Purchasing glasses online, missing a key ingredient

I am a bargain shopper. I appreciate a great deal just like any other person. I understand wanting to get the best price for anything. The problem is, sometimes a great deal isn’t what you think it is. Buying glasses online is like baking a chocolate cake without any chocolate. You’re missing a key ingredient; the optician!

Just yesterday I had a patient tell me he never understood how important the optician was in his seeing clearly. He came in to see me about a month ago because he could never see as sharp as he would like to. The optometrist did a complete comprehensive exam, corrected his vision to 20/20 with a new prescription, and referred him to me specifically. He told the patient to tell me what was going on. After talking to the patient for about 10 minutes, we came up with a plan for him. It would ultimately be 3 – 4 pairs of glasses (same amount as he currently had) but he wanted to start with one pair to see if he noticed a difference. When we talked yesterday, he openly said ” I know it’s a different prescription and that helps, but whatever you did….that’s the remarkable difference. I had no clue it was that important to see the right person for my glasses”

Of course I was appreciative of his words. I’m always happy when I can help someone see the best they can. I would like to believe every optician has the same goal.

Back to purchasing online, the optician is taken out of the equation. Let’s first talk about single vision lenses. I decided to purchase a pair of glasses on-line. I really wanted a pair of lime green frames big enough to fit my head. Not an easy task. I had been looking at the lines we carried but none of them had exactly what I was looking for. I finally found a pair and looked to see where I could get them. I saw they could be purchased online from Daniel Waters Eyewear. I won’t include a link because I don’t recommend purchasing glasses online. Now, if you want to take a chance and purchase a frame online because it is the frame you want, and take it to an optician for lenses, I have no issue with it. You are never guaranteed a good fit though so you may end up wasting your money. I know enough about frame sizing and shapes to know this frame was going to fit me.

The frame I chose was by Geek Eyewear

Geek Rad 09 Lime Green
Geek Rad 09 Lime Green Photo courtesy of Geek Rad Eyewear


Now, I ordered my lenses with a simple +.75 in both lenses. I knew I wasn’t going to be using the lenses, I had other plans for these beauties, but I wanted to see just how accurate everything was, so I treated it as if I was going to be wearing them.

The first question asked is for a PD (pupil distance) This measurement is the distance from one pupil to the other pupil. Now, a good optician will always measure this in relation to the center of your bridge because the distance from your bridge to your right pupil and from your bridge to your left pupil may be slightly different. Most of us are not perfectly symmetrical. In my case, I am more than a mm different.

When I put in my PD, I put in 62 which is my PD from pupil to pupil, but remember, I am not symmetrical so my PD for each eye is not 31. I selected single vision distance and regular plastic lenses. All of their lenses come with AR (Anti Reflective) coating. We will talk more about coatings at a different time. As an optician, I would normally take another measurement, an optical center measurement, for a single vision distance order, but it is not possible to do with online orders because you have to be wearing the frame in order to take it.  In theory, when a pair of glasses are complete, the optical center of the eyeglass lens should be directly over your pupil, especially in a single vision, distance only lens.

I placed my order. I received an email asking me which color I wanted. I found that a little odd since I had clicked on the lime green frame when I selected it. I did not see the email right away so it was a day or two more before I responded.

The glasses shipped out the day after I responded. When they arrived, I will admit the presentation was nice. They arrived in a case, a bottle of cleaner, and a lens cleaning cloth. The AR coating was visibly on the lenses.

Daniel Walters Online Purchase
Everything I received with my Daniel Walters order

So the optician in me immediately got to work checking the quality of the glasses.  To make sure the technical aspects are easy to understand, I will give you a little information.  A “B” measurement is the number of millimeters from the top of the lens to the bottom of the lens.  An OC (Optical Center) height should be specified when ordering distance glasses.  When it is not specified, the lab should cut the lenses with the pupil distance horizontally and to the middle of the frame vertically.  The vertical heights should be the same on both sides unless different heights are requested by the optician (again, it can not be done with an online order).   In looking at the picture below, you will see a couple of things.  First, you will see one set of three dots on the right lens (left side of the picture) and two sets of three dots on the left lens (right side of the picture).  As you can clearly see, the dots are not centered vertically in the frame.  They are extremely low.  I will explain how this could affect the wearer a little bit later in this post.  On the left lens, I first marked them up evenly with the right and then I moved the lens to mark it up to where the true actual optical center is.

Daniel Walters Eyewear marked up with optical centers located
Daniel Walters Eyewear marked up with optical centers located

As you can see from the next picture, there is a 4mm difference.  I would actually be ok with the difference (in theory) if it was centered vertically in the frame because then I would just assume they made the right lens wrong, but it isn’t so clearly they just have no care as to where they put the optical center.  Now, to be fair, the distance between one optical center to the other is 63 mm.  Yes, I ordered 62 mm, but the lab is allowed a 1mm variance, so it would be considered good.


Glasses not cut evenly
Optical centers not evenly placed on both lenses – Daniel Walters Online Orders

Next I put the frame on.  As you can see, my actual pupil is above the center of the frame.  This is the case with most frames today because it allows a better fit for progressive (no line bifocal) wearers, which a majority of eyeglass wearers are.  What my pupil is not, is it is not anywhere near the optical center!  If I chose to wear them, I could be causing some issues with my eye called induced prism.

