This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Judging Strength Of Light. One must not judge the strength of light by the appearance of the light entering the window; on the contrary, estimate the effect of this light on the darkest portions of the room. These portions should be the guide, as you must time for the shadows, regardless of high-lights. The necessary exposure may be estimated by measuring the light and floor space in square feet. The following rule may be employed. While it is by no means accurate, yet it is sufficiently close to serve its purpose, and with it the necessary exposure can be quickly figured:
63. First of all, it is presumed that the sun is not shining through the windows of the room in which exposures are being made; also that the source of light is diffused - i. e., the light employed enters the windows from the shady side of the building and, therefore, is not direct sunlight, nor does it approach the strength of direct sunlight. Second, consider how much space this source of light must illuminate. If, for example, the room you are to photograph is 12 by 18 feet, or 216 square feet (area of floor), and you have the full illumination from but one window, say 3x5 feet, or an opening 15 feet square, you would have to illuminate 216 square feet of floor through 15 feet of illumination space. In other words, 216 feet of floor space must be illuminated with 15-216ths; or, about l-15th as much light as would be available if the picture was made outdoors in the shade. Therefore, if the same light conditions prevailed indoors as outdoors, you would naturally give an exposure of 15 times that which you would give outdoors; but, as the interior light conditions are not the same as those outdoors, the difference must be estimated.
Estimating Strength Of Light. For ordinary work in rooms with medium light walls and light furnishings, it is safe to estimate that the light outside is more than 8 times as strong as the light inside of the window. To prove this estimate, place a piece of proof paper on a medium-strength negative, in a printing frame, and place it outside of the window to print. Note the time it will require to print this proof to the proper depth. Then, take the same negative with another piece of proof paper, placing the frame on a table inside of the window. You will find it will require 8 times as long to print the proof inside as it did outside; consequently, we figure that the light is 8 times as strong outside of the window as it is inside.
65. So, if with a U. S. No. 16 stop you can make a full timed exposure in the shadow of the house, outdoors, in 1/2 second, with the same stop and light conditions indoors, and with the object close to the window, the exposure should be 8 times as long, or 4 seconds. But, considering that one must illuminate the entire 216 feet of space with the 15 feet of window space, or about 15 times more space than the size of the window opening, you must give 15 times more exposure for the entire room than you would for an object close to the window, or 60 seconds.
Q6. Rule. - Divide the amount of window space into the amount of floor space, and multiply the result by eight times the amount of exposure necessary for outdoor work in the shade. This will give you the amount of exposure required.