## Tuesday, June 5, 2012

### Measuring Your Own Pupillary Distance

Pupillary distance (PD) is the fundamental optical measurement for eyeglasses. It's the distance between the pupils of your eyes. This is important because every prescription optical lens has an optical center. I'll let Allaboutvision.com explain it:

The optical center of your eyeglass lenses is the part that gives you the truest vision, and it should be directly in front of your pupils. To determine how to place the lenses in your frames so the optical center is customized for your eyes, the eyeglasses lab needs to know the distance between your pupils, or PD.
It can be tricky to measure your own PD, somewhat akin to trying to cut your own hair. Dispensers need lots of practice to be able to measure PDs correctly, and even experienced opticians have difficulty taking their own in a mirror.
Most online optical providers present you with several ways to go about this important task. Some suggest the simplest method, which is to have your prescribing eye doctor or an optician take the measurement for you. Alternatively, they offer step-by-step explanations of how to take your own PD in the mirror, or how to have a friend take it for you.
There is only one reason you would your want your own PD measurement: your want to buy glasses online, without the expense of an optician's expertise. Ok, fine. I'm a professional optician and I have reasons to hate that people do this. But I just love opticianry. What irritates me more than the erosion of respect for and practice of professional opticianry are the endlessly reused wrong methods of obtaining an accurate PD perpetuated all over the anything-for-a-dollar online glasses websites. So I'm going to tell you why their instructions are wrong, and then I'm going to tell you simply how to do it right.

There are two systems proliferated online to obtain your own PD: manually measure yourself, or via software manipulate an uploaded photo of yourself to produce the data.

To measure yourself the common instructions are (courtesy of random online glasses site justeyewear.com)
Have ready: a straight ruler*, a pencil, and a mirror
1. Facing the mirror, place the ruler on the bridge of your nose, bringing the start of the ruler directly below the center of one eye’s pupil.
2. Looking straight into the mirror, hold the ruler (keeping it steady and parallel to the floor!) and mark the location on the ruler of the other eye’s pupil.
3. Measure the distance in millimeters between the two marks. This is your PD.
4. Repeat this process a couple of times to ensure you have an accurate measurement.
If you have a willing assistant, you can simplify the process even further by having your friend measure your PD using a ruler. (Your job will be to stand still.)
Whether measuring with someone or alone, be sure to hold the ruler steady and parallel to the floor!

 Does this inspire you with confidence or what?

You may get close to your actual PD using this method, but prescription eyewear is neither horseshoes nor hand grenades, so maybe a more precision method is called for.

The second method is to upload a photograph of yourself with some sort of size reference device in the photo. This was initially developed as the iPhone app Pupil Meter using a credit card as the size-reference device. Since most credit cards are fixed size, once you know the size of the card one can calculate the distance between the pupils in the image.

The idea is sound. It's a simple ratio equation. But the app is notoriously flawed and inaccurate. Witness it's awesome 1 star rating and the many uncomplimentary comments.

The system caught on, though, and websites developed their own more sophisticated versions of the same measuring system. I've tried a dozen and the best-designed one I found is this PD Self-Test. This uses you own current glasses as the reference device. You just measure the size of the glasses and highlight your pupils in the image using their slick interface and it does the calculation.

I tried it out. Eight times. With eight different photographs. It measured me from a 61mm to 65mm PD. So call it accurate within ±4mm. How bad can that be?

The whole point of getting a PD measurement is to have the optical center of the lens centered in front of the eye.  If it is decentered, that induces prism: "A lens with prism correction displaces the image, which is used to treat muscular imbalance or other conditions that cause errors in eye orientation." Double vision and focusing difficulties can be caused because your eyes don't work together perfectly, and prism can be prescribed by doctors to correct that and force your eyes to focus together properly. However, prism that is not prescribed and unnecessary will do the opposite: it will create double vision and focusing problems.

