The problem with using afocal teleconverters is that they are fixed focus.
So, unless both eyes need zero dioptre correction, apart from astigmatism, one needs to wear glasses.
If one has little accommodation the view is out of focus for most.

The Nikon's above lose field when wearing glasses.
I can minimise this by using Minolta camera eyepiece correction lenses. I think it was the Minolta MD ones that fit perfectly into the recess at the back. The Minolta AF ones being a little big. This loses much less field than wearing glasses.

The Konica Minolta 1.5x (1.65x for me) loses no field with small glasses of the right correction, possibly my computer screen glasses.

So opera glasses that focus are better, but may not give the maximum field size. They may also vignet.
However, very large aperture afocal teleconverters probably give impressive monocular views. Possibly a Ricoh one was very large, I can't remember.

Regards,
David

We haven't mentioned the "field glass". These are bigger than the opera glass and were used by the military before the prismatic binocular came in. Does anyone have any experience with the field glass?

_________________
brian

Yes, Brian.

They are a pain.
Big and narrow fields.
They were made in vast numbers, and usually cost about £15 now.
Many names, often just the shop selling them.

They were still used in the 1920s and 1930s, but Porroprism binoculars are smaller, better and have wider fields.
To get a wide field, a field glass or opera glass needs large objectives.
Without huge objectives the fields can be tiny, especially if one wears glasses.

Regards,
David