Chapter II: 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.