It is very important that the optical centers of your new spectacles coincide with your own pupillary distance, i.e. the distance measured in millimeters between the centers of your two pupils. You can easily do this with a Perspex ruler and a mirror.
The measurement in Fig. 1 is exactly the same as the measurement in Fig. 2 due to the natural symmetry of the eye. Hence a measurement is taken from the outside edge of the iris (the coloured ring around the pupil) on the RIGHT EYE to the inside edge of the iris on the LEFT EYE or vice versa.
Close your left eye and look into the mirror with the ruler placed partially over the eyes with the zero mark in line with the outside margin of the iris in the right eye. Now close your right eye and open your left eye then read off the measurement in millimeters coinciding with inside margin of the iris in the left eye. This is your pupillary distance, which must coincide with the optical center of your new spectacles.
This can be done with a magnifying mirror if your near vision is poor or can be done by someone else using the same method without a mirror, with the measurer standing in front of you.
The measurer asks the person being measured to shut their left eye and to look into the right eye of the measurer. The ruler is placed over the bridge of the nose as in fig. 3 and the zero is lined up with the outside edge of the iris (right eye). The person being measured then closes their right eye, opens their left eye and looks at the measurers left eye and the measurement in millimeters is read off the inside edge of the iris (left eye) as in fig. 3.
This is not as complicated as it sounds and will ensure an accurately completed prescription.
If reading glasses are required then deduct 3mm from the final measurement.