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.
8 v! r# X  w* j2 I2 M, c$ Cvisual aids (glasses or contact lenses) must have vision of 20:20 (6:6) with both eyes open.
9 |$ k5 a! ?/ i& @  l
# o5 t) g& p  i' H7 |4 L8 R请问上面的意思是什么？相当于国内什么等级的视力？谢谢！