Thanks to Gillies link I found you shound not close an eye to set up
your binoculars


Binoculars (fig. 2-15) are the most commonly
used of all optical equipment. Although normally only
7-power, they give a wide range of vision and are best
suited for searching over a wide area or for following
moving targets. They require the use of both eyes and,
because both eyes do not always have the same vision,
it is better to adjust the focus for each lens individually.
Proper focus is essential. If the focus is off, eyestrain
is increased greatly, and you will not get maximum
efficiency from your optics.

To obtain the proper focus for each eye, turn both
binocular scales to the +4 setting. Hold the binoculars
firmly against your eyebrows. To get the focus for
your left eye (only one eye can be focused at a time),
cup the right hand over the right lens, cutting all light
to that eye. Be sure to keep both eyes open, however,
because closing one eye will give an incorrect focus.
Train the binocular on a small well-defined object.

Figure 2-15.-7 × 50 binoculars.

Slowly turn the eyepiece from the +4 setting until the
object stands out in sharp detail.
The reading on the scale gives you the correct
focus for your left eye. Now do the same for your right
eye. The chances are the setting will be different.
Repeat this step several times to make sure the focuses
are correct.

Once you get the glasses focused properly,
remember your settings so you can focus your
binoculars on a very dark night. The correct night
focus usually is a -1 setting from your day focus for
each eye.

The other adjustment for binoculars is the IPD
adjustment. All Navy binoculars have the IPD scale
on the hinges between the barrels. Find out what your
IPD is and remember it. It may already be in your
health record. If not, have a medical officer measure
you for it. When you have set your correct IPD scale,
you will see a complete circle, never overlapping
circles. At night, if you have the wrong IPD setting,
you will cut out a lot of light that should be going to
your eyes.

Take a look through a pair of binoculars not
adjusted for your eyes, then look through a pair that is
adjusted properly. Notice the difference. If possible,
always use the same binoculars.

Most glasses are treated scientifically to reduce
glare, but there are times when the direct rays of the
sun are so strong that it is almost impossible to
distinguish shapes and colors. To overcome this
handicap, glasses usually have colored lens filters that
can be inserted over the regular lenses, reducing glare
considerably.