The considerable increase in the efficiency which is brought about by increasing the length of the opening is unfortunately accompanied by a still more rapid increase in the time of ex posure, since the actual velocity of the shutter cannot be made very much greater than about 2 metres a second.
These numerical values hold also in the case of guillotine shutters having a rotating move ment about a centre coinciding with the apex of the angle of the sector opening.' In all other cases the efficiency increases as the apex is nearer to the diaphragm and the pivot farther away from it.
The rotating guillotine shutters of Lancaster (Fig. Ica) and Bertsch (Fig. 102), with effi ciencies of o-5o and 0-70 were used considerably at one time, but they gave exposure times much longer than those usually required in practice.
There should also be included in this category rectilinear or rotating guillotine shutters in which the moving part returns to its original position at the end of the exposure. If this type of shutter is used in front of, or behind, the lens, it gives one region of the plate a longer exposure than another. This fact has been made use of in landscape photography, the shutter being placed on the lens hood and opening from the bottom under the action of compressed air from a rubber bulb, and falling again under its own weight.
133. Roller-blind Shutters. A variation of the drop or guillotine shutter is the roller-blind shutter, which is often used in a great number of portable cameras. The shutter consists of a flexible blind wound on rollers at each side of the aperture (Fig. 103). This shutter, the use of which appears to have been first suggested by Relandin in 1855, was developed to its present form by Kershaw. It is always used either in front or behind the lens, the latter position being more convenient and one which facilitates changing the lens without changing the shutter.
A perfectly opaque blind of rubberized cloth is made of two bands in line with one another, and joined at their edges by bands in such a way as to leave between them an opening the height of which is usually one and a half times its width. This blind, which may be regarded as a single one with central opening, is wound at its lower edge on a roller containing a spring inside. To set the shutter the blind is wound on to the upper roller, either by pulling a small cord which is wound round a grooved pulley forming part of the roller, or by means of a small key projecting laterally from the roller.
The tension of the driving spring may be changed by means of a milled head situated at the lower part. 2 When this head is turned a dial is engaged on which is indicated approxi mately the corresponding time of exposure (for a shutter of about 2i in. opening, the exposures
usually vary from rirsth to 1/20th second, the speed scale indicating exposures spaced from r/ioth to /5oth second). The spring should never he left in tension when the camera is not being used. In order to release it, lower the spring catch, at the same time braking the milled winding pinion with the fingers, so as to prevent the spring from running down too violently.
A side lever can be moved to one of two positions, one corresponding to a continuous unwinding of the blind. At its other position the blind is stopped halfway during the whole time that pressure is maintained on the shutter release.
Several of this type of shutter have been fitted with an auxiliary blind used to cover the aper ture while the main blind is being set, but this is not generally done.
The efficiency of this shutter is independent of the degree of tension of the spring and is the same as that already given for the straight edged drop allowing for the fact that the motion is not uniform. This efficiency will be related to the local exposure time, the diameter to be considered being that of the effective aperture of the lens when the shutter is placed in front of the lens, or a diameter slightly less than that of the diaphragm when it is placed behind. The efficiency is slightly greater in this hitter position.
134. Modern Types of Guillotine Shutters. Shutters embodying the principle of the guillo tine shutter with rectilinear movement are used on many types of hand cameras, but in order that they shall have a reasonable efficiency and at the same time be fairly compact, the guillo tine is formed of two thin steel plates one of which covers the lens before, and the other after, the exposure. The time between the first contact of the first plate and the second contact of the second one with the aperture to be uncovered corresponds with the time of passage of the opening in a shutter of the simple guil lotine type. For long exposures, the two plates are worked independently ; for very short or instantaneous exposures the second plate is automatically released by the first one on its arrival at the end of its travel, or the first plate acts on an intermediate member which allows the second plate to move only after a pre determined time. When setting this type of shutter, the two plates either move in such a way that the lens is always covered, or act in conjunction with an auxiliary plate, which, having covered the lens during setting of the shutter, returns to its original position and takes no further part in the shutter's normal working.