I'm Zhao Xiaodao and I'm an ophthalmologist.
My friend Lao Wang is a short-sighted eye, and recently he came to me and asked, "Dr. Zhao, I heard that myopia is not presbyopia, and presbyopia is not myopia?" I looked up at him, he was quite naïve.
The relationship between myopia and presbyopia is not the same at all.
Let's first take a look at the visuals and how myopia occurs:
People who are nearsighted see clearly near but cannot see distant objects clearly because the natural focus of the eye is near and they lose the ability to focus at the distance. It is corrected by wearing a concave lens, denoted by a minus sign (-). If it is 200 degrees of myopia, then the natural focus of the eye is at the position of 0.5m, and by wearing -200 degree myopia glasses, you can adjust the focus to the distance and help you see distant objects clearly.
So how does myopia happen?
The occurrence of myopia is mainly caused by the growth of the eye axis. The eye axis can be understood as the anterior and posterior diameter thickness of the eyeball. In the eye that is facing the eye, the image will be projected on the retina after the light passes through the lens, while in the myopic eye, the image will focus in front of the retina after the light passes through the lens, causing us to not be able to see distant objects clearly.
Next, let's take a look at the visual effect and how it occurs in presbyopia:
People who have presbyopia see very clearly from a distance, but it is blurry when they get closer, because the natural focus of the eyes is at a distance and they lose the ability to focus closer. Therefore, some elderly people need to hold the mobile phone far away to see clearly, and presbyopia is corrected by wearing a convex lens, which is represented by a positive sign (+). If it is a presbyopia of 200 degrees, you need to wear glasses of +200 degrees.
If you don't consider the original power of your own eyes, the presbyopia power of different ages is roughly as follows:
Around 45 years old, presbyopia is about +150 degrees;
Around 50 years old, presbyopia is about +200 degrees;
Around 60 years old, the presbyopia is about +300 degrees;
How did presbyopia happen?
Presbyopia can be understood as the normal aging of the eyes, which is a state that everyone must face when they reach a certain age, and the ability to focus to close range is reduced due to the hardening, thickening, and elasticity of the lens due to the aging of the eye.
Generally speaking, the occurrence mechanism of "presbyopia" and "myopia" is different, in fact, they can coexist, and the statements of "myopia and agelessness" and "presbyopia are not myopia" may not be true.
When a myopic person is young, he can wear glasses to see near and far, and when a myopic person has presbyopia, then wearing myopia glasses, he can only see far away, and he is not clear when he sees near, and he must take off his glasses when he sees near.
Therefore, whether it is myopia, astigmatism, farsightedness or normal eyes, when we are old and our eyes are aging, presbyopia will inevitably come to our door.
However, between myopia and presbyopia, there is indeed a certain "offset algorithm".
Because the corrective myopia lens uses a concave lens, while the corrected presbyopia uses a convex lens, to a certain extent, myopia and presbyopia can indeed achieve partial "offset":
Scenario 1
If a person who is not nearsighted has presbyopia, assuming that the presbyopia is +100 degrees, then it is very simple, he only needs a pair of glasses to see closely.
Closer: +100 degree glasses
Look far: Don't wear glasses
Scenario 2
If a person with moderate myopia has presbyopia, assuming that the presbyopia is +300 degrees and the original myopia is -300 degrees, then he needs to take off his glasses and wear myopia glasses to see far.
Look close: just take off the lens (no need to wear glasses)
Vision: -300 degree myopia glasses
Situation 3
If a person with mild myopia at -100~200 degrees has presbyopia, this situation is actually quite embarrassing, and it will not be clear from far and near. Assuming that the presbyopia power is +300 degrees and the original myopia is -100 degrees, then he needs two pairs of glasses, a convex lens for near vision and a concave lens for far.
看近： +300度-100度=+200度花镜
Vision: -100 degree myopia
To put it bluntly, myopia will also be presbyopia, but contrary to the normal person's presbyopia experience, you need to wear glasses to see far, you must take off the glasses to see near, and you can no longer have a pair of glasses far and near, and in some cases, you may need two pairs of glasses (or gradient lenses) to see near and far.
Well, that's all for today's sharing.
I am Zhao Xiaodao
I am an ophthalmologist