Three-Dimensional Photography - Principles of Stereoscopy

THE STEREOSCOPE 53

scopic reproduction, exactly the same as that which separated the

similar images when the scene was viewed directly. Not only is this

true, but as the attention of the eye changes from object to object,

there is exactly the same continuous variation of diplopia which

would have been present in the real scene.

Thus we find that although the individual pictures of the stereo-

gram are sharply defined tliroughout, when viewed in the stereo-

scope there is the same “out-of-focus” effect in objects in remote

planes that is observed in real life. The diffuse definition observed

in direct vision in all objects not the center of attention, is not

actually a matter of the focus of the crystalline lens, but is due to

the doubling of the stereo images, an effect which is exactly re-

produced in the stereogram. Hence, it follows that those who sug-

gest using wide apertures when making stereograms to differenti-

ate planes as in planar photography, fail utterly to understand

the extent to which the stereogram duplicates conditions of direct

stereoscopic vision.

Accommodation and Convergence.-Accommodation refers to

the actual alteration of focus of the crystalline lens of the eye. The

Fig. 3-13. Individual vergence. In the stereoscope, one numbered circle
will have a dot over its center. This shows the individual tendency to
convergence or divergence.

ability of the individual to adjust his vision for both near and
distant objects is known as his “accommodation range.” At the
same time, when nearby objects are viewed, the axes of the eye-