I see binoculars such as an older vintage Nikon 7x35 and a 9.3 FoV with very small diameter oculars and then I'll see another pair with huge eyepieces but with a smaller FoV. So what is the optical advantage of larger eyepieces? Thanks.
[QUOTE=tpcollins;1896283]So what is the optical advantage of larger eyepieces? Thanks.[/QUOTE]
Mostly eye relief. Someone like Henry can give you more details on oculars.
Yep, long eye relief in combination with a wide AFOV requires a large diameter eye lens. No matter what kind of eyepiece formula is used the ray bundle emerges from the eyepiece in the form of a cone that matches the angle subtended by the AFOV. The cone comes to a point at the eye relief distance, so the cone length (the eye relief) for a given AFOV is determined by the diameter of the eye lens. Stating the same thing backwards; for a given eye relief value the AFOV is determined by the diameter of the eye lens.
Those old Nikon 7x35's had wide fields, but very short eye relief.