Posted 29 May 2015 - 07:47 PM
In those wide angle opera glasses the objective lenses are of very fast f/ratio. Or to put it another way, they're designed so as to be quite oversized, with the entrance pupil occupying only a fraction of its area for any one image point. For example, a 2.3X40, when used by an observer whose irises are 7mm, work as 2.3X16.1. The 'extra' objective is there only so as to permit that wide field. The wider the objective, the wider the FOV; that's how a Galilean system works because the exit pupil is *inside* the instrument.
Furthermore, these objectives are of the meniscus form, which is better for such a wide FOV. A standard achromat bino objective is not so 'bent' into a meniscus, and may not be able to ever deliver such a large FOV. Or it would require a more sophisticated eyepiece design, at least (any again, if feasible.)
Lastly, the f/ratio determines the maximum possible AFoV. A faster objective, as seen through the eyepiece, presents as an angularly wider, out-of-focus 'window'. A bino's f/3.8 objective cannot achieve the same large AFoV as can these opera glass objectives which are nearer to f/2.