1. Find out the value of the apparent field of view of the eyepiece. Every eyepiece has its own value of the apparent field of view and this value is supplied from the manufacturer. For the optical eyepieces PL16, PL26 & PL32 as mentioned in the above example, their apparent field of view is 52 degrees.
2. Find the value of magnification. This is calculated by dividing the focal length of your scope by the focal length of the eyepiece. For the Nipon 26-78x78 scope (focal length=780mm) with a 16mm eyepiece, for example, its magnification will be: 780/16=48.75.
3. Divide the value of the apparent field of view by the value of magnification. For the above example, 52/48.75=1.06 degrees. This is the value of the field of view for this scope using this eyepiece.