A certain number of the readers of this journal are occupied with
photography, and all assuredly are interested in this marvelous art,
whose progress is so remarkable. So it has seemed to us that it would
be of interest to treat of a question that is the order of the day. We
desire to speak of those photographic apparatus called instantaneous
shutters.
Numerous apparatus of this kind have been proposed to the public, and
several even have been described in this journal, but we have to state
that, despite the success in certain cases, none of them has proved
remarkable for its qualities and superiority. This is due, we believe,
to the fact that inventors, while showing arrangements that were often
ingenious, have not always taken into account the end that the shutter
is to subserve, and the qualities that it must possess in order to
attain such end.
In face of the progress made by extra rapid dry processes, the question
of shutters has become the most important, since cabinet-making, optics,
and photographic chemistry give us apparatus, objectives, and products
which, although they will doubtless be improved upon, satisfy for the
present all our needs.
What is understood by instantaneousness? To our knowledge, no definition
thereof has as yet been given. For our part, we propose to style
"instantaneous" any photograph that is taken in a fraction of a second
that our senses will not permit us to estimate. The shutter is the
apparatus which allows the light to enter the photographic chamber
during this very short time.
In order to examine the different rules that govern the question of
shutters, we shall take as an example the type styled the "Guillotine."
This apparatus, as every one knows, is a stiff plate containing an
aperture and passing over the line of the rays of light. Some place
it in front and others behind, while others again place it within the
objective. Let us examine and discuss what occurs in the three cases.
Suppose a rectilinear objective of the kind most usually employed in
instantaneous photography, and an object, A B, that we wish to reproduce
(Fig. 1), the objective being provided with any sort of diaphragm. The
point, A, sends a bundle of rays, a"b", to the first lens. Here they are
slightly refracted, and then go on parallel lines to the second lens,
where they are again refracted and form at A' an image of A. It is this
image that we see upon the ground glass, and which makes an impression
upon the sensitive film. The point, B, behaves in the same way and
gives an image at B', but, as will be at once seen, the image will be
reversed. In our figure, A corresponds to the sky and B to the earth.
If, then, the shutter passes in front of the objective, it will first
allow of the passage of the rays which come from the sky, then, on
continuing its travel, it will unveil the landscape, and lastly the
ground. As it is submitted to the law of the fall of bodies and has a
uniformly increasing velocity, it follows that the time of exposure will
uniformly decrease between A' and B', and that the sky will pose longer
than the foreground. Such a result is contrary to all photographic
rules, which require that objects shall pose so much the longer the
less they are lighted. This position of the "guillotine" shutter is
absolutely false, and must be altogether discarded. If the shutter be
placed behind the objective, it will follow, as a consequence of the
same demonstration, that the time of exposure will go diminishing from
B' to A', and that the foreground will be exposed longer than the sky.
The solution is logical, then, and will permit of obtaining excellent
negatives.
[Illustration: FIG. 1]
Let us now examine how the image, A'B', is formed. The point, A, appears
first, and becomes lighter and lighter up to the moment at which all the
rays that emanate from the point, A, are unveiled. The point, B', is not
yet visible. As the shutter continues its travel the point, B', appears
in its turn and becomes illuminated like the point, A'. At this moment
the objective is completely uncovered; the image, A'B', is perfect, and
possesses its maximum intensity. Then the point, A', gradually becomes
obscured and disappears; and the same is the case with all parts of
A'B'. The image is developed progressively from A' to B', and makes its
impression upon the sensitive plate successively--a fact which, as
may be conceived, may have its importance. If, for example, we are
photographing a ship that is being tossed about by the sea (and we
borrow this example from our colleague, Mr. Davanne), the image of the
top of the mast will not be formed at the same instant as that of the
base, and if the motion of the mast has sufficient extent it may take on
a curved form, due to the fact that it has effected a movement between
the moments during which its apex and base were being photographed.
Upon placing the guillotine shutter in the optical center of the
objective, what will occur? The shutter will permit the passage of an
equal fraction of the rays derived from A and B, that is to say, the
image will be complete from the first instant of the exposure. The
points, A' and B', will be illuminated precisely at the same moment. As
the shutter continues its travel, a fresh quantity of rays coming from A
and B will be admitted, and the image will be illuminated more and more
up to the moment at which all the rays can pass. It will then possess
its maximum intensity. Then a portion of the rays from A and B being
intercepted, the image will become darker and darker until complete
extinction. The image here, then, is not produced successively as in the
former case, but is entire from the beginning. In this case the image of
our mast cannot be misshapen, since it has been accurately photographed
at the same moment.
