STEREO PR0,JECTION 187In doing this it must be remembered that ordinarily the shutter
controls will be independent so care must be taken to set both
shutters for diaphragm and speed. A dual cable release, which is
available, serves to release both shutters with sufficient synchro-
nism for all but very rapidly moving objects. However to obtain
flash synchronism it will be necessary to adjust the release with the
utmost precision or one picture will be darker than the other.
However, such a camera does find many uses for which the con-
ventional model is not wholly suited.
CLOSEUP COMPENSATION
Compensation is often advised when mounting stereograms of
closeup objects. This has two advantages and some. disadvantages.
SIDEBmDs.-Because of the different spacing between closeup
and infinity images, and because infinity images are normally cen-
tered, each nearer pair of planes fail to match throughout, the dis-
crepancy growing as the planes become nearer and nearer. For very
closeup objects there may be a sideband of two or three milli-
meters unless prismatic supplementary lenses have been used to
overcome the defect.
These sidebands which change their width as the eye moves
from object to object must not be confused with the sidebands
which result from failure to register the aperture images upon the
screen. The latter are physical and of fixed width which can read-
ily be overcome as has been described. The former are variable,
are visual rather than physical and cannot be wholly overcome un-
less the whole scene lies in a single plane, a condition which would
render stereo valueless.
The only way in which these can be restricted is to move the
outer edges of the mask toward the center thus making the film
aperture narrower, and even so, this new window produces a re-
verse sideband for far distant objects. However, we may regard the
window, which is discussed elsewhere in this volume, as compensa-
tion for excessive sidebands. Such compensation is the reason for
having “normal” and “closeup” masks for mounting films.
PROJECTION COMPENSATION.-There is another type of compen-
sation used when mounting stereograms, particularly if they are
to be projected. This consists in moving the whole films slightly