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.
A Trial May Be Quite Valueless. Unless the photographer knows how much he has stopped down on account of the difference of distances in the subjects, and how much too great the optical defects of the lens, if any, he is not in a position to say definitely whether his lens is a good one or not. From this it follows that to try a lens on an ordinary outdoor subject, where there may be objects at all sorts of distances from the camera, as so many workers do, can be little or no test of the lens. In practice, of course, when you stop down on account of deficiencies in the lens, you also, at the same time, render it better able to deal with different distances in the subjects.
686. "What Stop Shall I Use?" - Few questions are more often asked of the photographer by beginners than the simple one "what stop ought I to use?" The mere putting of such a question is evident that the inquirer has no clear idea of what the stop is supposed to do. No doubt he thinks that if he uses the wrong size of stop his picture will be wrong, though in what way he does not know. It is like a certain amateur, who abstained from attempting anything but portrait work for a long time because he only had a portrait lens. Afterwards he found that it was one which would make excellent landscapes, as well as commercial pictures, when used with a fairly small stop, and that its principal drawback for such work was its bulk or size.
687. There is, of course, no definite rule designating the stop to use for different classes of subjects. For portraits the full opening should be used as near as possible, because it is necessary to reduce the length of the exposure. Landscape Photography does not make such demands upon the photographic worker. Good, fair definition is all that is required. With architecture, definition all over is important, and a small stop must be used, but with interiors the largest stop that will give clear definition is necessary to keep exposures reasonably short.
688. "The Largest Stop." - Perhaps it is best to summarize the whole subject by saying that the largest stop that will give the degree of definition required should in all cases be employed, except when you deliberately stop down to accent shadows.