10 THREE-DIMENSIONAL PHOTOGRAPHYless objectionable than loss of definition by poor focus. It is desir-
able to avoid all blur, of course, but if conditions necessitate a
compromise between small aperture and motion-stopping expo-
sure, it is advisable to favor the small aperture even at the cost of
a slight blur of motion.
On the other hand “follow shots,” in which the exposure is
made while swinging the camera to keep up with a moving ob-
ject, are often surprisingly successful. If the object is sharp and
the background is sufficiently blurred to rob it of identity, the
stereogram may be very good indeed. So if you blur the back-
ground, blur it a lot!
INTERRELATION OF FAcToRs.-Field depth demands a small
aperture. Motion demands fast shutter speed. Shadow detail
demands full exposure. Exterior scenes demand the use of a
filter. All of these combine to limit the possible exposure some-
what rigidly. Often compromises must be made. The first thing
is to use a lighter filter, or perhaps no filter, although this is
undesirable. Next sacrifice some speed even at the expense of
motion blur, Then manipulate the aperture, even to losing defi-
nition in the far background. Finally, rather than sacrifice shadow
detail, either go to a faster film or try the same subject later
when the light is more favorable, With experience you will learn
to compromise these factors so that with hardly any discernible
loss you will find it possible to make a good stereogram of any
normal subject.
When color film is used, its slow speed further complicates the
problem.
EMULSIONS.-h stereography, differences which are so subtle
as to escape notice altogether in planar photography, become
painfully obvious. Therefore, it is desirable that the positives
have the greatest possible photographic quality. In fact, real suc-
cess will never be achieved by the amateur until he has learned
to make good transparencies. The print should be brilliant and
full toned, with detail in the darkest shadows and the brightest
highlights.
Ordinarily, while our most sensitive emulsions are perfectly
satisfactory for routine planar photography, it will be found that
emulsions of somewhat less than the greatest sensitivity will give
the best satisfaction. The ideal emulsion is a slow, fine-grained