SHUTTERS 127. Preliminary Remarks. Although in the earliest days of photography the word shutter was used to signify the cap of a lens, and then at a later date the different arrangements (flaps, etc.) worked directly by hand, the word is now used to describe any mechanism by means of which light can be allowed to pass through an aperture during a certain definite time.' Except for very special applications, the question of a special shutter never arose until the advent of very sensitive photographic emulsions and very large-aperture objectives, since it was unneces sary as long as exposures required to give a good image were of the order of a minute.
Certain shutters of very simple mechanism, the motive power of which is obtained from air compressed by squeezing a rubber bulb or by the pressure of a finger on a trigger, work auto matically at each pressure without the necessity of any special preliminary operation (Everset shutters). But in the case of shutters controlled by springs, and these are the only ones which allow light to pass for a very short time, the driving springs must be set every time an exposure is to be made.
128. Different Positions for the Shutter. If we consider the luminous beam which, after passing through a lens, forms an image in the plane PP, we see at once that the cross-section of the beam taken as a whole is smallest at the diaphragm DD (this is actually the aperture of the diaphragm), and that on the other hand each beam, considered separately, has its minimum section in the actual plane of the sharp image.
The velocity of the moving parts of a shutter being limited according as these parts start from rest and stop as soon as the aperture is closed again (excepting some special types of shutter for aerial photography), it is obviously of im portance, in considering very short exposures, to uncover successively the different beams where they have the least section, e.g. by placing an opaque screen pierced by a narrow slit which can sweep over the whole surface of the image, as near as possible to the image in the position indicated by (r) in Fig. 95.
In using such a focal-plane shutter we must distinguish between the local time of exposure, which is the duration of the admission of each of the beams,' and the total time of exposure.
this being the time taken for the slit to move across the whole of the image (the direction of movement is generally chosen parallel to the short sides). During the total exposure the relative positions of the camera and the subject may change, and if this happens the different parts of the image will not correspond to one and the same phase of the movement. It will be seen in studying this type of shutter that the image deformations which result are generally negligible, except in cases of extremely rapid movement of the object or the camera. In such cases neither will any other of the usual types of shutters give a sharp image.
For exposures of one-hundredth of a second or more, it is generally better to allow the different beams used in the formation of the image to enter the camera simultaneously, and it is therefore better to place the shutter in the immediate neighbourhood of the diaphragm, as at (3), in such a way as to allow the construction of the most compact shutters. As the local exposure time is here the same as the total time, a sharp image will not be deformed, and the exposure received at each point of the sensitive film will be roughly proportional to the illumina tion at that point if there were no shutter.
For reasons of convenience (easy adaptation of a shutter to any lens and any camera without having it specially fitted, greatest liberty in the choice of mechanisms, which can thus be of extreme simplicity, etc.), the shutter is some times placed at other places than those indicated above, behind, or in front of the objective in one of the positions (2) or (4). In these positions the shutter suffers from the disadvantages of both the focal-plane shutter and the diaphragm shutter. The local exposure time is no longer equal to the total exposure, whence arises the possibility of the deformation of a sharp image (the risk of this, however, is considerably less than with the focal-plane shutter) and the risk of inequality in the local exposure times (some times done purposely in photographing a land scape in order to decrease the exposure of the sky).