Eye Glasses Measurements (PDs)

Our doctors provide you with the best care possible. To provide you with the most precise and comfortable vision that your eyes deserve, proper eyeglasses dispensing and fitting are also an important part of the eye care process. Licensed professionals such as optometrists and opticians are trained to:

• Assess your visual needs and provide counseling.
• Make proper eyeglasses measurements.
• Adjust and fit the eyeglasses on the patient's face.

2. Eyeglasses measurements

Specific measurements are needed in order to dispense glasses. The measurements depend on your prescription, how high it is, presence of astigmatism, type of lenses (bifocals, progressives), and type of frames (curvature of the frame, of the patient's face).

PD (pupillary distance) measurement: the distance between the pupils. It has to be measured by a pupilometer, based on the corneal reflex method, in order to be the most precise. It has to be a monocular measurement (one eye at a time, from the nose to each eye's pupil).

Why? Only a deviation or difference of 0.5mm or 1mm can sometimes change your effective visual perception and comfort. It is especially more important on some prescriptions (medium to high RX, astigmatism, eye muscle deviation, etc.), and on some types of lenses (bifocals or progressives, aspheric lenses, high index lenses).

Monocular measurements are especially important on some RX or lenses. For example, if your total binocular PD is 70mm, your right eye may be 33mm and left eye 37mm. Glasses made with a total PD of 70mm, compared with glasses made with 33mm right and 37mm left, will feel different and have different visual properties, especially on some types of RX or lenses.

PD measurement is not the only measurement needed for proper eyeglasses dispensing. PD measurement is not part of a standard regular eye exam, as PD and other glasses measurements are only taken at the time of dispensing by a licensed optician at an optical store, with your final chosen frame sitting on your face.

Proper preparation of eyeglasses also requires other accurate measurements, such as:

• Vertex distance: the distance between your eyes and the back of the eyeglasses frame. It can only be measured with your physical frame sitting on your face. It is needed for medium to strong prescriptions (more than 4 dioptres). Why? If your frame sits closer to or further from your face than intended, the effective power your eyes receive is different, and the prescription on the lenses needs to be calculated and changed accordingly.
• Bifocal or progressive height measurements: the position of the bifocal or progressive lenses. It has to be measured with your frame on your face, as it varies with the size of the frame and how the frame sits on your face. A deviation of as little as 1mm in the progressive height can cause visual discomfort and change your effective vision.
• Single vision lens height (optical center): the measurement from the center of the pupil to the bottom of the frame. It can only be measured with your frame on your face. It is especially important for medium to high prescriptions, presence of astigmatism, aspheric lenses, high index lenses, and other eye conditions. An incorrect optical center can induce prism effects and eye strain.
• Base curve: the curvature of the lenses. The base curve sometimes has to be specified and changed depending on the patient's RX, frame, or shape of the patient's face. An incorrect curvature can induce or create more peripheral astigmatism, and can change your perception, magnification, and comfort.

To provide you with the most precise vision and comfort that your eyes deserve, these measurements have to be done by a trained, licensed professional such as an optometrist or optician working in an optical store. Contrary to some beliefs, your chosen eyeglass frames have to be on your face in order to make these measurements with precision.

4. Possible symptoms from inaccurate measurements

Visual or systemic symptoms, such as blur, eyestrain, diplopia, or headache, can develop if the spectacle parameters are inaccurate or proper dispensing procedures are not followed.

Optometry standard of practice: "The measurement of a patient's pupillary distance (PD), fitting height for multifocals, selection and fitting of an appropriate frame, selection of an appropriate lens material, and selection of an appropriate lens coating are considered part of the eyeglass dispensing service and not part of a regular eye examination."