THE QUESTION OF ILLUMINATION
Accustomed to working out-of-doors during the bright days, the
amateur’s first difficulty in attempting photography of any kind
at home is to realize the immense difference between the intensity
(photographic power) of light indoors and out. After making an exterior
view with an exposure of 1/25th of a second, it is incomprehensible
that, coming indoors to photograph a well-lighted room, one must
multiply the outdoor exposure hundreds of times to get a satisfactory
negative. This is why at-home photographs are, usually, so wofully
under-exposed, so lacking in gradation, and so excessive in their
contrasts. There is so much light out-of-doors, and it is so widely
diffused; indoors the illumination is so unequal, and its distribution
so unsatisfactory. The eye readily accommodates itself to the wide
difference between this and that; the lens and plate, on the other
hand, are painfully exact in their statement of it.
The first step toward success, therefore, must be to familiarize
oneself with the altered conditions of illumination. We must know how
light acts when confined, as in a room; and how feebly it penetrates
the shadows thrown by objects in its path within a circumscribed area.
This is the bottom problem in all photography indoors, whether we
use daylight or artificial light. When it has once been mastered,
everything else will be easier, and our percentage of failures will
be materially lessened. This familiarity must come by observation and
experience—well-worn advice, perhaps, but very much needed, as the
average at-home photograph tells us. The simplest way to get this
experience is to make a series of interiors of the home and, as a
contrasting example, an exterior view of the house. This will teach us
many things worth the knowing. First, however, let us see what may be
learned by observation alone. An actinometer will be helpful, in fact,
it is an indispensable aid in all indoor work with the camera. Lest
the strange name make the novice fearful, I hasten to explain that an
actinometer is simply a little instrument which measures the actinic
(photographic) power of the light.
Exposure meters, which most amateurs know about, are actinometers,
fitted with scales or tabulated figures applying what the actinometer
says to give the exposure required under certain conditions, with
this or that plate and lens aperture. These exposure meters save
considerable time and prevent failures. Every amateur should possess
one, and will be richer when he is familiar with its purposes and
practical use.
A simple actinometer may be made at home for experimental purposes.
To make it, procure a few 4 x 5 inch pieces of slow bromide paper.
Immerse each sheet for five minutes in a ten per cent solution (45
grains to each ounce of water) of potassium nitrite (not _nitrate_).
This should be done by lamp or weak gaslight, and the paper dried in
darkness. When dry, cut the sheets up into strips one-quarter by three
inches, and store them in an air-tight box, such as the tin tube in
which platinum paper is sold. Expose one of the strips to diffused
daylight until it darkens to a slaty-blue color, and match this with
watercolors on a piece of white paper. This will form what is known as
the _standard tint_. Now get a piece of thin white glass, and a piece
of cardboard, each one by three inches. At each of the four corners of
the card paste narrow strips of thick paper, so that the card (when,
later, attached to the glass) is slightly separated from it except at
these four raised corners. Across the top of the card, paste a piece
of the standard tint about one-half inch wide. Fasten the card and
glass together with black needle paper so as to exclude all light, but
leaving the ends open. Cut an opening one-quarter by three-quarters
of an inch through the black paper at the top of the glass, so as to
show the standard tint beneath. Now slip a strip of the sensitive test
paper through between the card and glass until it meets or crosses the
standard tint, and the actinometer is ready for use.
If we expose the actinometer to diffused light (at a window), the
sensitive test paper gradually darkens until it matches the standard
tint. By noting, with a watch, the time required for the paper to reach
the standard tint in color, we get a figure in seconds or minutes which
represents the _actinometer time_ of the light at that hour and in that
place. In this way we measure the intensity of the light. If we compare
this actinometer time with the time required to get the standard tint
in another place, where there may be more or less light, we get a
definite idea of the relative intensity of the light in both places.
If, for instance, our first observation gives us the standard tint in
six seconds, and a second observation in another room requires twelve
minutes to give the tint, we know that the light there is 120 times
weaker than at the window where we made the first observation. Hence it
follows that if at the window one second is the correct exposure for
a certain plate and lens aperture, we must give 120 seconds, or two
minutes exposure, with a plate of the same rapidity and the same lens
aperture, in the room where we made the second actinometer test. This
shows the usefulness of the actinometer as a light measurer. If we make
a tour of the rooms in the house between 11 a.m. and 1 p.m., when the
light is fairly constant, the actinometer times given by the tests here
and there with the actinometer, will give us a fairly correct idea of
the relative exposures needed in each room.
