One of my friends in states answered my question the other day. I post her answer here. From the answers and comments I got I start to realize maybe this is another culture difference I never think of before.

> **How is your eyesight** would be the correct way to ask, but we do not ask other people that question unless there is a specific need to
> ask. 
> 
> No one would ever ask someone here **"What is your eyesight"**. So
> forget that phrase entirely.  If someone sees you are having trouble
> reading, they might ask if they need reading glasses.  We do not ever
> have any conversation in general about eyesight.  Someone might say,
> "I have to get a new prescription for my glasses today so I will be at
> the eye doctor's all afternoon", to tell what they are doing.  We
> would only ask if the conversation went to the topic of two people
> with problems wanting to talk about maybe who their doctor is or where
> they buy their glasses or might exclaim, "Oh, I can hardly see this. 
> I might  need a new prescription."
> 
> Rarely would you ever ask if someone is near-sighted or far-sighted. 
> I have never asked anyone that in my life nor have they asked me.  You
> certainly would never talk about what your prescription actually is
> about the diopter or whatever the word was that you used.  We would
> never in any case at all go into the technicalities of our
> prescription because none of us know it. We just don't talk about it
> here and if someone tried to have a chatty conversation with you ,
> that would never be the topic.