Times of the Islands Winter 2019/20

Times Kevin_Times Kevin 9/18/18 10:51 AM Page 1
of the adjustments has been inadvertently reset. In doing
so, you will maintain the dioptric difference between your
eyes regardless of the distance to the target—500 feet
or 5,000,000,000 miles. (Only 2 to 3% of the population
have the same setting in each eye.)
Spatial accommodation
Spatial accommodation is a collimation (alignment) issue
that, more often than not, originates with the binocular
but which can be caused by the binocular’s IPD (interpupillary
distance) to be misplaced relative to the separation
of the user’s eyes.
For example, just placing a binocular to the eyes
is inadequate unless positioned in such a way that the
binocular’s exit pupil is placed precisely in front of the
pupils of the observer’s eyes. If the observer has an IPD
of 69 millimeters and the binocular’s IPD is set to match,
all is well. If not, the observer must use some degree of
eye-straining spatial accommodating, even if the binocular
is well collimated.
The Internet is replete with articles telling observers
how they can “easily” correct misalignment by tweaking a
few through-the-body/prism-tilt screws, with most such
instructions omitting other alignment conventions and
the repairs often needed to allow any of those conventions
to work.
There are, however, stipulations of which the exuberant
screw-tweaker needs to be aware. IF only one side
of the binocular is misaligned, IF that side is the one
adjusted, IF the error is small, IF the individual’s physiological
accommodation is adequate, and IF the distance
to the desired target is far enough this—conditional alignment—may
be enough to make the instrument perform
well or even excellently. Even so, while that is adequate
for some users, it leaves others, who didn’t have all those
IFs in their favor, frustrated with a less than crisp image.
In addition, without specific knowledge, the same
procedure can push the binocular ever farther out of alignment
and can, in some cases, damage the instrument.
And although rarely, if ever, seen in print, understanding
spatial accommodation is critical to getting the best view
from the binocular.
So, if you find that your binocular gives you a double—or
even an uncomfortable—image you should
consider that the binocular may be misaligned or you
don’t have the telescopes spaced properly for your eyes.
In order to find out, bring the binocular to your eyes and
seek the best view of a target at least a mile away. A
streetlamp will work fine. Then slowly move the instru-
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