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.
Horizontal Swing. The horizontal swing is attached to the regular view camera and is seldom used for ordinary work. However, when taking a view along one side of a narrow street, if difficulty is experienced in sharply focusing foreground and distance, the horizontal swing will aid you. Here again, you must exercise care or you may swing the wrong way, as it is very easy to become confused. If you will bear in mind to always swing toward the portion of the object at the greatest distance from you, you will never make a mistake. Until one becomes thoroughly familiar with the use of swing-backs it is a good plan to practice and experiment with the swing both ways, carefully watching on your ground-glass the effects produced.
Linear Perspective. The picture that represents an object on a plane surface appearing precisely as the eye views it from a given point, is a simple definition of linear perspective. By following this rule in an effort to produce such a picture, you will be able to judge for yourself when pictures have their proper linear perspective. It is a fact that quite often the lens will produce a perspective which to the eye would appear very displeasing. This is accounted for by the fact that the angle of lens used is frequently at variance to the normal angle of the eyes, so that a view which might not appeal at all to the eye may, when photographed, appear most pleasing. The same scene if photographed with a longer focused lens would, like the eye, require a greater distance to produce the proper linear perspective. It is advisable, therefore, in order to judge the linear perspective with the human eye to become familiar with the angle of the lens employed. As compared with the eye this can very readily be accomplished by observing the focus on the ground-glass from the same point of view as that of the eye.
Perspective Lines. By Illustration No. 19 we will endeavor to illustrate prespective lines in the most practical manner possible. Line AA is what is known as the horizon line. The simplest explanation of the horizon line for a landscape view is that it is a line which apparently separates the sky from the earth. For an architectural view the horizon line is that line which when looking forward appears to be on a level with the eye, and to which all other lines are seen to converge. While the horizon line never changes and is always on a level with the eye, yet the appearance of the view, or building, can be improved by the proper selection of view-point, thereby raising or lowering the horizon line in the view. It must be borne in mind that photographing a building from a low view-point, thereby supplying a low horizon, will tend to heighten the building, and on the contrary a high view-point shortens it. Much depends upon the judgment of the photographer in the selection of view-point, as to the location of the horizon in the view, in order to truthfully represent the building photographed. The proper location of the horizon in architectural views is governed entirely by the selection of viewpoints.