Monocular PD and what it means

Monocular PD is the distance between the bridge of your nose and the iris of either of your two eyes. It is the opposite and half the binocular PD which is the distance between the two irises of your two eyes. Your PD measurements can be taken by:

  • Contacting your optometrist
  • Using PDCapture technology used by various online practitioners to measure it

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What is Monocular PD? What are importance of monocular PD?

People often order glasses by simply providing their binocular PD. However, binocular PD is known to produce lenses that roughly align with the applicant but doesn’t ensure a 100% exact fit. This is due to the fact that binocular PD comes in low 60s and even in high 50s and low 70s at times. This is where the monocular PD comes into the picture. Apparently, monocular PD produces more accurate spectacles because its numbers come in pairs (one for every eye) which are often in the low 30s and high 20s. For this reason, monocular PDs tend to position the clearest region (which is the optical center of your lenses) more precisely in the middle of your eyes for a more comfortable vision and clearer image.

Fabrication of spectacles calls for the need to take monocular PD measurements to relate to the frame. Fortunately, most people’s center bridge tends to coincide with the bridge of the frame. Even if there is variation, it is so tiny that it goes unnoticed and doesn’t affect the eyewear’s usability. In some extreme cases such as one side of an individual’s nose being steeper than the other, one side of the nose bulging over the other side or the nose or other problems with which distorts the symmetry of the nose, fixed pads tend to be added on the nose to correct the imbalance.

Near Monocular PDs

It is often assumed that the distance PD taken on the spectacle plane is always same at any two distance from a person’s cornea. However, this isn’t true in monocular pds perspectives. The distance between the plane’s converging lines is known to vary with the vertex distance.

Anatomical variations

There are situations whereby an individual’s nose or glabella (the area separating the eyebrows) or both aren’t centered appropriately relative to the lateral sides of the head. For instance, the eyes may not maintain an equidistant distance from the glabella’s center either at an angle or straight down.

A. Nose centered: The monocular pds here are equal. The frame will automatically center on the face.

B. Nose off-center of the right of the patient: Monocular pds can be measured only after fitting the frame.

C. Nose centered: The left monocular PD of the patient is smaller than the right monocular pds.

D. Glabella is centered while the nose slopes slightly to the right:The frame is adjusted and pushed to the right. Monocular pds can only be measured after fitting the frame.

Just like binocular pds, the process of studying monocular pds is an intricate affair. Although most individuals fall within the default margins, there is quite a significant number of individuals who face’s asymmetry calls for adjustments.