What is PD?

  • Post Date: February 24th, 2021 at 3:00 PM
  • Modified Date: June 24th, 2024 at 10:30 AM
  • by Alex E

Key Takeaways

  • Pupillary Distance (PD) is the distance between your pupils, measured in millimeters.
  • Critical for aligning optical centers of eyeglass lenses with your eyes.
  • Incorrect PD can cause prismatic imbalance and eyestrain.
  • Professional tools, like a pupilometer, ensure accurate PD measurement.
  • Far and near PD measurements differ and are often noted separately.
  • DIY methods (using a ruler or photo) can be inaccurate.
  • Accurate PD is essential for proper vision correction.

The PD, which stands for pupillary distance, is the distance between your pupils in millimeters. You have a Right Eye (OD) PD and a Left Eye (OS) PD, which can be different and are measured from the center of the nose to each eye independently.

Why is the Pupillary Distance (PD) important?

Your PD measurement dictates the horizontal placement of the optical center of the eyeglass lens. This allows the optical center of the lens to be aligned in front of the optical center of your eye during the manufacturing of eyeglass lenses.

What is the effect of an incorrect Pupillary Distance (PD) measurement?

If the PD is incorrect, then the actual location of objects you are looking at will move, an effect called ‘prismatic imbalance.’ This usually feels like a pulling sensation, making your lenses uncomfortable to wear and causing eyestrain. The higher the prescription, the more a PD error will be noticed.

Can I measure my Pupillary Distance (PD) myself?

While it is possible to measure your own PD, it is not recommended. The required accuracy of this measurement is one millimeter per eye. The methods available online to do so, from the credit card picture, to the rulers, have an error higher than the ANSI lens production tolerances. This means that there is a high chance that measuring your PD yourself will cause deteriorated vision and eyestrain, especially as the Rx gets stronger.

How is the Pupillary Distance (PD) written? What is the nomenclature?

There are technically four different PD measurements as follows:

  1. Far PD (OD, Right Eye)
  2. Far PD (OS, Left Eye)
  3. Near PD (OD, Right Eye)
  4. Near PD (OS, Left Eye)

Eyeglass lens laboratories only need the Far PD measurement to produce lenses. When the Right Eye PD equals the Left Eye PD, then sometimes the PD is written as just one number. This is called a ‘binocular’ PD measurement. It is best practice to always write each eye individually, called ‘monocular’ measurements. If there is a PD written with two PD’s, like 55/52, or 29/28, this can be either a Far PD/Near PD, or a Left PD/Right PD, which can be determined by the size of the number as Far/Near is ‘binocular’ and Left/Right is ‘monocular.’

How is a Pupillary Distance (PD) measured?

There are many ways that the industry measures a PD. The most accepted method is using a Pupilometer, which is a device held up by an optician in front of a patient, giving monocular Right and Left eye PD measurements. The other accepted method is marking the pupil center with a permanent marker by isolating one eye at a time, but this method is very difficult to master. It is not recommended to measure a PD using a picture as this method does not account for convergence and gives a large error that can cause eye strain.

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