The optical structure of each model of binoculars is different,
so even if the magnification rating is the same, how much view
the pair of binoculars can pull in will differ. The width of the
view you can see through the binoculars is called the field of
view. For bird watching in a large forest, using a wider field of
view will be more useful.
1) Real field of view
This is the view through the binoculars (Fig. 5-a), and it is
measured from the center of the objective lens and expressed
in degrees (angle). The lower the magnification the binoculars
have, the wider the real field of view—and the higher the
magnification, the narrower the field of view. Because of this, it
is hard to compare the real field of view of binoculars with that
of binoculars of different magnification rating.
2) Apparent field of view
This is the value of the real field of view multiplied by the
magnification (Fig. 5-b). For example, if 10x magnification
binoculars have a 5x real field of view, the apparent field of
view will be 50˚. This value represents the field of view which
you will see looking through the binoculars. It is comparable
even among binoculars of different magnification. In general,
if the apparent field of view is more than 65˚, it is considered
a wide field of view.
Apparent field of view = Magnification x Real field of view
3) Field of view at 1.000 meters
The field of view, measured in meters, which you can see 1.000
meters in front of you.
Field of view at 1.000 meters = 1.000 x 2 x tan (
(Fig. 5) Looking through 10x/5˚ binoculars at a subject 1,000 meters away
c Field of view at
b Apparent field of view: 50˚
Real field of view
a Real field of view: 5˚