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.
* a& F4 A- |* Y- i2 Bvisual aids (glasses or contact lenses) must have vision of 20:20 (6:6) with both eyes open.
' B( B; W# ]$ l* k% O
9 K: p7 c( ]9 W  e0 }/ y: I& q请问上面的意思是什么？相当于国内什么等级的视力？谢谢！