A ship that looks like it’s 1/2 mile away may
actually be twice, or more than twice, that distance from
you. Sometimes objects that seem to be half the distance
to the horizon may actually be considerably closer.
Knowing your height above the water helps you to
estimate ranges. For example, at a height of 50 feet, the
distance to the horizon is about 16,000 yards (8 miles);
at a height of 100 feet, the distance is about 23,000 yards
(11 1/2 miles). Practice estimating distances to known
objects. Until you become proficient at estimating
ranges, use phrases, such as “close aboard,” “on the
horizon,” and “hull down.”
Ranges are reported in yards and spoken digit by
digit, except that multiples of hundreds and thousands
are spoken as such.
USE OF BINOCULARS
Using binoculars for searching isn’t always better
than using the naked eye. Several factors govern when
and how binoculars should be used. For example, in fog
binoculars should not be used. At night, they should be
used quite often. Another factor is their field of view,
which is about 7°. Depending on the type of search, such
a narrow field may hamper proper scanning techniques.
Adjusting Binoculars
Three adjustments are required to obtain proper
focus and to gain maximum benefit from the
light-gathering quality of binoculars—two adjustments
for focus and one for the proper distance between lenses.
To properly focus your binoculars, you should do
the following:
1. Set both eyepieces to the +4 mark. Place the
binoculars firmly against the eyebrows and
locate a small, well-defined object about 1/2
mile away.
2. Cover one lens. (Do not touch the glass.)
3. Slowly turn the other eyepiece until a sharp
image is obtained, then back off as far as
possible without losing the sharpness. (Keep
both eyes open; closing one will give an
incorrect focus.)
4. Note the reading on the scale; then repeat the
previous procedures two or three times to obtain
the exact setting. Follow the same procedure for
the opposite eye.
The final adjustment is to establish the
interpupillary distance (IPD), which is the distance
between your eyes. Move the barrels up and down until
you see a single circle (fig. 3-7). Then note the reading
on the IPD vernier between the barrels. An incorrect
IPD setting will strain the eyes and waste part of the
binoculars’ light-gathering ability.
You won’t have your own personal binoculars.
They are passed from watch to watch. Therefore, it’s
important for you know your focus and IPD settings so
that the binoculars may be properly adjusted at night or
when there are no objects on which to focus in the
daytime. For nighttime use, the focus setting is one
mark less than for daytime.
Daytime use of binoculars depends upon the type of
search being conducted. Surface lookouts should use
them to scan across their sector—they should then use
the naked eye on return sweeps. Sky lookouts should
use them only to identify a contact detected with the
naked eye.
The binoculars should be used more frequently at
night than during daylight, but searches should still be
made with the naked eye. You often can see objects,
particularly moving ones, out of the corner of your eye.
These objects might not be detected with the binoculars
because of their narrow field of view.
Binoculars should never be used in fog, rain, snow,
or thick haze.
3-16
Student Notes:
Figure 3-7.—Proper IPD setting.
