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G'day syousef,
Magnification = focal length of objective lens / focal length of eyepiece
Altering the focal length of the objective lens (which changes its curvature) is one way to change the magnification of a binocular. The Nikon SE series of binoculars follow this pattern. They come in 8x32, 10x42 and 12x50 versions which all use the same eyepieces and prisms, just the objective lenses (and the portion of the binocular holding them) differ. However, many manufacturers offer series of binoculars that use common objective lenses and prisms but different focal length eyepieces to give different magnifications, e.g., the Fujinon FMT-SX 7x50 and 10x50. Any combination of the two approaches is possible but manufacturers want to use as many common parts across a range of binoculars as they can to keep costs down.
The apparent field of view (AFOV) of an eyepiece depends on several things, e.g., the curvature of the lenses within the eyepiece, their spacing, the glass types used and the aperture of the field stop.
Some of these parameters are linked by the formula Chris gave you:
AFOV ~= True field of view (TFOV) x Magnification
All the best and happy observing!
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