Fig. 104 shows diagrammatically how such a shutter works (the driving springs, ratchets, and the shutter release not being shown). In the position of rest (position I), the plate A rests against the stops cc and engages, by means of the pins dd, the plate B, the solid part of which covers the aperture. To set the shutter, plate A is drawn towards the left, and in so doing covers first of all the opening in B, and then drags B with it until it is stopped from going farther by the studs ee (position II). The release sets A free, so that under the force of the driving spring it moves to the right and in so doing uncovers the aperture (position III). After it has passed completely over the aperture and the opening in plate B, it engages the latter and pulls it over so as to cover the aperture again (position IV). The greater the length of the notches in which the stops dd slide in A relative to the diameter of the aperture, the greater is the efficiency of the shutter.
135. Flap Shutters. After different types of flap shutters worked directly by hand had been for a long time in use, J. W. T. Cadett, in 1878, made the first model of such a shutter which was raised by a pneumatic bellows. This very clumsy piece of apparatus was fixed to the lens hood. A much less clumsy model was made by C. Guerry in 188o (Fig. 105), and they are still being made for use by portrait photographers. To Guerry also is due the excellent idea of putting this shutter inside the camera 1 so that in portraiture the sitter may not realize the exact moment when the exposure is made. The flap, which is extremely light, is covered with black velvet, and is fixed to the bottom of a box which is itself lined with velvet in such a way as to be light-tight. The shutter opens by pressing on a pneumatic bulb and remains open as long as the bulb is pressed. For long expo sures, in order to avoid maintaining the pressure continually, a tap which is placed on the tube joining the bulb to the shutter may be closed.
It will be seen from the drawing that the foreground will be exposed longer than those parts of the subject which are situated at a height greater than that of the camera. This may be an advantage in landscape photography so long as it is not necessary to give exposures less than one-third of a second, which is the minimum time which this type of shutter can give.
If we represent by unity the exposure for beams which strike the top edge of the plate, the exposures on the axis and at the lower edge of the plate are respectively o•23 and 0-15, provided that the flap re-descends immediately it has uncovered all the useful field. Since the efficiencies in these three regions are respectively 92 per cent, 83 per cent, and 50 per cent, the illuminations will be 92 per cent, 19 per cent, and 7.5 per cent of what they would have been during the total time of exposure if no shutter were used (H. Wurtz, 1906). This disproportion
is considerably less if the shutter is kept fully open for some time ; if, for example, the total time taken in the actual opening and closing is one-third of a second and the duration of full aperture is one second, the relative values of the illuminations become 98 per cent, 8o per cent, and 66 per cent.
Joubert suggested in 188o the use of a second flap connected with the first and remaining always parallel to it. By means of this double flap shutter (Fig. ',Do) considerably shorter exposures may be given, but the illumination is now a maximum in the centre of the plate, where it is about 40 per cent greater than at the upper and lower edges. In the form in which it is often used by numerous portrait photo graphers, the second flap can be disconnected so that the shutter can be used with the single flap only.
Mention must also be made of a type of shutter which combines the simple flap and the roller blind shutter in one (Tauveron, 1919), the latter being automatically released at the moment when the flap gets to its top point.
In this way the two drawbacks of the roller blind shutter in portraiture are obviated, since, on the one hand, the flap prevents the lens being uncovered whilst the blind is being set, and on the other hand, the noise which the blind makes is only heard whilst it is ing, and it is then too late to have any effect on the sitter.
136. Various Other Types Derived from the Flap Shutter. In con nection with this type of shutter there are others which may be mentioned designed chiefly for use behind the lens, and of which only one appears to have survived.
The multi-flap shutter, shown diagrammatic ally in cross-section in Fig. 107, consists of a great number of light flaps or plates pivoted around horizontal axes and just overlapping one another in the two extreme positions. The chief drawback of such a device is that the light emerging from the lens is never completely transmitted. The flaps, being always parallel to one another, allow the central beams to pass almost completely when they are parallel to the optical axis, but they then intercept an appre ciable part of the oblique beams. This is the reason why this shutter has an efficiency not more than about 33 per cent. Although with one shutter of this type (Krauss, 1894) exposures as short as Ii400th second could be made, it was nevertheless soon abandoned.' Two other types of shutter may be mentioned, viz. the rotating plate, placed between the lenses and turning on one of its diameters, a device which resulted in the outer parts of the lens being used almost exclusively, and the " plug " shutter, something like the plug of a tap, with which the central portion of the lens was chiefly used. Both are far too cumbersome to be placed between the lenses of modern objectives.