Have blurry vision and need glasses to see? Your prescription mostly likely falls into one of two categories: you are nearsighted or you are farsighted. But what's the difference between these two categories? The easiest way to remember the difference is it's in the name!
Nearsighted individuals see “better" for the up close near distances. Farsighted individuals see “better" for the far away distances.
That's the most simple explanation for the differences between the two main categories of eye prescription. Let's explore more in depth the differences between the two:
Nearsighted people see up close
In the most simple terms, individuals with nearsighted (also known as myopic) prescriptions see better up close. Glasses or contact lenses are used to correct their vision to see off in the distance. Having nearsighted vision is very common. Close to a half of all adults have nearsighted vision!
However, there is quite a large range of prescriptions. Some people require very little to see off in the distance. Some people require a very high prescription.
The amount of prescription someone requires actually affects what they can see up close. And it can actually be calculated!
Roughly calculate your prescription
Here’s a fun science experiment.
- Grab a measuring stick that can measure in centimeters or meters.
- Hold some small print close to you.
- Gradually push that small print further away.
- Measure the distance at which that small print becomes blurry.
- Divide 1 by this distance in meters; This is roughly your prescription!
The way we measure prescriptions is in the unit ‘diopter’. This unit is calculated from 1 / the distance (or the inverse of the distance) of where objects are in focus for you.
Example:
Those with higher prescriptions, such as a -3.00, can only see 0.33 meters in front of them (1 / -0.33 meters = -3.00 diopters); but those with lower prescriptions such as a -1.00 can see all the way up to 1 meter in front of them (1 / -1 meters = -1.00 diopters). As you can see, those with very high prescriptions may need to hold things very very close to read (such as right in front of their face).
What's going on in the eye
For perfect vision, light must focus at a single point on the retina in the back of the eye. Anything that causes light to focus elsewhere causes blurred vision.
In nearsighted eyes, light focuses too much and too early in front of the retina. If you bring an object closer and into focus, you are actually “pushing" the focus of that object onto the retina to make it clear.
Glasses that correct nearsighted prescriptions use lenses that prevent light from focusing too early so that it can correct your vision. These lenses reduce the focusing power of the eye and thus the prescription has negative numbers.
Nearsighted or myopic eye, graphics by Gumenyuk I.S., CC BY-SA 3.0, via Wikimedia Commons / cropped from original
Farsighted people see off in the distance
You may even still have 20/20 vision with a farsighted (or hyperopic) prescription; you just have a harder time seeing up close.
However…
This one is a little trickier to understand. The reason being is that there are many farsighted people who see totally fine both in the distance and up close. The reason for this has to do with the natural lens inside our eye.
But before we get into that, let's see what's going on in the eye
Light focuses beyond the retina
Farsighted individuals have light which doesn't focus enough. Objects don't come into focus on the retina. Instead these objects are focused behind the retina.
To correct farsighted prescriptions, glasses use lenses which bend and focus light more so that vision hits the retina perfectly. These lenses add focusing power to the eye and thus the prescription has positive numbers.
Farsighted or hyperopic eye, graphics by Гуменюк И.С., CC BY-SA 4.0, via Wikimedia Commons / cropped from original
Our natural lens
Note the shape of the lens used in the glasses. The shape is very similar to the natural lens in our eye. In fact, many people don't notice they are farsighted because the natural lens inside the eye is doing all the work.
When we are young, our natural lens is flexible. It's able to change shape to focus light. When someone who sees perfectly in the distance looks up close, this lens changes the focus in the eye to also allow clear up close vision.
When you look up close, you need more focusing power and the natural lens changes shape to add more power to the eye. And it does it all seamlessly without us noticing that it’s working.
At baseline, farsighted people require more power to focus things onto their retina. But this natural lens delivers and not only adds extra power to focus through that farsighted prescription, but also enough power to focus up close. The natural lens is able to do all the work that a pair of glasses can do; allowing farsighted individuals to often have good vision without any glasses at all (unless the farsighted prescription is too big).
That is, until one key event in life.
Presbyopia
This natural lens is only flexible when we are young. As we get into our 40s, this natural lens starts to become inflexible. You lose the ability for this lens to adjust your focus up close. Things become harder to read. This is called presbyopia and it affects everyone when their vision is corrected.
When this lens is becoming weaker, those farsighted individuals who could see perfectly fine for both distance and up close start to notice trouble with their vision.
The first thing that they notice (like everyone else) is that their up close vision becomes more blurry. Because their lens is already doing the work to correct their farsighted prescription, it has less reserve to focus up close. These farsighted individuals will notice more difficulty with their up close vision at an earlier age (and require reading glasses) since the lens has a greater amount of work to do.
As their lens becomes even more inflexible, it can no longer compensate for their farsighted prescription. Distance vision starts to become blurry (and up close vision becomes even more blurry).
In the end, both distance vision and up close vision can become blurry for farsighted people; but distance vision will still be less blurry than their up close vision.
Summary
Although both nearsighted vision and farsighted vision both cause blurriness, there are some fundamental differences between the two. In the most simple terms, the nearsighted individuals can still see up close but everything is blurry in the distance. The farsighted individuals may be fine for years and years until eventually their up close vision becomes blurry and they see better off in the distance.
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