Field of view is an area you see when looking through the optical product. Although it primarily depends on the build of the eyepiece, it is hugely affected by magnification. If you look through two binoculars of the same model but with different magnification, you can see that the one with 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 with binoculars, spotting scopes and other optical products it's measured at 1000 m.
With binoculars a field of view with more than 140 m at 1000 m distance is considered a wide angle, while with riflescopes it is with a field of view over 38 m at 100 m. Wide angle is particularly useful in 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 housing doesn’t automatically mean wider field of view.
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.