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.
visual aids (glasses or contact lenses) must have vision of 20:20 (6:6) with both eyes open.
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请问上面的意思是什么？相当于国内什么等级的视力？谢谢！