The field of view is an area the observer sees when looking through the optical product. Although it primarily depends on how the eyepiece is built, it is also hugely affected by magnification. If you look through two binoculars of the same model but with a different magnification, you can notice that the one with a lower magnification factor will have a wider field of view. So, when comparing binoculars, you must compare the ones with the same magnification. With riflescopes, the field of view is being measured at 100 m, while ith binoculars, spotting scopes, and other optical products it is measured at 1000 m.
With binoculars, a field of view with more than 140 m at 1000 m distance is considered a wide-angle one, while with riflescopes, a wide angle refers to a field of view over 38 m at 100 m. A wide angle is particularly useful when bird watching. It is also important to mention that the size and lens diameter of optical products are not indicators of their field of view – bigger binoculars do not automatically mean a wider field of view.
The field of view can be expressed in two values – degrees or meters.
One degree is 17.5 m at 1000 m / 1.75 m at 100 m.
If you divide the field of view given in meters by 17.5 you get the field of view in degrees.
If you multiply degrees with 17.5 you get the field of view at 1000m.