Page:EB1911 - Volume 21.djvu/541

what is more important, how far they can be de ended on for regularity. There are many simple ways in which the actual time of exposure from opening to closing can be ascertained sufficiently closely for practical purposes. They depend upon the measurement of the trace left on a sensitive plate by the passage of a brightly illuminated object revolving at a known speed or falling vertically through a known distance, when photographed with different speeds of the shutter against a dark background. These, and the more elaborate methods for obtaining more accurate determinations of the shutter-exposure periods and o the corresponding effective exposures —i.e. showing the actual effect of the shutter t rough its different phases from opening to closing—have been described by Sir William Abney in the work already mentioned, by A Londe in La Photographic moderne and La Photographze mstantanée. An apparatus for testing shutters at the National Physical Laboratory was described by J. de Graaf Hunter in the Optician, 1906.

1. Flap Shutters.—The simple flap shutters consisting of a hinged flap opening upwards in front of the lens, though favourites in early days for landscape work, and still useful for intermittent exposures or as sky-shades for securing cloud effects or increasing foreground exposures, have been almost superseded by quicker and more compact forms. They are used with single and double flaps for portraiture and studio work, for which purpose they are made to act noiselessly and not attract the attention of the sitters. Guerry's (figs. 59 and 60) is a good example of the type. W. Watson's “Silent"

Fig. 59.—Guerry's Single-flap Shutter.

Fig. 60.—Guerry's Double-flap Shutter.

shutter is hemispherical in form and collapsible, the two wings opening out an folding together, when actuated by a special “ Antinous ” release, and R. & J. Beck's is another form, a single lifting flap with pneumatic release.

2. Drop Shutters.—The old simple drop shutter, in which a plate having an opening in it falls in front of the lens aperture, has been superseded y the more compact and quicker-working roller-blind shutters, which act on much the same principle. It had a theoretical interest in connexion with the effect o different forms of aperture circular, square, or elongated-used with shutters of the lateral type, but it is now generally recognized that a more or less extended rectangular opening, of at least the full width of the lens aperture, is best for securing the even admission of light from all parts of the image with shutters of the rectilinear lateral type, to which this and similar shutters, in which a single opening passes across the lens aperture, belong. In Busch's “sky shade” shutter (1907), fitting on the front of the lens a single leaf moves vertically upwards and descends again, giving less exposure to the sky.

3. Combined Drop and Flap Shutters.—In early dry-plate days several forms of this kind of shutter were brought out, under the names of Phoenix, Phantom, &c., but are now little used. In these shutters, in addition to the drop slide, there was also a lifting flap, which on release opened from below, and, having fully uncovered the aperture, released the drop slide, which fell and closed the shutter. They were useful and effective in the smaller sizes, but heavy and cumbrous in the larger Sgeed could only be estimated very roughly by the use of india-rubber bands for giving tension.

4. Rotary Shutters.—These are of the latera type, and consist of a circular metal disk revolving on an axis eccentric to the axis of the lens, and furnished with a radial sector-shaged opening, which passes laterally in front of the lens aperture w en the tension of a

Fig. 61.—Rotary Shutter.

spring is released (fig. 61). They are used in various patterns in cheap hand cameras, usually in front of the objective, though they may be placed behind it or between the component lenses. So long as the opening is at least equal to the size of the lens aperture, the illumination is sufficiently even, but the openings are usually elongated so as to give a longer period of full opening. Working by a spring the are more portable and convenient than drop shutters. Beck’s “Celverex " between-lens shutter (1906) is of this type, the disk being revolved by a spring and the variations of exposure obtained by altering the size of the opening passing over the lens aperture, and not the tension of the spring It is speeded for exposures of 1/10, 1/20, 1/40, 1/80 sec.; also “bulb” and “time.” It is fairly accurate and consistent in action, but loses efficiency at the highest speeds by the diminution of the opening.

5. Roller-Blind Shutters.—For general use the well-known roller blind shutter of the single lateral type, as made by Thornton-Pickard and others, is undoubtedly one of the most popular and efficient. It possesses most of the qualities laid down as essential to a good shutter, gives good illumination, appears to be fairly regular in its action and can be used for time or instantaneous exposures. It consists of a light mahogany or aluminium box, arranged so that it can be fitted in front of or behind the objective. It is made in different sizes, and each size can be adjusted to smaller objectives (fig 62). It is also made with a disappearing cord, and in an improved pattern, the “Royal,” all the fittings are inside the box. By pulling the cord

Fig. 62.-Thornton-Pickard Roller-Blind Shutter with automatic exposure appliance.

Fig. 63.—Mechanism of the Thornton-Pickard Roller-Blind Shutter.

A, Upper roller.

B, Lower roller.

C, Cord.

D, Black curtain.

H, Aperture in curtain.

R, Rubber ring adapter.

an opaque black curtain with an elongated rectangular aperture is unro le from the lower roller on to the up er one, and held by a coiled spring on the lower roller (fig. 63). pressure on a pneumatic bulb inflates a second smaller bulb, raising a lever which 'releases the spring, and thus brings the blind down with a rapidity which can be adjusted by turning a handle actuating the spring, the corresponding speed being shown on an indicator. For time exposures, pressure on the bulb opens the shutter, and another pressure closes it, but an arrangement is now made by which time exposures of 1/8, 1/4, 1/2, 1, 2, 3 seconds can be given automatically, the pressure of the bulb opening the shutter, which closes of itself at the expiration of the exposure required. The theory of shutters of this type has been very frilly discussed by Coventry (op. cit. p. 50), who shows that for any given tension of the spring the actual exposure decreases as the size of the lens aperture diminishes, while the effective exposure remains constant for all apertures. This is peculiar to the lateral shutter. He also shows that with plates of very different rapidities, though the exposure may be the same, the actual exposure effective is less with the rapid plate and a small stop than with the slow plate and a large stop; consequently the blur due to the movement of the object would be proportionately less on the ra id plate than on the slow one. Also that or any given lens the smaller the shutter the more rapid the exposure can be made, though with the same lens a larger shutter is capable of giving a more efficient though less rapid exposure. It is better, therefore, for moderate exposures, to have a larger shutter than the size of the lens requires. Sir William Abney had given diagrams of the action of a shutter of this kind in his book referred to; they show clearly that the centre of the plate gets more exposure than the margins; but practically this is not very noticeable, and the action is very regular.

6. Focal Plane Shutters.—These are also roller-blind shutters with mechanism similar to the foregoing, but arran ed so that the slit in the curtain may move rapidly close in front of the sensitive plate, exposing different portions of it in turn, the intensity of the exposure being regulated by the width of the slit, whether adjustable or not, and the rapidity with which it is moved by the unwinding of a spring. The advantages of these shutters are now being fully appreciated, the principal being that they are quite independent of the lens, so that one shutter will serve for different lenses, and an suitable lens may be used at its full intensity, without the loss of, efficiency inherent in the ordinary forms of lens-shutters. They thus add effectively, if not actually, to the speed of a slow lens, or if a lens be stopped down there is less loss of efficiency, with a gain in increased depth and definition. They are particularly well adapted for the very short exposures required in photographing near and quickly moving objects, racing horses, divers, &c., and many reflex and other hand cameras are fitted with them. They are constructed in different forms, either for short exposures with high speeds alone, or for short and prolonged ex osures; with a sin fe slit of fixed or variable width moved at regulfited speeds, or with a series of slits or openings varying in width, their speeds being adjusted by the