According as to whether the release must be kept pressed down during the whole exposure, or whether it can be released as soon as the shutter is opened and be pressed again for closing, we say that the shutter gives bulb exposures or time exposures. On many shutters the positions of the index corresponding respec tively to these two methods of working are arbitrarily indicated by the letters B and T, the first letters of the words bulb and time. On others which only allow time exposures the positions are denoted by T and I, standing for time and instantaneous, this latter word being used to denote all exposures less than one-tenth of a second. 2 131. Summarized Description of Some Types of Shutter. The number of different types of shutter which exists is far too great for them to be described here ; it will, however, be possible to indicate the chief peculiarities of some characteristic types, chosen for preference from those most commonly used.
As far as possible in the descriptions which follow, the historical order will be taken, so that the reader may be able to follow the evolution of the shutter, and to compare it with the progress of photographic technique.
132. Simple Drop Shutter. In its simplest form, the guillotine, or drop, consists of an opaque screen (having a cut-out portion at least however, be reduced to a single opaque sector, making a complete revolution each time.
The opening was originally a circle of the same diameter as the aperture to be uncovered, but in 188o Joubin showed the advantage of having an opening of which the edges actually responsible for the opening and closing were straight.
The efficiency of the guillotine type of shutter for different shapes of the opening was studied by J. Demarcay (1891), whose conclusions are given below.
In the case of guillotines having a rectilinear movement and openings of one of the forms A, B, or C (Fig. 99), the efficiency for differ ent relative proportions of the height a of the opening to the diameter d of the aperture will be expressed by the values given in the following table, assuming the screen in the plane of the diaphragm, and given a uniform movement.
equal in size to that of the aperture to be uncovered), falling under its own weight or helped by some kind of spring which causes it to pass rapidly before the lens (Fig. 97). Such an arrangement was used in 1845 by Fizeau and Foucault for photographing the sun, but it was about 1855, after the introduction of the wet collodion process, that it was generally em ployed, first in the form of a very clumsy arrangement fixed to the lens hood, the velocity of fall becoming less according as the slope of the frame in which it was mounted was less.
It afterwards appeared in the form of a metal plate passing between the lenses of the objective and helped by an elastic band (Jamin, 1862).
Between these times (1858) the real guillotine or rectangular guillotine had been replaced by a circular one (Fig. 98), much less clumsy, in which the old movement (which, geometrically speaking, is a rotation around an axis at an infinite distance away) was replaced by a rota tion around an axis situated a short distance from the circumference of the mounting, the cut-out sector being used either in front of the lens hood or in the immediate neighbourhood of the diaphragm. The opaque screen may, The opening with convex edges (form C) gives a greater efficiency, but gives predomin ance to the influence of the marginal rays ; the opening with rectilinear sides is preferable. Considering specially this latter case, it is easy to trace the effect of the length of the opening on the efficiency. In Fig. 100, where distances measured from A along the line AD are pro portional to the time from the instant when the aperture begins to be uncovered, and where distances along AB are proportional to the areas of the aperture uncovered, the curve AGH corresponds with the opening period and JKD with the closing period of the aperture, whilst the straight line gives the time during which the aperture is fully uncovered. The efficiency is equal to the ratio of the total shaded areas to the rectangle ABCD. In the case of a uniform, movement the curves AGH and JKD are sym metrical about their mid-points G and K. The area of each of the curved triangles AHL and MJD is therefore equal to that of the rectangles A'BRIL and MJC'D'. The efficiency is thus the ratio of the areas of the rectangles A'B'C'D' and ABCD. It will be readily seen that the efficiency becomes greater as the dis tance LM (determining the duration of full aperture) increases.
If, as in almost all shutters of this type, the velocity is not uniform, the efficiency decreases as the difference between the extreme velocities gets greater. In the case of a movement having a uniform acceleration (a shutter falling freely), he values of the efficiencies of a drop shutter with rectilinear edges fall respectively to o-38 for a = d, and to 0-49 for a = 2d.