ELEMENTARY STEREOGRAPHY 9a formless smudge. Therefore, give that exposure demanded by
the deepest included shadow of significant size.
This exposure will very often be from two to three times the
average exposure, (which is in addition to any factor used because
of the reflector, if used) and the differential reading will show
that the highlights are being seriously overexposed. It will be
necessary to compensate during development, often to the extent
of using only half the normal time. In stereography the best re-
sults are obtainable only by going back to the old principle of
exposing for the shadows and developing for the highlights.
It is difficult to imagine an exterior scene which can be stereo-
graphed successfully without a filter. An extensive series of filters
is not needed, but the set should contain the medium or dark
yellow, yellow-green, medium orange and tri-color red. Of these
the medium orange will probably be the most often used.
DEFINITION.-AS a rule the stereogram should be visually sharp
throughout. Any obvious unsharpness, or hint of softness, is
usually unacceptable.
It has been repeatedly suggested that the stereo depth be en-
hanced by softening the definition of the background. Such a
suggestion indicates a lack of knowledge of the most elementary
stereoscopic principles.
The stereogram duplicates in every way the appearance of the
original. When this stereogram is viewed, assuming it to be
needle sharp throughout, the eyes see sharply only khat object
upon which they are focused, and as the vision shifts from object
to object, other objects in the field of vision take on exactly the
degree of unsharpness they would have in real life. This fact is
one of the fundamental principles of stereography. Therefore, any
attempt further to simulate the characteristics of direct vision only
introduces an unnatural effect and a loss of stereoscopic acuity.
It is therefore advisable always to use the smallest aperture
compatible with other essential factors. When a large aperture is
essential, try as far as possible to avoid any great depth in the
field, and focus carefully to include as many planes as possible
within the field depth.
MOVING OBJEcrS.-This, of course, refers only to the use of the
simultaneous exposure either with reflector or double camera.
Strangely enough, a slight blur caused by the moving object is