So unnecessary prism is bad. But nothing can be measured perfectly, even the most accurate eyewear has some margin of error. How much is too much? Well let's figure out what a pair of glasses I would order myself using the above PD Self Test measurements would be like. If my PD ended up being decentered by the test's 4mm error how much prism would I get?

The Prentice Rule is a simple optician's equation to calculate exactly that. The formula is:
$P = cf$
P is the amount of prism in diopters D
c is decentration in centimeters
f is lens power in diopters D

We are looking for the prism P. I know the decentration is 4mm, which is 0.4cm. My Rx lens power is a moderate 2.5D (-2.50, but signs don't matter for this calculation). P = 0.4*2.5 = 1.0D. To put this in perspective 0.25D is typically the smallest increment prescribed by doctors, generally because it is the smallest amount that will have noticeable impact on vision in most people. 1.0D is four times that. That is literally suffering four times the prescribed dosage. The ANSI standards for prism deviation in a spectacle lens is no more than 0.67D, or at slightly higher power ±2.5mm. The lenses made off that PD calculation at 4mm decentration and 1.0D fail both standards and are worthy only of the garbage bin.

Here's a kicker. My actual PD, measured many times by every method from a ruler held to the face to the Visioffice digital measuring system, is 69mm. The online calc didn't even come close. If I had used it's average result of 63mm, that would not possibly, but exactly cause my resulting glasses to be decentered 6mm, inducing a whopping 0.6*2.5=1.5D unnecessary prism, more than double ANSI standards. Those glasses would certainly cause double vision, strain, and focusing failure.

How can the system be that off? Well, because you have two PDs. When you focus on objects within 20 feet, your eyes converge and the distance between your pupils decreases. How far away is the cameraphone or webcam you're using to take the image for the online systems? 2 feet? 4 feet? Those systems are measuring your near PD, not the 20+ foot distance PD you want in your glasses. They don't even get the near PD right either.

 Look at this handy diagram! It's depicting cameras, but the concept is the same for eyes.

There is no accuracy to online PD systems. It is improbable that glasses made from theses PDs even accidentally end up being correct.

So you've hung out through my lecture and you want to know how to do it right. It's ridiculously simple. I don't know why no online opticals recommend this. I'm sure people simply assume a slick looking digital system like the one above is more accurate than any manual system, and are inspired with confidence in their cut-rate online supplier. But manual is simply the best for doing this on your own. Here it is:
1. Wear your glasses. (Or any glasses if you don't have your own. Even just try on demo glasses at the mall if you are that bold.)
2. Have a felt-tip maker handy.
3. Focus on a single object in the far distance (anything farther than 20 feet works, but farther is better).
4. Raise the marker to your right lens and precisely put a dot on it directly over the distant object.
5. Repeat for your left eye. If done correctly, with both eyes open the two dots should overlap into a single dot over the distant object. If not repeat making the markings until they do form a single dot.
6.  Measure the distance between the two dots on your lenses with a millimeter ruler.
7. That's your distance PD.
8. If you need a near PD for reading or computer glasses, just do the same procedure but focus instead on the object you will be looking at, either reading material or computer monitor.
I've done this myself many times and always replicated the same 69mm result ±1mm. It works because the dots you are putting on the lenses mark the points through which you are actually looking, which is just exactly what PD measurement is used to determine.

I don't mind educating about this because despite the online opticals' claims, PD is not the only barrier between you and high quality eyewear. There are 30 other ways the online guys can and will ruin your glasses. Only a knowledgeable and skilled optician can make great eyewear. For excellent vision, a perfect comfortable fit, and beautiful eyewear that will last at least a year, you just gotta see an optigeek like me.

But if you want to bring me your own PD measured by my method, I'll take it. But you wouldn't mind if I just double-checked that PD myself first though, right?

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JeffD said...

