When you purchase glasses online, you will inevitably need to produce your PD numbers. These numbers are not necessarily part of your prescription for glasses and often times, they are not even measured during your eye exam.
So what do you do when the website you are ordering your glasses from is asking for them, but you have no idea what they are?
In this article, I will show you how to measure your PD for eyeglasses, using nothing more than a few objects we all have at home.
What Does PD Mean?
PD stands for the pupillary distance, and it refers to how far apart your eyes are. It is called the pupillary distance because to measure it, you take the distance between the center of each pupil.
Why is the PD Important?
The PD is required when making glasses because it allows the glasses manufacturers to align the center of the lenses with where the eyes will be behind the frame.
It is important to understand that the vision through lenses is not the same through every part of the lens. For the prescription to be accurate and for the vision to be clear, the eyes visual axis must be looking through the ‘optical center’ of the lenses.
If the PD is not taken into consideration when making glasses, it may result in viewing through a spot on the lens other than the ‘optical center’. This can make the vision uncomfortable. You may experience blurriness, eye strain, double vision, headaches etc.
The larger and more complex your prescription is, the more important it is to have accurate PD measurements.
Different Types of PD
There are 6 main types of PD:
1. Distance binocular PD
2. Distance monocular (right) PD
3. Distance monocular (left) PD
4. Near binocular PD
5. Near monocular (right) PD
6. Near monocular (left) PD
When people refer to the ‘PD’ without any other classifiers, they are by default referring to the distance binocular PD. This is the most important one, and it’s the distance between the center of the two pupils when eyes are looking at least 20 feet (6 meter) away.
The distance monocular PDs are the distances between the center of the face to the center of the pupil on each side. These are also known as the ‘half PD’s’ but they are not simply the distance binocular PD divided by two! Most people’s faces have slight asymmetry causing the left and right eyes to be at different distances from the center mid-line of the face.
The near binocular PD is the distance between the center of both pupils when the eyes are looking at 16 inch (40 cm). Looking at this distance causes the eyes to turn in towards each other slightly, causing the distance between the two eyes to get smaller.
Similar to the distance half PDs, the near monocular PDs (or near half PDs) are the distances between the center mid-line of the face to the center of each pupil when the eyes are looking at 16 inch. Again, these are usually a little asymmetric.
Which PD Do You Need To Order Glasses Online?
You can get away with ordering glasses by simply providing your distance binocular PD. This is a single number that’s usually in the low 60s, but can be in the high 50s to low 70s. If that’s all you have, it will do. It will roughly align the lenses to your eyes, but it will not be 100% exact.
For more accurate glasses, it is better to enter the distance monocular PDs. These are two numbers (one for each eye) that are usually in the low 30s or high 20s. Using the distance monocular PDs will position the clearest part of the lenses (the optical center of the lenses) precisely at the center of your eyes for clearer and more comfortable vision.
If you are ordering from a website that does not take 2 PD numbers, it may be better to find a site that does.
If you were not given a PD at your eye exam or you were only given your binocular distance PD but want to order glasses using your monocular distance PDs (for better results), you can:
- Return to your eye docto’rs to have it measured
- Read on to learn how to measure it yourself
What You Need To Accurately Measure Your PD
You don’t need any special optometric equipment to measure your PD. All you need is a standard ruler (that shows centimeters) and a camera. Cellphone cameras are fine for this purpose.
You will also need to know a little bit about the anatomy of the eyes, which I will cover in this article.
How To Measure Your PD
1. Position your ruler against your forehead. Make sure to place the centimeter side facing out (so that it’s visible to the camera).
• Try to position the ruler as close as possible to your eyes without obstructing them.
• Take extreme caution not to poke your eyes with the ruler.
• Try to take the picture as square-on as possible. Avoid taking it at an angle.
2. Have a friend take a picture of you, or take a selfie of yourself with the ruler against your forehead.
***Very Important: For distance PD, the picture must be taken while you are looking at least 20 feet (6 meters) into the distance. DO NOT look directly into the camera***
3. Upload the picture to your computer and open in it any basic picture editing program. For this purpose, I like to use Paint (for PC) or Photoshop.
4. Cut a rectangle at the edge of the ruler that spans a distance larger than the distance between both eyes. Then drag it down to just below the eyes.
5. Use the mm scale to measure the distance between the center of both your pupils in millimeters (mm).
*TIP: If you measure the distance from the edge of the iris (the colored part of the eye) to the corresponding edge of the iris on the other eye, it is also the PD. Measuring this way is more accurate then trying to estimate the center of the pupil (the black circle in the center of the eye)*
6. For monocular PDs, draw a vertical line down the very center of the bridge of your nose. Measure the distance from that line to the center of your pupil on the right and the left side.
*TIP: The nose often curves slightly to one side. Do not center your vertical mid-line with the tip of your nose. Place it in the center of the bridge of the nose, which is the hump that forms directly between the two eyes.*
*TIP: If the right and left distance monocular (half) PDs (33mm and 31mm) do not add up to the distance binocular PD (64mm), something went wrong with your measurements. You’ll have to double check your work.*
In the instructions above, I suggested that you take a picture with your ruler on your forehead just above the eyes. This is just to make it easier when your trying to measure the distance between your eyes later on in your photo editing program.
However, if you’re very proficient at photo editing, as long as the ruler is anywhere in the photo and the the scale is visible, you will be able to take the measurement. The only thing you need to keep in mind is to keep the ruler in the same plane of the eyes. Do not hold it away from (or behind) your face.
What Are The Near PDs For?
The near PDs are for reading glasses, and some progressive (no-line bifocal) lenses. To measure the near PDs, you would repeat the exact same steps above but just change 1 crucial thing:
*Instead of taking the photo while you are looking into the distance, you have to take the photo while you are looking at something 16 inches (40cm) away.*
The easiest way to do this is to position the camera at 16 inches from your eyes, and stare directly into the camera as you take the picture.
Note that in the example above, both near monocular (half) PDs (32mm and 31mm) add up to the near binocular PD (63mm).
Also note that in the two examples above, the distance binocular PD (64mm) is bigger than the near binocular PD (63mm). This is what you should expect. If your near PD is larger than your distance PD, you may have made a mistake along the way.
To have this explained a different way, check out 39dollarglasses.com’s simplified explanation.