请教这里的视力标准如何换算成国内的数值？

nils_li

an uncorrected visual acuity of at least 20/40 (6/12) with both eyes open.
" Y9 W8 a- k$ s' g# Vvisual aids (glasses or contact lenses) must have vision of 20:20 (6:6) with both eyes open.
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  b+ z7 E' Y6 L# s7 m8 [/ ~$ P3 X2 ]' ^请问上面的意思是什么？相当于国内什么等级的视力？谢谢！