Hi, I have a 31 nagler and and just received a Ethos 13mm.
with a laserpointer you can easy measure the FOV by pointing the laser through the eyepiece on to a board,
After some playing with the laser you will notice the edge of the field on the board, mark this edge of the fieldstop (of the eyepiece) ON the board,
slide the laser to the opposite of the fiedstop and mark it also.

Measure the distance between the eyepiece focal point and the board.
Measure also the distance between the markings.

With basic mathematics (and a calculator with tangent) I calculate for the 31 Nagler (first measure): 84 degree
(second measure): 83 degree.
For the Ethos (first measure): 99.96 degree
(second measure): 100.2 degree

Simply a 31 Nagler HAS a 82 degree field, and a Ethos HAS a 100 degree field.

Holland from the Netherlands