Buying Glasses Online – PD & Fitting Height

March 28th, 2009 by andum | Posted in Buying Online, Manage Money, Save Money, Vision Care

photo credit: oskay

How often have you heard someone tell you how much he or she paid for their prescription glasses? How often have you told someone how much you paid?  These statements never seem to suggest that the glasses were cheaper than expected.  In response to the high cost of glasses, buying glasses online is becoming commonplace, with price being the primary driver for the increasing success of this option.

I am not going to tell you to or not to buy your glasses online, but I am going to provide you with some points to consider before hitting the submit button on your next online eyeglass order.  So, caveat emptor (buyer beware).

Eyeglasses require a prescription. This prescription will include measurements that will allow each eye of the patient to see as clear as possible at distance or near or for both distance and near.   Some patients may require other specialized features that are prescribed to aid the eyes in working together (you will need to include these values in your online order).  Prescriptions typically include an expiration date, a date specified either by state regulations and/or the prescribing doctor.   This is it – there is rarely inclusion of other information that is necessary to fabricate the pair of glasses.  This additional information is typically gathered by the optician when eyeglasses are being ordered.  So, if you are going to order glasses online, you should know more about this ‘additional information’.

Pupillary distance (pd): The pd is the distance between your eyes and is used to place the optical center of the lenses in front of your pupils.   Here are four points about this value to consider when ordering glasses online:

1. The measurement for the pd may be done simply with a ruler or a device called a pupilometer.   If you have your pd from your optician or eye doctor, you can use these values.  But if you have to measure the pd at home, you will need a ruler marked in millimeters.  The process to measure pd requires that the ‘patient’ be effectively looking at distance.  This is done by measuring each eye separately and not by simply having the ‘patient’ look  at the person taking the measurement and recording a single value.  This would be a near measurement and will not account for differences between the right and left eyes.  If the measurement taken is of the near pd, the distance pd will be underestimated by up to 3 mm.  (A common way determine where to place the center of the lenses is to properly mark the location of the pupils relative to a pair of eyeglass lenses and then measure these marks for the right and left eye.) 
2. PDs are typically measured for each eye, as the face is often asymmetric, with the difference between the eyes as much as 4 millimeters (mm).  Since the pd can be different for each eye, you should question how accurate your glasses will be if the online form only has space for a single value and you know that the pd for your right eye is different than the left eye.  If you can only enter a single value, this means that your glasses will be fabricated as if the pd for each eye is the same.
3. If the pd used to fabricate your eyeglases is incorrect – it doesn’t match your actual measurement – then prism is induced.  This induced prism has the effect of making your eyes work harder, especially if your eyes already struggle to work together.  The amount of prism induced is a direct function of  the strength of your prescription and the amount that the pd is incorrect.  If you have a high prescription, it is especially important that the pd is correct.
4. PD is especially important when ordering no-line bifocals (also called progressive addition lenses), since the channel that connects the distance vision zone to the near vision zone in these lenses is narrow.  If the pd used to prepare the glasses is incorrect for one or both eyes, you will not be looking through the clearest portions of the lens, resulting in less than optimal vision or even unacceptable, blurry vision.

Fitting height. Fitting height is the measurement of the distance from the bottom of the lens to either the middle of the pupil for no line bifocals or, for bifocals with lines, to another location, such as the lower lid margin.  Why is this measurement important?  If this measurement is incorrect, you will not be able to comfortably access the portion of the lens that you need to use for distance, intermediate or near.  You will have to compensate by lifting your head up or down to a less than optimal position to see well.  Here are a couple of points to consider when placing your online order for bifocal glasses.

1. The fitting height is dependent on the frame and the way that the frame fits on the face.  It is not a value that is dependent only on the frame.  If the online ordering form does not ask for the fitting height of the glasses, then the glasses will be fabricated based on an average value for fitting height for the specific frame.  Take a look at a few faces and see how different just one variable that effects fitting height – nose bridge width – differs among people.  If the fitting height is too high, you will have to drop your head to see clearly.  If the fitting height is too low, you will have to tip your head back to see clearly and may even run out of lens to see clearly at near.  The accuracy in preparing the correct fitting height is more important for glasses that do not have nose pads, since the only way to adjust the fit of these glasses is to wear the glasses lower or higher on your nose.  Frames with nosepads have more room for adjustment, increasing the chance that errors in fitting height can be compensated for without compromising the way that the frames were intended to fit on your face .
2. Let’s suppose that you possess the frame that you will be ordering online or have a reasonable facsimile.  For no-line bifocals, the fitting height is measured from the bottom of the lens to the pupil center.  This is not a measurement that you will likely get correct without some training, as the accuracy of the measurement is very dependent on head position during the measurement and proper identification of the bottom of the lens.

There are other issues to consider when ordering glasses online – including prescription verification and lens and coating options – I will discuss these in a future post.

Ordering a pair of glasses is typically done following an eye exam at the doctor’s office or a nearby retail location.  I suspect that the frequency of eye exams is less for people who have switched to ordering eyeglasses online (the online sites that I visited did not require proof of a current eyeglass prescription).  The recommended frequency is generally every two years, with an eye exam recommended more often with increasing age and for children, in the presence of underlying health concerns or if the person is at a greater risk of eye disease.  Since you may be saving money by buying your eyeglasses online, you ought to consider taking these savings and apply them to regular exams.

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