photo credit: net_efekt

Some visitors have found my previous post on buying eyeglasses online because they were searching for help in figuring out why they could not see with the glasses that they had purchased online or they wanted help with measuring their PD.  In this post I will list some of the reasons why your vision might not be best with your new glasses and help you troubleshoot.  I also have included some links at the end of this post that will help you measure your PD accurately.

Before you read further … it is always a good idea to figure out if your vision is poor with one or both eyes.  To check this, alternately cover each eye and compare the vision with each eye while looking at distance or near – where ever you may be having a problem.

Prescription is Not Correct

Consider this a possibility if the new pair of glasses is your first pair with the new prescription.  If there was an error in determining your prescription or when writing it down, it is more likely that one eye is wrong than both eyes.  If you suspect that the prescription is wrong, you will have to return to the eye doctor that provided you with the new prescription or find a new doctor.

Glasses Not Made Correctly

Eyeglasses sold in the United States have to be prepared to documented accuracy standards.  Of course, people make mistakes when fabricating glasses that are not caught during the inspection process or purposely release the glasses thinking that the error is either not significant or will not be noticed by the patient.  It is more likely that the error will be made with one lens rather than both lenses.  One way to check this error is to find an optician or eye doctor’s office with a dispensary and ask them to verify the glasses against the prescription.  I would suggest that you return to the doctor’s office that provided you with the prescription – they might not like that you did not buy your glasses from them but they will still want you as a patient.

Distance PD is Not Correct

The PD is the measurement that specifies the point in the lens that you will be looking through when looking straight ahead.  Since the PD is not routinely provided with your prescription, many patients will have to figure out how to measure their own PD in order to complete the prescription (see links below).  I reviewed the difficulty in getting this correct in my earlier post linked above.   Please note that this is an issue more likely to effect patients with higher prescriptions or when ordering no-line bifocals.  To summarize, errors can occur when measuring your PD, in fabrication or when the PD for each eye is not identical and the online form assumes that the PD for each eye is equal.

Here is one way for you to check to see if the PD is incorrect.  Cover one eye and look a little bit to the left and then right and see if your vision improves.  If it does, this may mean the prescription is correct but the PD is wrong for one or both eyes.  If your vision does not improve, either re-measure your PD and compare against the values that you provided when ordering the glasses or have the PD of your glasses verified.

Near PD is Not Correct

The near PD is used when ordering reading glasses.  If you have ordered reading glasses and you have a high prescription and you have provided your distance PD, then you will not be looking through the optimal point of the lenses.  On average the near PD is 1.5 mm less than the distance PD for each eye or 3 mm for both eyes (binocular).  Most bifocal eyeglass orders do not require a near PD.

Fitting Height is Wrong

This only applies if you have purchased bifocals.  It is somewhat easy to check and see if the fitting height is incorrect – simply move your head up and down and see if your vision improves.  If your vision improves when tilting your head down, the fitting height is too high (number too big).  If your vision improves when you tilt your head back, then the fitting height is too low (number too small).

PD Measurement Links

Although most of the online eyeglass retailers provide assistance in measuring your PD, not all provide a method that is very accurate.  Below is a link for printing your own millimeter ruler and two links to help your friend measure your PD (I don’t recommend trying to measure your own PD).  I suggest that you follow the directions in both links and compare the results.

• printable millimeter ruler
• good description for having friend measure PD
• near PD measurement method – add 3 mm for distance PD

I have reviewed some of the more common reasons  for having trouble with new eyeglasses – there are others.  If you have questions or want advice, please post a comment and I will reply as soon as possible.

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.