Are you a glasses wearer? Or are you looking to get one? If so, today we are going to talk about something that is very important when choosing a pair of glasses that fits your eyes perfectly. This is a measurement called 'Pupillary Distance' . You may not have heard this term before. However, it is very important to keep your vision clear.
What is this 'pupillary distance'?
Simply put, Pupillary Distance , or PD for short, is the distance from the center of your pupil (the black circle in the middle of your eye) to the center of your pupil in your other eye. This is usually measured in millimeters (mm). Eye specialists sometimes refer to this as the Interpupillary Distance (ID) .
When you are given a prescription for glasses, this PD value is very important. Because the lenses of the glasses need to be adjusted exactly in front of your eyes. That is when you get the clearest vision possible. If you are buying glasses online, you will definitely need this PD value along with the prescription from your eye exam.
Can I measure this at home? How do I do it?
Yes, you can try to measure your PD at home if you want. However, for the most accurate measurement, it is best to have it measured by a trained eye care professional . Because even a small change of a few millimeters can affect your vision.
However, if you want to try it yourself, all you need is a ruler marked in millimeters (mm) and a mirror. There are special rulers available online that are specifically designed to measure PD. There are also some smartphone apps that claim to help you take this measurement. However, it is difficult to be very confident in the accuracy of the measurement, whether you measure it yourself or with an app.
If you want to measure yourself, here's how:
This can be a little difficult, so you need to be patient.
1. First, stand in front of a mirror in a well-lit area. Keep your face about 18 inches away from the mirror.
2. Now, hold the ruler up to your face so that you can see the millimeter markings. You want to hold the ruler as if you were looking at a wall, just below your eyes.
3. Close your left eye.
4. Now, looking only with your right eye, place the zero (0) mark on the ruler exactly in the middle of your right eye's black circle. Imagine that there is a line that divides the black circle exactly in two, and the zero should be on that line. Don't move the ruler at this point, okay? If you do, you'll have to start over.
5. Okay, now open your left eye and close your right eye.
6. Now, looking only with your left eye, find the millimeter mark that is exactly in the middle of your left eye's black circle. As before, if you imagine the black circle being exactly in two, the closest millimeter mark to that center is your PD value.
Remember: Measuring this way alone is a bit difficult, and mistakes can happen. So get help if possible.
It's even easier if you measure with someone's help!
It's even easier if someone helps you. Here's how to do it:
1. The person measuring your PD should sit in front of you, with about 18 inches between your faces.
2. He should hold the ruler up to your face, so that he can see the millimeter markings. The ruler should be under your eyes (like you are looking at him from above a wall).
3. Now, look straight into the left eye of the person measuring your PD. From this point on, try not to move your head.
4. The person measuring should position the zero (0) mark on the ruler so that it is at the outer edge of your right eye's black ring (that is, the edge of your right eye's black ring that they can see from their left). Do not let the ruler move at this point, or you will have to start over.
5. Now, look straight into the right eye of the person measuring your PD.
6. The person measuring should look at the millimeter mark closest to the inner edge of your left eye's black ring (that is, the edge of your left eye's black ring that they can see from their right). That millimeter mark is your PD value.
Why is this 'pupillary distance' (PD) so important?
Think of it this way: the pupils in your eyes are like the lenses of a camera. Light enters through these. Then, this light passes through the lens inside the eye and falls on the retina at the back of the eye. That's when we see.
However, if you have poor eyesight, this light is not focused properly. That's why we see some distances blurry. Here are a few examples:
- Farsightedness/Hyperopia - can see clearly up close but cannot see far away.
- Nearsightedness/Myopia - Can see clearly at a distance but cannot see up close.
- Presbyopia - a condition that affects many people after the age of 40.
- Astigmatism - Light is not focused properly due to a change in the shape of the eye.
When it comes to glasses, not everyone will get the same type of glasses. The reason for this is that many factors affect your vision, such as the severity of your vision problem, and the number of degrees of astigmatism you have.
By accurately measuring your PD and adjusting the optical center of your lenses so that they are centered in the center of your cornea, you will get the most out of your lenses. This will give you clear, comfortable vision.
