What is the common answer to the question: What's your eyesight?

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.

I suspect the answer you're looking for is "20/20 vision," or the average level of vision. An article from the American Academy of Ophthalmology offers this explanation:

“A person with 20/20 vision can see what an average individual can see on an eye chart when they are standing 20 feet away,” says Dr. McKinney. McKinney is an ophthalmologist and glaucoma specialist at Eye Health Northwest, Oregon City, Ore.

[...]

For example, if you have 20/30 vision, it means your vision is worse than average. Twenty feet away, you can read letters most people see from 30 feet.

But 20/20 vision is not perfect vision. A person can have 20/15 vision, which is sharper than average. If you have 20/15 vision, you can see a line in the eye chart at 20 feet that the average person can only see when they are 15 feet away. The goal of glasses or contacts is to bring a person’s vision to 20/20.

If you look at the article you'll also find it features a chart similar to the one you have posted.

This phrase is absolutely commonly used and understood, enough so to appear in aphorisms like "hindsight is 20/20," meaning "it is easy to see what one should have done after the fact." It is read "twenty twenty."

If I'm not mistaken, metric countries often use 6/6 vision, with meters, rather than feet, in the same way.

"What is your eyesight?" is asking for a description of vision, of the working of the human eye. A suitable answer would be "Eyesight is the ability to perceive light".

"HOW is your eyesight?", or more commonly "How good/strong are your eyes/glasses?" is asking on the quality of your sight, as compared to an average human. The normal answer would be "Good", or "Fine as long as I wear my glasses", or "I'm quite nearsighted". Or of course "None of your business!!", because it is a very personal question.