, producing a curve, one portion of which will practically be a straight line. The position of this line with reference to a scale of exposures given on the diagram decides the rapidity of the plate, while its length indicates the “capacity” of the plate for the truthful rendering of tone. The elaborate investigations by which these results were obtained are of great interest, and were published in the Journal of the Society for Chemical Industry (1890, 1891), and later ones in the Photographic Journal (1898). A complete account of the system by V. C. Driffield was published in 1903, as No. 56 of the “Photo Miniature Series.” The sensitiveness shown on the H. & D. scale is directly proportional to the speed number given. The method has been adopted by several dry-plate makers in 