Hi Roger - Hope I don't overload you, but here goes:
The word is “binocular,” not a “pair of binoculars,” which means two binoculars. The prefix “bi,” from the Latin bini, means double or two. Add it to “ocular,” from the Latin oculus, meaning eye, and you’ve described a two-eyed instrument. A binocular.
A common binocular glass is BK 7, the “B” meaning borosilicate. “K” stands for Kron or Crown, one of two main types of optical glass. Another popular Crown glass is BaK 4, a barium-silicon compound. BaK 4 prisms are denser, with better light transmission and clarity than BK 7. While BaK 4 is usually more costly than BK 7, you won’t see much difference through binoculars. They’re both prism glasses. You can tell prism type by looking into the objective end of a porro-prism binocular. If the exit pupil appears round, it’s BaK 4. If it looks square, it’s BK 7.
Porro prism binoculars allow better depth perception. Good roof prism glass uses phase-correction-coated prisms, called “p-coated,” which corrects the phase shift that occurs when light leaves uncoated roof prisms. Without p-coating, even the very finest roof prism binoculars don’t match the optical sharpness of the finest porro prism models.
The size of the field depends not only on eye relief, but on the size of your ocular (not objective) lens and the magnification. When you look through a glass, your eye becomes the apex of a cone of light whose base is the perimeter of the ocular lens. From the side, that cone is seen as a two-dimensional triangle. If the angle at the apex (your eye) is 30 degrees, (a 30-degree angle subtends 150 feet at 100 yards) and your glass is an 8x, then dividing 150 by 8 yields a field of 18.75 feet.
Real angle of view (RAOV) is used to determine field of view (FOV) and apparent angle of view (AAOV). To determine a binocular’s FOV at 1000 yards, multiply RAOV by 52.4 feet. AAOV is determined by multiplying the RAOV by the magnification. A binocular with an FOV over 400 feet or an AAOV over 60 degrees is considered a wide angle.
Your eyes can resolve about 60 seconds of arc, or one moa. Magnification gives your eyes more resolving power, but as magnification exceeds resolving power of the glass, you get a larger image that is no clearer. Rayleigh’s constant, 114.3, divided by the objective lens diameter in millimeters, yields the maximum resolution in seconds of angle. To get the maximum power at which the eye can use this resolution, we divide 60 seconds of arc (the naked eye resolution) by the maximum resolution to get the magnification. 50 mm = 26.2x, 32 mm = 16.8x.
For good definition (the optical quality most of us call sharpness), the diameter of the objective lens should measure, in millimeters, at least four times the magnification. For the finest definition, an objective at least five to six times the magnification helps enormously.
I think a 7x or 8x would be better. The 8x40 or 8x42 seems to be the overall favorite for birdwatchers, too. The 9x25 has just a little too narrow FOV for football from the top row of the bottom level.