I'd really like to thank you for this post. I've had growing problems with prescriptions and glasses over the past few years as I've more recently been getting them from the VA and WalMart. Although I can't say that either one is the cause, I can't seem to be able to wear any one pair of glasses for very long and I've returned many pairs after either complaining that my eyes aren't adjusting to them, or after complaining and then learning things like the lenses are in crocked or my PD is wrong.

I looked at a lot of online videos and considered the iPhone app but then saw this and decided it makes sense. My eye's won't lie. I took out 3 pairs of glasses, all that fit differently and did this over and over with all of them. I cam up with a PD between 61 and 65 but was able to be comfortable with the decision that my PD is 63. Not because it was right in the middle though but because it was most common amongst all my glasses. I even took off the glasses and manually changed a mark to try 62 and 64 to assure they did not line up. The system here worked very well.

Thanks again.

Info Opticians in Dulbin said...

Working as an opticians is not so easy and reason behind this is the experience need to perform the job sharply otherwise this might be harmful to the eyes visibility.

Daniel Livingston said...

JeffD I'm glad you liked the info and I hope it helps you on your next pair.

It's true this is no replacement for a skilled optician, but it can be a beneficial replacement for a poor optician, or none at all.

Tom Sellers said...

I modified the method a bit: I used 8mm stick on dots and a pair of safety glasses. using a circle on the computer monitor, I placed the dots and then moved them with my fingernail until they had no discernible overlap while looking at the circle. Then I measured them with vernier calipers. Next used a streetlamp outside. For the near vision I got 61 and for distance I got 64. The website you said was the best was quite a bit out, just as you found (58/60). The distance reading was same as Zenni's ruler method and 1mm off what my spouse got using the calipers while I stared at the smoke detector. So the lesson I took away from this is I may have been OK with the distance prescription using the online directions and the printout ruler, but the PD for the readers would have been out by 3mm. One other thing you may want to mention to your readers, if you mark your lenses with a marker, make sure it's not a permanent marker !

Daniel Livingston said...

Glad you got some good measurements Tom.

One of the best features of my technique is verifiability. As long as you are looking directly through the dots on your lenses, and you measure the dots correctly, you know you have an accurate PD.

Even permanent marker comes off with alcohol. Better yet invest in good quality anti-reflective lenses which are so slick and easy to clean that even permanent marker wipes off with a dry microfiber cleaning cloth.

Professional Optician Dublin said...

Great information...
It s really very interesting to determine my PD. The way you have explained it has made it a game rather than a serious study.
Thanks...

HLR said...

Interesting but the thing I don't get is once I have had my eye test I don't see an optician again. i.e. a salesperson sells me some glasses and when I pick them up the sales person check they feel ok but I don't see an optician again, so why shouldn't I use an online shop to get glasses from?

Also instead of charging us stupid amounts for the glasses they should charge us the time for the optician and sensible prices for glases.

Daniel Livingston said...

HLR, the optician is the one aiding you in selecting eyewear and then fitting them. If you shop the malls and chain stores, yes they seem more like simple salespeople than opticians.

A true optician not only helps you select frames, but also educates you on the best frame choices, and more importantly designs the best lens for your frame, Rx, and vision needs. Eye doctors do not do this, frame salespeople do not do this, online glasses vendors do not do this.

Therefore, even with a good Rx, correct PD, and a frame you selected yourself, your vision may be poor or uncomfortable, and will DEFINITELY not be as good as it could be if your eyewear was made start to finish by a skilled optician.

Buying online or in the discount chain opticals may be cheaper (though often not), but you sacrifice the comfort and quality of your vision and eyewear fit. These business just cannot provide what and optician does.

Chelsea Weaver said...

Ok,so I'm not sure what's up with my glasses and the PD is the only ting I didn't have when I ordered. The description of prismatic effects doesn't really seem to be what is happening to me, but I could (easily) be wrong.
I don't have poor vision and these glasses were intended to be used for looking at a board in the classroom without squinting. I do have astigmatism and there were corrections for it. I ordered two pairs and both of them do this. When I look at things that should be rectangular, both the top and bottom edges converge to the right. I asked a friend to put them on (yes his rx is different) just to see if he saw the same effect and he did. Is it just poorly made (both pairs) or is it something amiss on my rx? Thanks!