Is 'Nasopupillary Distance' something like this?
Yes, there is another important measurement besides PD, which is the 'Nasopupillary Distance' . This is the distance from the middle of your black eye to the middle of the bridge of your nose.
Imagine, four out of five people do not have completely symmetrical eyes and noses. That is, the `Nasopupillary Distance` is not the same for both eyes. Most of the time, this difference is less than 2 millimeters (2mm). However, for about one in six people, this difference can be more than 2 millimeters (2mm). Since this distance is small, many people may not even notice it or know about it. However, this can also affect the design of glasses, especially multifocal lenses.
What happens if the 'Pupillary Distance' (PD) measurement is incorrect?
Imagine what would happen if you were fitted with a pair of glasses that were incorrectly measured for your PD. You would likely start to experience discomfort like this:
- Eyestrain
- Headache
- Distortions in your vision - things appear distorted or distorted
Although these symptoms are not very dangerous, they can cause you a lot of discomfort. Also, your vision may worsen due to unnecessary strain on your eyes. That's why it's important to have the right PD.
Does this 'pupillary distance' (PD) change from time to time?
When you are young, this PD changes as you grow up. That is, the PD you have when you are young is not the same as when you are young. However, this PD value usually does not change after you become an adult.
However, there are very rare cases where it can change. For example, if you have to have surgery on your eyes or face, your PD may change slightly after that surgery.
What is the average 'pupillary distance' (PD)?
The average PD value for an adult is about 63 millimeters `(63mm)`. However, this varies from person to person. Most adults have a PD value between 50 and 70 millimeters `(50mm - 70mm)`. A small number of people's PD values can vary outside of this range. Some people may have a value as low as 45 millimeters `(45mm)`, while others may have a value as high as 80 millimeters `(80mm)`.
In what situations should we not rely on our own judgment?
If you only use your glasses for one purpose (for example, reading or light work), your PD should be fine for that purpose. However, there are some cases where it is necessary to measure your PD very accurately . These are the cases where:
- If you need multifocal lenses: These are lenses such as `progressive lenses`, `bifocals` (lenses that can see both far and near), or `trifocals` (lenses that can see all three distances, intermediate distances, and near). To make each part of these lenses exactly, you need to know the PD value very accurately.
- If you need to continue wearing glasses for both near and far work: Because when we look at something close, our PD narrows by about 3 millimeters (3mm).
- If you have any misalignment of the irises of your eyes (such as strabismus): In such cases, it is essential to have your PD measured by a specialist.
In cases like this, it is best to have your PD measured by an ophthalmologist or trained eye care professional rather than measuring it yourself.
Why isn't this PD mentioned in my glasses prescription?
This is a problem that many people have. In some countries (for example, the United States) there is no law that requires the PD value to be stated when giving a prescription for glasses. However, some states do have such laws. In Sri Lanka, usually your ophthalmologist or the glasses manufacturer will measure this PD value and keep it with them.
They may be able to give you a PD value if you ask. However, some places may charge a separate fee for it. However, in order to get a pair of glasses that fit you perfectly, it is essential to have this PD value measured accurately. Therefore, it is a good idea to ask your eye doctor or eyeglass shop about this.
So, here are the take-home messages:
Okay, so here are some things you need to remember from what we've talked about:
- Pupillary Distance (PD) is the distance between the black rings of your eyes.
- This PD value is very important for making glasses that give you clear, comfortable vision .
- While you can try to measure this at home, it is best to have it measured by an eye care professional for the most accurate measurement.
- If the PD measurement is incorrect, you may experience discomfort such as eyestrain, headaches, visual distortions , and your vision may even deteriorate.
- The correct PD value is essential, especially if you use multifocal lenses or if you have a condition like strabismus.
So, the next time you go to get your glasses fixed, pay a little attention to this PD value. Because even a small difference of a millimeter or two can have a big impact on your vision! Two healthy eyes are a great value in life.
` Pupillary distance, glasses, eye exam, vision, eye health, PD measurement, interpupillary distance
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