In reply to jethro kiernan:
Those look like they take up a fair bit of real estate in your field of view (though the same can be said of most bifocals I suppose).
I don't have great uncorrected eyesight in low light (pupils dilated, bigger aperture, less d.o.f.) but find if I shine a bright light on a map I can read it. Otherwise I tend to be wearing my (varifocal) glasses inside my goggles.
If map reading as part of an assessment or exercise, a compass with a big magnifier on it might also be useful... or a credit card fresnel sheet (though faffy, it would just look like you were using a compass, and you could tie to your compass lanyard).