Before buying eyeglasses, make sure you know your focal distance.
What is cortical distance?
Pupillary distance or PD refers to the distance in millimeters between the center of one pupil and the center of the other. It is a necessary measurement so that the optical centers of your eye lenses coincide with the colic distance. Thus, maximum visual performance is achieved.
Where is the cortical distance indicated?
The specific measurement is given in your prescription by your eye care professional. Usually, it is written on your prescription in the field PD or else Cortical Distance.
How much can my choral distance be?
In adults: 54-74mm
For children: 43-58mm
Is the focal distance different for far and near?The answer is
Yes!
Pay special attention to completing the choric distance of the recipe. The cortical distance for far distance correction differs by 3mm from the corical distance for correction
of the close distance.
B.C. If your focal distance for far is 63mm, then for near it will be 60mm.
Read more:
How to read / fill your prescription
Types of LensesSize
Details 