Center of my pupil in comparison to optical centers Online purchase
Center of my pupil in comparison to optical centers – Daniel Walters Eyewear online purchase

Induced prism is complicated.  I don’t want to get too technical on this post because I want everyone to be able to understand it.  Prism bends the light.  Sometimes patients have a need for prism such as a lazy eye or double vision.  Prism moves the optical center of the lens in a way that it forces those muscles to work more so the patient sees evenly (no double vision).  Induced prism occurs when eyeglasses are measured or made wrong, resulting in prism being in the lenses when it wasn’t wanted.  If you are a technical person and want to know more, click here for an interesting article on induced prism.  The amount of prism induced can be calculated by a formula called Prentice Rule.

Center of my pupil in comparison to actual optical center marked up
Center of my pupil in comparison to actual optical center marked up

Prentice’s Rule is something all licensed, certified, or good opticians know and understand.  Prentice’s rule was named after an optician; Charles Prentice.  It is a formula to determine the amount of induced prism in an optical lens.  The formula is simple.  P(prism) = cf/ 100  c is decentration (the distance between the pupil center and the lens’s optical center, in millimetres  f is the lens power.  So taking all of this into account.   P = 5 x .75/100  P = .0375  So, in this pair, it is not even quite half a diopter of prism (prism is discussed in diopters)  That isn’t a lot but imagine if my prescription was a -3.00 instead.  The amount of prism would be quite a bit more.

bad online eyeglass purchase with mark ups
Noticable difference between eyeglass lenses cut and center of pupil – online eyeglass purchase

So, how does all of this affect you?  Prism affects the muscles in your eyes.  First, you have to understand, you may pick up a pair of glasses and have induced prism and you wouldn’t even realize it.  Now some patients can tell right away, they may have a slight double image, they may feel a little dizzy, but depending on the amount of induced prism, you may not even know it’s there.  If you don’t know it’s there, then it’s not a problem, right?  WRONG!!!  The best way I can explain it is, imagine you go to the gym and you’re working out your triceps.  You start out lifting 10 pounds with your left arm only.  You don’t do anything with your right arm.  Then you eventually add 10 pounds to your right arm but whatever you lift with your right, you add 10 more pounds to your left.  You get to 80 pounds with your right so you’re lifting 90 pounds with your left.  Now, you go to lift a 100 pound object and you start out doing great, but your right arm isn’t quite as strong as your left so you feel it inching lower and lower until you are forced to put it down.  The left muscle just got worked differently than your right for too long.

Now, imagine you go to the optometrist for a comprehensive eye exam.  You walk away with a prescription and you take it to an optician who fits you with a great pair of SV distance only lenses, digital, with a smart blue filter because you spend a lot of time on your cell phone and tablet.  They are measured perfectly, you come in to pick them up and you can’t see clearly.  You’re seeing a slight double image.  The optician tries to figure out what’s going on, recheck their measurements, everything is correct.  You make an appointment for a recheck and the double vision isn’t making sense.  Then they have to begin a series of tests to rule out any medical issues because double vision when you don’t have a history or double vision can be caused for a medical alarm.  Your old glasses are rechecked a little more in-depth and it is discovered you’ve been wearing a pair of glasses which have caused induced prism for the last year  / year and a half.  You then have two choices, work with the doctor and the optician to begin the slow process of reversing it, which could end up costing you a little more because you may have to buy a couple sets of lenses over the course of 12 – 18 months OR you have the doctor now prescribe some prism in your new glasses and you will have to always make sure your glasses have prism in them going forward, which means you will have to always go to an optician who is skilled at fitting prism.  All of this time and energy could have been avoided if you would have just had your glasses done by a skilled, professional optician in the first place and not taken the “cheap” route.

In addition, a lot of offices are starting to charge patients a discovery / troubleshooting fee when you purchase your glasses online and you can not see out of them.  Most optometrists will do a prescription recheck on a patient who purchases glasses from another optical location if they did the original exam but some optometrists are saying no, they won’t do it for free if you purchased online because it is resulting in too many patients coming back in and they are finding it isn’t the prescription that is the problem, but it is the way the glasses were made.

This next picture shows an actual pair of glasses purchased by one of my patients (before he was my patient).  He came in because he was struggling to see out of his new glasses.  I marked them up to double-check all measurements and as you can see from the picture, where the center of his pupil sits is not where the center of the lens is and his prescription is a lot higher than +0.75 so he was noticing the difference.

Bad online glasses
Patient’s online purchase indicating his pupil and optical center of lens.

As I said at the beginning, I understand wanting to save a few dollars here and there, but when it comes to your eyes, sometimes it’s a little better to spend a little more on the front side so you’re not dealing with issues later.  Don’t leave the chocolate out of the chocolate cake.  Find a great optician who understand his or her profession really well and trust them to take care of you and your eyes.  As I often say, a prescription is only half the solution, your optician is the other half!  When you purchase online, you take away that key ingredient, the optician!

Have a great day everyone and see the world as clearly as you can! 🤓

If you liked this article and want to read more, please follow me on Facebook @PaulaTheOptician or subscribe to this blog.  You can find my other writing, not related to eye care and optical on my site:  HaliPawz  Where I enjoy writing reviews, ADA adventures with my mom, as well as just some humorous things that happen to me.