HYPERSTEREO 2G7roughly approximate, you may consider 1500 feet to be 1/3 mile
without seriously affecting the result.
Of course, if the object is a mountain, it would be wrong to
show it only 1/3 mile distant, but still you must have it within
1/2 mile or it will be very flat, Therefore, the chosen apparent
distance is controlled by the object to some extent.
Assume the mountain is five miles away, and you wish to bring
it to within 1/2 mile. Parallax is a directly proportionate factor.
Double the parallax and you halve the distance, treble it and the
distance becomes 1/3. Therefore to make the distance 1/10 (1/2
mile from five miles), the required base will be ten times normal
or about two feet.
The only other factor is that of foreground. Keep the nearest
foreground at least seven feet distant. As 7xio=70J the nearest
permissible foreground would have to be 70 feet away when you
use a lox base.
Keep these two factors in mind and you can use hyperstereo
freely and with excellent results. Remember, you can use a three
foot foreground in normal stereo, so you could use a 30 foot
foreground with lox bases, but the greater distance is advisable in
hyper stereo.
In practice a reflecting camera, either stereo or mono-lens, will
be found the most satisfactory, although excellent work has been
done with the usual 35mm camera. The ground-glass is ruled in
crosslines, if this has not been done by the maker. These lines
serve to align the subject in the two successive exposures, and if
key objects at the extreme limits of the screen are observed and
exactly duplicated as to position, a continuous base perpendicular
to the axes of the lenses may be maintained with surprising accu-
racy. Because of the great distance of the subject, there is prac-
tically no observable difference at the edges resulting from stereo
parallax.
LONG-FOCUS LENsES.-It may be asked why a long-focus lens
should be used if base alone determines apparent distance,
The original purpose of hyperstereo is to impart relief to some
object whose size is such that a remote camera position is essen-
tial. As long as the subject has this characteristic, hyperstereo
alone is the solution. In fact, to add long-focus lenses would