The calculation of exposures, however, depends not only on the
intensity of the light, but also upon the character of the subject,
the speed of the plate, and the lens aperture or stop used. Of these
four essential factors the home-made actinometer gives us but one, so
that its usefulness is very limited. What we need is a series of tables
enabling us to apply the actinometer figures to the varying conditions
of each exposure proposed. These calculations are part of all exposure
meters or exposure tables: hence it is better to buy an exposure
meter in the beginning, and save ourselves the worry of calculation
and guesswork. For outdoor work I prefer Todd’s _Tables_ before any
meter; but for indoor work, Watkins’ or Wynne’s meters will be found
more satisfactory. There is little choice between these two useful
instruments, but I have mislaid my Watkins, so we will use the Wynne
meter wherever needed in the preparation of these pages.
Let us take a Wynne meter, and make a few tests about the house in
order to get a good grasp of this important detail of illumination and
exposure. It is a November day, at noon. The sun is high and the sky is
filled with white clouds. We propose to use a rapid plate, about speed
F90 on Wynne’s list. Exposing the meter to diffused light, _outdoors_,
we get the standard tint in 24 seconds. This is called the actinometer
time. Placing the speed number of the plate (F90) against the figure 24
on the dial of the meter, we see at a glance that the exposure required
is 3/16ths second, with the lens at _f_/8. The exposure required with
every other diaphragm is shown on the meter-dial at the same time.
Going _indoors_ we expose our meter in a well-lighted room, facing the
windows, and get the tint in 8 minutes. The exposure indicated with
_f_/8 is 1/16th minute, say 4 seconds; with _f_/16, 15 seconds; and
with _f_/22, 30 seconds. In another room, less abundantly lighted, we
get the standard tint in 27 minutes, indicating exposures of 2 and 4
minutes with stops _f_/22 and _f_/32 respectively. In an upper hallway
the actinometer time is 64 minutes, necessitating an exposure of 7½
minutes with _f_/32, supposing the same brand of plates to be used
throughout.
By this time the sun has come out and we return to the well-lighted
parlor to see whether we dare attempt a group of two children. A light
background and a white side reflector are arranged near a window to
form a small studio. The group will be two feet from the window, and
lighted by the midday sun filtered direct through a white muslin blind.
The plate is an extremely rapid brand, listed by Wynne at F111. Exposed
in the shadow of the group the meter gives us the tint in six minutes.
This indicates an exposure of 2 seconds for a normal subject with the
lens at _f_/8. We remember the rule to multiply normal exposure by 1½
for portraits indoors, and give 4 seconds as a full exposure. Without
sunlight, and with the blind lowered from the top, the actinometer
time is 8 minutes, and the exposure, with _f_/8, is six seconds.
Substituting a subject in dark dress, and a medium background, the
actinometer time is sixteen minutes, and the exposure must be nine or
ten seconds. Or it is night, and we want to photograph the table set
for a small supper party before the guests arrive. We desire only the
table and its arrangement; all else may be hidden in shadow. It is
lighted by a four-jet chandelier, and on the table is a befrilled lamp
with a pink shade. We expose the meter, facing the light source, in
the shadow of the rose-bowl near the centre of the table. Using the
lighter of the two tints on the meter-dial, the actinometer time is one
hour and, taking the speed of our non-halation plate as F45 when used
with this lighter tint, we give an exposure of 16 minutes with _f_/22.
Had we used a “backed” orthochromatic plate, an exposure of 12 or 15
minutes would have been ample. The quality of the illumination is an
important factor here. With ordinary gaslight rapid isochromatic plates
will shorten the exposure. With incandescent gaslight, and either plain
or orthochromatic plates, the exposure will be shorter than required by
ordinary yellow gaslight.
Let us now see what the meter will tell us about the falling away of
the illumination in a room as we work at various distances from the
window. At four feet distant, we get the standard tint in 3 minutes
(meter facing the light of the undraped window); at 8 feet distant in
12 minutes; and at 12 feet distant in 20 minutes. In the middle of the
room the actinometer time is 15 minutes; this with _f_/22 indicates an
exposure of one minute, with a plate marked F90 on Wynne’s list. But we
want to get all the detail under a certain black oak table. Placing the
meter in the deepest shadow where we want detail, we get the tint in
24 minutes, so that our exposure must be 3 minutes with _f_/22 (normal
exposure multiplied by 2), and we must control the contrasts in the
negative by development or reduction with ammonium persulphate.
In this way we may gain a practical knowledge of the problem of
exposure indoors without waste of plates or material, and when we
come to photographing interiors will be able to handle them more
intelligently than was possible without this knowledge.