Binocular and monocular PD describe the same pair of eyes at different levels of detail. Binocular PD gives one total separation; monocular PD preserves the right and left positions separately.
For precise centration, the useful question is often not only “how far apart are the eyes?” but where does each eye sit inside the chosen frame?
01 · BINOCULAR PD
Binocular PD records one total eye separation
A binocular PD of 64 mm means the measured total separation is 64 mm. It does not tell you exactly how far each eye lies from the frame or facial midline.
For simple cases with near-symmetry, one binocular value may be enough for some lens orders, but it cannot describe right-left asymmetry.
02 · MONOCULAR PD
Monocular PD records the right and left sides independently
Monocular PD records each eye position from the centre reference used by the measurement system. The exact reference convention can vary, but the purpose is to preserve separate right and left location data.
Assuming 32/32 from a 64 mm binocular value is therefore not equivalent to actually measuring 31/33 mm.
03 · ASYMMETRY
Right-left differences are common real-world conditions
The nose, eyes, ears and facial width are not perfectly symmetrical in many people, and a frame can naturally settle slightly off the geometric facial centre.
If both lenses are laid out from one assumed midpoint, one eye can end up more decentered than the other. Monocular data allow the layout to reflect that asymmetry.
04 · FRAME PD
Wearer PD and frame centre distance are different quantities
Using the boxing system, lens width A plus DBL is often used to estimate the frame’s geometric centre distance, sometimes called frame PD. It is a property of the frame, not the wearer.
The difference between that frame geometry and each monocular eye position helps determine the horizontal decentration required during lens layout. Changing frames therefore changes the decentration even when the wearer’s PD does not change.
05 · WHEN IT MATTERS
Monocular data become especially useful in progressives, high powers and asymmetric fits
Progressive lenses commonly use distance monocular PD so each fitting cross is centred to the corresponding eye. High prescriptions can also be more sensitive to unwanted prismatic effects caused by centration error.
Monocular PD is not automatically “premium” or necessary for every simple lens. The point is to use the level of positional data the prescription and lens design require.
SOLUTION GUIDE
GLANOS measurement and fitting check
One total separation value.
Separate right and left eye positions.
Frame geometry, not wearer anatomy.
Relate each eye to the frame centre.
Do not force right and left into equal numbers.
GLANOS LENS CHECK
One total distance cannot show where each eye sits inside the frame
The practical difference between binocular and monocular PD is not just one number versus two; it is the amount of positional information available for lens layout.
CAUTION
Do not create monocular PD by simply dividing binocular PD by two
Natural right-left asymmetry is common. When separate positioning matters, measure each eye directly.
GLANOS EYEWEAR
Eyewear discomfort has a reason.
binocular PD · monocular PD · pupillary distance · frame PD · lens centration · eyewear measurement