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to obtain proper focus and correct distance between
lenses.
To obtain proper focus, observe the following steps:
1. Set both eyepieces to the +4 mark.
2. Place the binoculars firmly against the eyebrows
and locate a small, well-defined object about l/2-mile
distant.
3. Cover one lens (do not touch the glass).
4. Slowly turn the other eyepiece until you see a
sharp image, then back off as far as possible without
losing the sharpness. (Keep both eyes open; closing one
will give an incorrect focus.)
5. Note the reading on the scale, then repeat the
above procedure two or three times to obtain the exact
setting.
6. Follow the same procedure for the opposite eye.
The final adjustment is to establish the inter-
pupillary distance (IPD), which is the distance between
your eyes. Move the barrels up and down until you see
a single circle, as shown in figure 3, 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 will not have your own personal binoculars—
they are passed from watch to watch—so it is important
Figure 3.—Proper IPD setting.
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