THE QUESTION OF LIGHT AND ILLUMINATION
Thus far we have gained a general understanding of the different papers
and the characteristics desirable in negatives. Before we take up the
actual manipulation of bromide paper there are a few elementary principles
bearing on the important detail of illumination which we must master.
These may necessitate a little thinking, but a practical grasp of them
will make our after-work much easier, and ensure that fairly good prints
from poor negatives will be the rule instead of the exception.
In the first place we have often read that a strong light overcomes
contrasts, while a weak light increases them. Yet how many of us realize
when we come to make prints by any process exactly what this means; in
other words, how many of us apply the rule in everyday practice? It is
very easy to see what is meant by the rule if we will take an ordinary
negative, such as a landscape with clear sky, and hold it first six inches
from a gas-flame and then six feet. It will be found in the first case
that the sky portion is translucent while the clear glass will, of course,
be clear; in the second the sky will be opaque and the clear glass still
clear. The contrasts have been rendered greater by removing the negative
further from the light-source. As this is true in the extreme case given,
so it is true in a smaller degree where the distances are only slightly
varied, as well as where we deal with the graded portions of the negative
instead of with only clear glass and the densest portions. It is this fact
that we utilize in bromide printing; and it is because we have such
unlimited control over the strength of our light that it is possible with
it to get equally good prints from a wide range of negatives. It is very
much simpler and more practicable to regulate the strength of the light by
increasing or diminishing its distance than by interposing sheets of
paper, ground glass, or opal, as is occasionally done with other
processes.
The necessity, however, for occasionally changing the strength of our
light in this manner may seem to introduce an element of uncertainty into
the problem of exposure; but there is another rule which brings it back
again to simplicity itself, and enables us to quickly calculate equivalent
exposures at varying distances from the light-source. This rule is: "The
intensity of illumination varies inversely as the square of the distance
from the source of light." For instance if a given negative requires five
seconds exposure at one foot from the light, it will have an equivalent
exposure if exposed for twenty seconds at two feet, the square of one
being one, and of two being four.
It remains then only to apply these two rules to our actual work with
bromide paper. The shadows in a certain negative will receive full
exposure, say, in eight seconds at one foot from the light; but the high
lights of the negative are so dense that no light will penetrate them at
that distance from the light in that length of time. Hence a stronger
light must be used, or the action of the same light continued for a longer
time; but the latter will not do since the effect would be to over-expose
the shadows. Hence, knowing that a strong light overcomes contrasts, we
move the negative to the distance of six inches, where the rule tells us
the equivalent exposure will be one-fourth that at twelve inches, in this
case two seconds. Here the shadows get no more light, but it is possible
that the high lights of the negative will be penetrated by reason of the
additional force of the light.
On the other hand we have a thin, flat negative requiring for the shadows
two seconds exposure at one foot from the light. Knowing that a weak light
increases contrasts we move the negative three feet from the light, and
instead of two, give eighteen seconds exposure, the rule telling us that
this is equivalent. Thus we are enabled to regulate the strength of our
light to suit the character of our negative. But a standard distance of
one foot will not suit with all kinds of lights or with all sizes of
negatives. If, for instance, our light is a Welsbach burner, giving an
intense and comparatively white light, we will find that a normal negative
will print too flat if exposed at one foot. In such a case two or even
three feet would be a better standard. Experience with our light will,
however, furnish the best standard, always taking a standard negative for
the tests.
[Illustration: Fig. 1]
The size of the negative also has its influence on the unit of exposure.
For instance, we may have a half-inch oil-burner, in which case we would
probably have to expose a standard negative at four inches in order to get
the proper contrasts. But this is out of the question with a negative of 5
× 7 or over, as a reference to the diagram, Fig. 1, will clearly show.
Here we find that while the centre of a negative is four inches from the
light the extreme edges will be over five inches from it, the rule as to
intensities telling us that the light at the edges will be only 16/25 of
that at the centre. This would result in a marked falling off of light at
the corners, and would necessitate a constant motion of the printing frame
throughout exposure, which is not wholly satisfactory. The remedy would be
to use a stronger light at a greater distance. But another reference to
Fig. 1 will show that if a 5 × 7 negative be held at seven inches from the
light the difference will be only as 49 is to 56, which can in practice be
disregarded, though it would be better to have it even less. Hence we see
that it is never safe to have our unit less than the base-line of our
plate, and it is better to have it even greater, as we will frequently be
obliged to halve the distance to overcome contrasts. It follows from this
that the larger our negatives the stronger must be our light.
Now all of these considerations may make very dry reading, but the reader
who has followed them closely will see how vital they are to successful
work. It should not be thought, however, that every exposure on bromide
paper must involve an arithmetical calculation. On the contrary, once the
proper distance from the light for the normal negative has been
ascertained, it will be found that nine negatives out of ten will require
no change in the distance from the source of light. This, of course,
presuming that we classify our negatives and enlarge from those of the
same quality at the same time.
One great objection to the use of bromide paper is that it must be handled
in a dark-room. But this objection is not as serious as it may seem. An
ordinary living room at night furnishes a delightful place in which to
make prints, if we handle our solutions with reasonable care. The ruby
glass can be removed from the dark-room lamp, and the orange glass used
alone. But in this case, as indeed with the ruby light, care must be taken
to guard against too much light. Development should be conducted at a
distance of several feet from the light, and when almost completed, the
tray can be brought close under the light to enable the worker to stop it
at exactly the right moment. Ordinary bromide paper is about as sensitive
as the process or slow dry plate or the average lantern-slide plate, and
requires as much care as either, but not nearly so much as the most rapid
dry plates. If fogging is noticed, of course additional precautions should
be taken at once.