Daniel Livingston said...

Chelsea, that is a hallmark effect of astigmatism correction in glasses. Poorly made lenses can make this effect worse, but if they are made well and your Rx is good, the distortion will disappear with wear as your visual system accommodates to the different perspective.

Chelsea Weaver said...

Thank you so much for the answer! I got the cheaper pairs just because they weren't going to be used all the time and I have so much trouble finding a pair I like to wear :-) I'll be back into my eye doctors' office Monday to see if I can find a set of frames and get their optician to make me better lenses!

Eye care said...

Thanks for explaining the importance of using an optician. I don't think most people realize how important seeing an optician is, if you want glasses that are comfortable to wear and allow you to see as well as you are supposed to with the glasses on.

I think sometimes people think that once they have the correct prescription, that's all that's necessary. As you have explained here, other factors are in play.

Daniel Livingston said...

I appreciate the compliment, thanks!

Anonymous said...

Daniel
I recently ordered new frames and lenses after an eye exam through the local Army base optical shop. I thought the sales person was helpful. When the glasses came and I wore them for a few days, I developed dizziness and severe headaches. I took the glasses back. This time I was helped by the manager who had 16 years experience (Optician??) PD was 5mm off the original measurement! Next time I will find the certified optician. What do I look for? License? Certificate?

Daniel Livingston said...

If the PD was off from the measurement, the fault lies with whomever made the glasses and whomever failed to inspect and reject them, not necessarily the optician that helped you in the first place. The manager likely didn't do anything except correct the fabrication error, which he as manager is responsible for seeing to it those errors never happen in the first place.

An optician with ABO Certification, or in those that require it state licensing, is always desirable, and often more than experience. To ensure the best quality you have to find an optician you trust can do the job right. Good eyewear is not a commodity, it's a service, and you need to select your optician like you would a minor contractor. There's always someone doing it cheaper, but to do so they have to cut corners and you usually only find out the problems after the fact. A good optician will make the eyewear themselves and take pride in the finished product, and would never let a pure laziness error like 5mms off get through to you.

Allan said...

Hi Daniel, this post is the most helpful pupillary distance article on the internet--full stop!

I have a silly question: do all trained, licensed opticians measure pupillary distance with the method you described here? Mine seemed to just use a very fancy caliper/ruler--after reading your article I got quite suspicious of him!

Daniel Livingston said...

Thank you!

Opticians don't use this method at all. This is a way to subjectively self-assess a generic PD good enough for simple single vision glasses. Modern technology enables opticians to design extraordinarily precise and custom fitted lenses beyond anything one can get online with superior visual quality and comfort. To do so we employ sophisticated measuring equipment that discerns PDs unique to each frame (which they are) as well as several other fit factors that impact visual quality, none of which is captured by this or any generic PD measurement method.

This method works as well as any simple PD measurement technique, but good opticians have already advanced well beyond such basic vision correction and provide superior measuring accuracy, personalized lens design, and visual clarity.

Anonymous said...

Thanks for the info. I decided to risk buying online. I walked into a glasses store and paid them \$20 to measure my PD.

The savings I'm looking at is \$400 and in my opinion worth the risk. Maybe I'll get bad glasses, maybe I won't but \$400 is too much to pay for the service. I would have gladly paid extra, but not 4 times as much.

Mb C said...

Wal-Mart Vision center had measured me at 62 pd a few years ago and I recently just re-ordered glasses at a higher end store and they measured me at 64 pd - if the 62 was right and I get new glasses at 64 would that be terrible? Would I notice a huge difference do you think?

Daniel Livingston said...

That depends on your Rx. 2mm isn't huge but for higher powers and anything other than single vision lenses it will cause problems. Regardless always accept the PDs measured by your lens maker, because at least if they are at fault they will fix it.

