Calculate TFOV using Eyepiece Field Stop and Telescope Focal Length
Where:
True Field of View (TFOV) is the actual angular width of sky visible through your eyepiece-telescope combination. It represents how much of the night sky you can see at once.
For variable power binoculars that use standard eyepieces, you can determine the maximum usable field stop to prevent vignetting by calculating with known eyepiece parameters.