The scaling may be off a bit, but the geometry suggests (a) that the bulk of the binocular is not in the visible field, even a Porro, and (2) an appreciable amount of unmagnified information within the 120º overlap area of the retina can enter though the corrective eyeglasses beyond the annulus.
For the most part, unmagnified information beyond the eyeglasses themselves probably projects onto the 30º monocular fields on either side. This will vary with the eyeglass diameter. Finally, because of the steep angles involved, very little size change or fore-aft movement can result in a large change in how the full field appears. Hence, each individual may take a bit of fitting before everything works just right.
I'm still fascinated by whether or how the brain integrates magnified and unmagnified images in the overlap area.
Ed
Hm, tricky one...
Your drawings show a front view towards the eye and a view from the side.
Mine have only shown the view from above, where the visual field expands towards right and left. And, yeah, a cycloptic view to be correct, where no nose is in the way and left and right eye are made one.
At the moment, I have no porro in my possession, but frankly I'd guess that the prism housings + the hands will intrude on those, the lateral parts of the visual field.
More so with shorter physical length of the oculars, or with longer eye relief where the diameter of the eyepiece will not hide them.
But I may be wrong, it has occurred before.
The spectacle lenses' width is of interest only for the lateral parts of the visual field. But nobody will receive a focused image of the medial parts of the bins, either because of the short distance (1" to 2") or because they are way out of the eye's zone of sharpness when you look through the bins.
The zone of sharp vision sits in the 2 degrees absolute center of the visual field.
The height of the spectacle lenses might be of significance should they allow to glance upwards when the bins are held in place, provided their ocular diameter is not too large. When looking straight forward, the peripheral parts of the lenses carry no additional information that is useful for the sense of vision.
Extremely large lenses would however be hard to wear, and grease from the eyebrows would deposit on them immediately.
Possibly, a thick spectacle frame might pose some noticeable intrusion. It has happened to me, but it was actually in the very image after being pushed-up by the bins.
//L