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.
Exposure. The tendency among all beginners of hand camera work is towards under-exposure. It is a remote contingency that the beginner will over-expose. The older worker, accustomed to time exposures and the use of restrainers in development, dreads the under-exposure and tries to avoid it. By using rapid plates, a quick shutter and a large aperture in a good light, he overexposes. While the beginner has not learned how to hold the camera still, or is afraid that if he does not give a very short exposure there will be signs of movement, due to his own unsteadiness with the camera, he will fire off his shutter at the one-fiftieth of a second with his hand camera what he would give a whole second to with his tripod camera. He should not forget, while making his exposure, that there is no possible condition that would emancipate him wholly from the relative values of light, speed of plate or film, speed of lens and shutter, and size of stop. All these are factors that must be contended with. They represent so many natural laws. Though light is most brilliant at midday, the angle at which the lengthening shadows fall between two and four P. M., and the soft clear light of the early morning, between nine and eleven A. M., give the best results in fully exposed negatives.
633. This question of light and color is an interesting one for the beginner, to whom all sunlight is white light. But to the scientist, light is made up of several colors - red, yellow, orange, green, blue, indigo, violet. Now the various colors act differently upon the sensitive silver salts in the emulsion of the plate or film. Blue, indigo and violet rays work the most rapidly; green rays are slower, and the red, yellow and orange rays are the slowest to affect the plate. Therefore, a landscape in which the prevailing color is green, from the grass and trees, with blue from the sky above, will require a shorter exposure than if we were photographing red and yellow flowers together. If we undertake to photograph an oriental rug or an oil painting we will have, perhaps, all the colors in the spectrum. In that event there are colors to which the bromide of silver is most sensitive, and also those which have the least effect on this salt. If we give the proper exposure to one color we over-expose another, and vice versa. What must be done? There are several remedies, but they are all based on one general principle, viz., that of correcting color value by reducing them all to a common or uniform value. Special plates, orthochromatic plates, for instance, used in conjunction with a ray filter or color screen make it possible to reproduce the various colors in monochrome in their correct values; but the time of exposure is then lengthened very materially. (This is also described at length in Volume III of the library.)
634. In lighting, reflection is another prime factor to be considered. A view, for instance, taken over a body of water, a lake or ocean, will only require one-half the exposure of an ordinary landscape in the open, because of the reflected light from the water. The same is true of snow scenes. Furthermore, the quality of light in a dry climate is much quicker than it is in a moist climate.