The true place for the guillotine shutter, then, from a theoretical
standpoint, is in the interior of the objective. Are there any other
advantages to be gained by so placing it? Yes; it is easy to understand
that for the same time of exposure, and consequently for the same
result, the aperture may be so much the smaller in proportion as the
optical center is approached.
The luminous rays, in fact, form in the objective a double truncated
cone whose upper base is equal to the diaphragm, and the lower one to
the diameter of the lenses. If the aperture be equal to any diameter
whatever of one of the cones, the result will be the same; but, for
the same period of exposure, it will evidently prove advantageous to
approach the diaphragm. The ratio of the apertures that give the same
results at the optical center or behind the objective is as that of the
diaphragm employed to that of the back lens. If the diaphragm is
one centimeter and the lenses four centimeters, an aperture of one
centimeter in one case and of four in the other will give the same
result.
We shall see further along that it is advantageous to employ apertures
equal to several times the diameter of the diaphragm or lens. Now, from
what we have just said, an aperture, equal for example to four times the
diaphragm, will be only 4 centimeters, while the corresponding aperture
behind the lens must be 16. The dimensions of the first will be
practical, and those of the second will give too cumbersome and too
fragile an apparatus. But why must the aperture be larger than the
diaphragm employed? This is what we are going to demonstrate. Let us
make the aperture equal to the diameter of the objective, and see
what occurs at the different periods of the exposure. For the sake of
clearness, we shall suppose the velocity uniform.
It is evident, _a priori_, that a perfect apparatus will be the one that
will allow the light to act during the entire exposure with a maximum of
intensity. Is it thus, when the aperture is equal to the diameter of the
objective? Evidently not. Let us consult Fig. 2. We here see the shutter
progressively uncovering the objective. The light will increase from A
to C up to the moment when the objective is entirely uncovered, and will
then immediately decrease up to B. The objective has operated with a
maximum of light for only a short time. We are far from the ideal result
in which the maximum of light, CD, should exist during the entire
exposure, and form the upper plane precisely equal to AB.
[Illustration: Fig. 2.]
If we cannot obtain such a result in practice, we must nevertheless
aproximate to it. We shall do so by increasing the shutter. Up to C' the
apparatus will operate as before, but from C' to D' the aperture will be
complete, and from D' to B' will decrease as has been said.
Let us give A'B' the same value as AB, that is to say, let us increase
the velocity in the second case in order that the time of exposure shall
be the same; we shall at once see that in the first case the object will
be completely uncovered for only a very short time, while in the second
the exposure will be perfect for a very appreciable period.
The time of exposure which is absolutely active, we propose to call
effective time of exposure in contradistinction to the total time of the
same. The more we increase the value of C'D', that is to say, that of
the effective time, the more the ratio, C'D'/A'B', will approximate to
unity, and the nearer we shall reach perfection. The correlative of such
elongation of the aperture is an increase in velocity which will always
bring the total exposure to the same figure, whatever be the aperture
employed.
If the aperture be equal to two diameters, the effective time will be
equal to half the time of the total exposure; and if it is equal to
three diameters, the exposure will be good during 2/3 of the total time.
This amounts to saying that the effective time of exposure is equal to n
times the diameter--1, the velocity being supposed always uniform. If
we place the shutter within the objective, it is the diameter of the
diaphragm that it will be necessary to say. The effective time will be
equal then to n diaphragm--1.
From what precedes it results that in no case should the aperture be
inferior to the diaphragm, since the former would otherwise absolutely
suppress the effective time in giving a lower plane corresponding to
an insufficient quantity of light. Moreover, an aperature of this kind
would prove injurious to the quality of the image by successively
uncovering rays which do not form their image identically at the same
point. We are now, then, in presence of results that are absolutely
positive, and they are as follows:
1. The guillotine shutter should be placed in the interior of the
objective and as near as possible to optical center, that is to say,
behind the diaphragm, since the latter is precisely in the optical
center.