Real field of view is the angle of the visible field, seen without moving the binoculars, measured from the central point of the objective lens. The larger the value is, the wider the viewfield available. For example, binoculars with a wider field of view are advantageous for locating fast-moving wild birds within the viewfield. This also applies for finding small nebulas or a cluster of stars in astronomical observations.
Apparent field of view
Apparent field of view is the angle of the magnified field when you look through binoculars.
The larger the apparent field of view is, the wider the field of view you can see even at high magnifications.
With the conventional method used previously, the apparent field of view was calculated by multiplying the real field of view by the binocular magnification. (With this formula, apparent field of view wider than 65˚ is called wide field of view.)
After revision, Nikon's figures are now based on the ISO 14132-1:2002 standard, and obtained by the following formula:
tan ω' = Γ x tan ω
Apparent field of view:2ω'
Real field of view:2ω
(With this formula, apparent field of view wider than 60° is called wide field of view.)
For example, the apparent field of view of 8x binoculars with a 7.0° real field of view is as follows:
2ω’= 2 x tan-1 (Γ x tan ω)
= 2 x tan-1 (8 x tan 3.5°)
Field of view at 1,000 meters
Field of view at 1,000 meters is the width of the visible area at a distance of 1,000 meters, which can be seen without moving the binoculars.
For example, with 8x42 7.0° binoculars:
Field of view at 1,000m = 2 x 1000m tan (7.0÷2) = 122m