The most commonly used optical equipment is a
set of binoculars (figure 2). The size of binoculars deemed
most useful for marine work is 7 x 50; that means the
glasses have a magnification of 7 power and an objective
lens 50 mm in diameter. This ratio of magnification is a
satisfying compromise between the need for magnification
and the reduction of angle, or field of view, that results as
the magnification is increased. Large objective lenses have
excellent light-gathering characteristics, making them
particularly suitable for night use.
To gain maximum benefit from the light-gathering
quality of binoculars, you must adjust the binoculars
Figure 2.7 x 50 binoculars.