98 THREE-DIMENSIONAL PHOTOGRAPHYnate portions of the camera field which contain distracting ob-
jects; to limit the field to the object of primary interest. This is
usually more of a commercial or record type of masking than
pictorial, But this leads into the general field of masking.
Planar pictures are usually masked to a rectangle, less often
to round or oval and at times “fancy” masks are used such as a
star, diamond, leaf, fish or the like. We are not concerned with
any question of taste or advisability; only with the fact that these
various masks can be used with stereo if desired. As a matter of
interest it may be added that the keyhole mask often used in
planar photography is more effective in stereo because the window
placement plus the shape gives the effect of really looking through
a keyhole.
The actual masking is done just like the window masking. The
two openings must be of the same size and shape, and the two
must be carefully aligned. This is usually accomplished by care-
fully drawing the outline upon a piece of tracing paper which is
then used as a printing mask.
Masks are positive or negative. The negative mask is the most
popular because without further mounting it provides a black
frame for the stereogram.
The negative mask is made by cutting out from opaque paper
two shapes corresponding to the mask openings. These are aligned
and attached to a piece of tracing paper. Guide lines are marked
upon the paper.
This negative mask is placed in the printer and a sheet of paper
exposed behind it. On the back of the sensitive paper, the out-
line of the mask is traced. This paper is then exposed to the trans-
posed stereo negative, the pencilled lines serving as a guide to
locate the centers of the object symmetrically within the mask
openings.
The choice of mask is largely determined by the characteristics
of the negative. If the negative is thin and open, so that it gives a
dark to black background, a positive mask is used (Fig. 6-i), one
with the mask openings cut from opaque paper. If the back-
ground is medium to dark in the negative (medium to light
printer), the negative mask is used. Thus through tonal contrast
the shape of the mask is visible.