I would like to know the field of view I can archieve with my scope if I am going to buy an eyepiece, especially a wide angle eyepiece. Apply the formula below, you can get a rough figure of the true field of view (TFOV). Why is it just a rough figure? There are still several factors to be taken account but I think this is good enough. There is a more precise formula to do the same work.
TFOV = AFOV of Eyepiece / Magnification
The apparent FOV (AFOV) of an eyepiece is provided by the manufacturer. To know the magnification, please refer to Calculate Eyepiece Magnification.
Apparent Field of View of Several Eyepiece Designs
Orthoscopic – 43º
Plossl – 50º
Radian – 60º
Panaoptic – 68º
Nagler – 82º