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What's The Difference Between Short Sighted vs Long Sighted?

What's The Difference Between Short Sighted vs Long Sighted?

Have blurry vision and need glasses to see? Your prescription mostly likely falls into one of two categories: you are short sighted or you are long sighted. But what's the difference between these two categories? The easiest way to remember the difference is it's in the name!

Long sighted individuals see “better” for the long far away distances. Short sighted individuals see “better” for the short up close 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:

Short sighted people see up close

In the most simple terms, individuals with short sighted (also known as myopic or nearsighted) prescriptions see better up close. Glasses or contact lenses are used to correct their vision to see off in the distance. Having short sighted vision is very common. Close to a half of all adults have short sighted 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!

The way we measure prescriptions is in the unit ‘diopter’. This unit is actually calculated from where objects are in focus for you (it is the inverse of how far away that focus is from you). For short sighted individuals, objects are in focus up close (which is why they can see up close). Their prescription is the inverse of how close that is from them.

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.

Short sighted or myopic eye

In short sighted 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 short sighted prescriptions use lenses that prevent light from focusing too early so that it can correct your vision.

Correcting a short sighted or myopic eye

Short sighted or myopic eye, graphics by Gumenyuk I.S., CC BY-SA 3.0, via Wikimedia Commons / cropped from original

Long sighted people see off in the distance

This one is a little trickier to understand. The reason being is that there are many long sighted (or hyperopic or farsighted) people who see totally find both in the distance and up close. The reason this is the case 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

Long sighted or hyperopic eye

Long sighted 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 long sighted prescriptions, glasses use lenses which bend and focus light enough so that vision hits the retina perfectly.

Correction of long sighted or hyperopic eye

Long sighted 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 glasses. The shape is very similar to the natural lens in our eye. In fact, many people don't notice they are long sighted because the natural lens inside our eye does all the work.

When we are young, our natural lens is flexible. It's able to change shape to focus light. For long sighted people, this means that the natural lens is doing all the work that glasses would do. But this natural lens is important for everyone.

For someone who sees perfectly in the distance (whether naturally or through the use of glasses), light is focused perfectly on the retina. Remember how short sighted people were able to “push” the focus of an object onto their retina by bringing it closer? If you bring an object up close when everything is already perfectly focused onto the retina, it will “push” the focus behind the retina. If nothing else happens, this will make that object blurry.

But that natural lens adjusts to bring that up close object in focus; and it does it seemlessly without us noticing that it's working.

Presbyopia

But 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.

But when this lens is becoming weaker, those long sighted 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 long sighted prescription, it has less reserve to focus up close. These long sighted individuals will notice more difficulty with their up close vision at an earlier age (requiring 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 long sighted 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 long sighted people; but distance vision will still be less blurry than their up close vision.

Summary

Although both short sighted vision and long sighted vision both cause blurriness, there are some fundamental differences between the two. In the most simple terms, the short sighted individuals can still see up close but everything is blurry in the distance. The long distance 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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