Unprecise rule of thumb

WIDE 

 Distance from 2 inches to 1 foot 

Equivalent FOV:  example 6 inches distance has 6 inches quare FOV

Medidum

Distance from 2 inches to 1 foot

Half size FOV:  example 1 foot distance has 6 inches quare FOV

Narrow

Distance from 2 inches to 6 inches

1/3 fov sizes: example 6 inches distance has 2 inches quare FOV

Long range narrow

Distance from 3 inches to 5 feet

1/3 size FOV: example 5 feet distance has around 1.5 feet FOV