Naturalistic Photography for Students of the Art.

by P. H. Emerson

He goes on to tell us that we see things singly when the two images of that thing are projected outward to the same spot in space, and are therefore superimposed and coincide. Objects are seen single when their retinal images fall on corresponding points—that is, objects lying in a horizontal circle passing through the point of sight and the central spots are seen single. Now “all objects at the same or nearly the same distance, but a little to the right or left, or above or below, are also either seen single, or else the doubling, if any, is usually imperceptible.” This surface of single vision is called the horopter.

There are, then, two adjustments, the focal and the axial, the one an adjustment for distant vision, the other for single vision, and connected with these is the adjustment of the pupil, which contracts and expands, not only to light, but also to distance and nearness of the object. Therefore, three adjustments take place when we look at anything. Connected with these laws are the laws of direction and corresponding points. Thus we see our perfect image can only exist in one place at once, that all between the eye and the object and beyond the object is indistinct, and that the further off an object is the more luminous does it appear. Two objects, too, may be seen as one.

F. Perspective.

Depth, Size, and Solidity.

Perspective.

The next question is, “To what is due the appearance of solidity and depth?”

Depth, or relative distance, is judged of by a combination of four kinds of perspective.

1. Focal or monocular perspective.—Objects at the point of sight are sharp, but all objects beyond or within this distance are dim. Distance is judged partly by the act of focussing the eye by acting, as we have said, on the lens. As this power only acts within twenty feet, it is evident that things can only be in focus in one plane.

2. Mathematical Perspective.—Objects become smaller in appearance and nearer together as they recede. This is another aid to the judging of distance. The true rendering of this perspective in photography depends on the correct use of the lens, as will be explained.

3. Aërial Perspective is the perspective due to the scattering of light by aërial turbidity, for the atmosphere always contains floating particles of matter. As the objects recede this curtain of turbidity becomes thicker and the distant objects grow dimmer and bluer. This is another aid to the judging of distance, but any one not accustomed to count on this effect may easily misjudge, as we have done before now to our cost in Switzerland, where a peak miles away has, at times, seemed to be in the next valley.