208 THREE-DIMENSIONAL PHOTOGRAPHYgreater the departure from the ideal monocular, pinhole defini-
tion.
This phenomenon gave rise many, many years ago, to a very
clever device. A lens cap was made which included a vertical bar
across the center of the lens. The width of this bar was from 1/4
to 1/3 the total lens diameter. This gave a clear differentiation
between the two stereo images when placed over any lens. When
focusing the camera, it was only necessary to bring the two images
of any object into close coincidence to produce a sharp focus upon
that object. This was the forerunner of the rangefinder. It is as
good as any focusing device when used with any camera equipped
for visual focus, although the larger the lens diameter the better
the result.
The writer introduced his own variation of that device by
applying filters of complementary colors to the two sides so that
the image which is being focused not only comes into super-
position but exhibits natural color, while other objects are tinged
a distinct filter color, red or green, for example.
Thus it is that an image, although not visibly diffuse, does have
a dual, even multiple, character. The presence of this funda-
mental difference of stereo parallax, although in no way giving to
the planar photograph any degree of stereoscopic relief, does give
to the image a degree of planar realism never achieved in the
small-lens image. It should be emphasized that this effect is one
of diameter alone, not of relative aperture, nor of focal length,
nor of degree of correction. It is simply the presence of images
whose natural parallax includes that degree of parallax normal to
vision. The “realism” of the portrait is simply an unconscious
recognition in the planar image of those fundamental elements
which are normal to vision prior to the synthesis of the stereo-
scopic image.
STEREO PARALLAX AND SOFT Focus.-Soft-focus portraits have
always been popular, but their origin is not widely known. The
admired absence of razor-sharp outlines should not be the pres-
ence of actual image diffusion. It should be the quality imparted
by the old-fashioned, huge portrait lenses which had such aper-
tures that they did include this normal degree of stereo parallax.
The “soft focus” is simply the difference between the parallactic
images at the point of best registration. It is a visual effect normal