Measuring accurately and positioning the lens correctly is extremely important for those with requirements of high lens power. Pupillary distance is calculated in millimeters; it is the distance between pupils of an individual and this is essential in correctly mounting frames and eye glasses in spectacles.
Pupillary Distance or PD is the distance measured between the outer edge of one pupil and the inner edge of the other pupil, normally taken in millimeters. Some opticians state this in single figures such as 60 millimeters while others break it into 2 sets of figures, for example, R31 and L29 - here it is the distance between each eye and the center of the nose.
Simple forms of measuring PD include:
- Take a millimeter scale, ask a friend to hold it just above the center of your eyes, now measure the distance between the inner and outer edges of your eyes. For accurate measurements, take care to ensure that both parties are sitting comfortably at the same level, preferably resting the chin so as to avoid shaking and thereby inaccurate measurements. Seated opposite each other, and keeping ahead of the person being measured, the friend will have to hold the scale by their left hand and place it above the center of your eyes. Closing their left eye, measure the distance between the outside edge of the left pupil and the center of the nose. Next, with the right eye closed, get the friend to measure the distance between the inside edge of the right pupil and center of the nose. Closing one eye is very essential to avoid parallax error in measurements. All through the measurement process, the person whose PD is being measured is required to sit still, fix sight on a particular object directly in front of them and not move their pupils at all. Preferably, do not blink much, since that could vary the vision and corrupt measurements taken.
- While the method mentioned above is quite effective, it is quite amateurish and is not recommended at all, particularly if you want to get prescription glasses based on such sort of measurements. Since it is a matter relating to the eyes, a vital sense of sight for a human being, it is highly recommended by the writer that everyone consult an ophthalmologist or at least an optician who will take these measurements accurately with use of highly sensitive and accurate instruments. There are apparatuses that electronically sense positions of the pupils and the nose and give out very accurate readings.
While getting the PD of an individual, take care to note that the measurements are stated differently by opticians and ophthalmologists depending upon whether the person's sight problems are classified as varifocals, high prescriptions or prisms. Normally, it is only in cases of single vision that the measurement is quoted in single value; or else it is mentioned in varying values for each eye.
Take care not to rely on old or outdated prescriptions while getting your spectacles, always getting newer and professional measurements on prescriptions.