Anonymous said...

ok, not everybody who wants to buy frames online is some cheapskate with a disdain for skilled opticians. i actually really like my optician and wish he would take my PD measurement … but he won't! this is all because i looked at every pair of glasses he has (a couple hundred) and didn't like any of them. on the other hand, there is an online store that has very stylish (not cheap) frames that i'd love to use. i understand that making glasses is part of his business, and i would love to give him that business if i could, but i really just can't stand the frames he has (they're all outdated by about 3 years, none of them matches the shape of frames i've traditionally used, etc.) so, i've resorted to the cc-verifying method in the past because i didn't have any other option. that said, this year i'll be happy to use the felt marker method. thanks for the post.

Daniel Livingston said...

You don't need your PD to buy frames online. If you truly like and trust your optician, buy whatever frames you like and take them to him to have proper Rx lenses put into them.

I understand in my shop even the 1,000+ frames I feature don't cater to everyone, so I gladly accept the opportunity to fit lenses to whatever frame my customers want to bring me.

The optician's primary responsibility is comfortable vision. I would love to supply complete frames and lenses for all my customers, but if that's not possible I'll gladly acquiesce the frames for the more important Rx lenses. I believe \$200 lens technology in a \$20 frame is superior eyewear compared to \$20 lenses in a \$200 frame.

If your optician doesn't have what you want, then get the frames with which you will be happy. Barring egregious fit errors on your part, a good optician will provide you the best vision possible regardless of you frame choice.

Anonymous said...

I would believe the argument that opticians/RDOs care about people's vision IF they gladly provided the PD when requested by a patient.

As I see it, and some opticians freely admit online, they are attempting to protect their income stream.

The hypocrisy of "I'm not giving you the PD" (when I've already paid for that as part of my exam) "because I care about your vision" just doesn't jive when the PD is not provided upon request. If your "care" about my vision was truly your concern, provide the PD when asked. Otherwise, you appear greedy to me. And now you've lost more than your income stream -- you've lost my trust. And you can be guaranteed that I won't be coming back.

Anonymous said...

I would like to add to the comment immediately above: this is not addressed to anyone in particular (Daniel Livingston), it is a general comment.

Daniel Livingston said...

There are several reasons opticians may not give out PDs for use elsewhere, and some do advocate "protecting" the profession, but as my advice here should make clear I am not such an advocate.

PDs are not accurately measured during your exam, only an approximation is made to fit the lenses in front of your eyes, satisfactorily only for a fixed straight ahead gaze and just the few minutes of time you spend looking at the chart. I guarantee you do not want those numbers being made into eyewear.

I do not provide PDs for eyewear I do not make, for two reasons. The most obvious is that my skill as an optician will provide excellent accuracy perfect for any Rx, and if that's what you want it’s a paid professional service like any other. If that skill isn't valuable to you, then just use my method and do it yourself.

The most important reason I don't give out PDs, even for a fee, is because obtaining good measurements for lenses is the responsibility of the eyewear maker. If I'm not making the eyewear myself, I want no part in the process. There's dozens of ways someone else can screw up eyewear even with accurate PDs, and I won't contribute to the eyewear when I can't guarantee quality.

Anonymous said...

http://pd.warbyparker.com/ provides pretty accurate PD's - do you agree ?

Daniel Livingston said...

Clearly didn't read the article and are just posting ad-spam, because obviously I don't agree because I tested the credit card method and it sucks. All the online methods are horrendously flawed. My DIY is easily superior.

Anonymous said...

Thank you for this very informative post. I wish I had read it last week before ordering my glasses online. They arrived yesterday and are unusable. I get the prism effect and am sure it has to do with the PD measurement. For me, it was more of the convenience factor in ordering online but clearly this is an area where personal attention is needed. Fortunately this was not a very expensive lesson but my great deal just ended up in the trash bin. See a real optician people!

Daniel Livingston said...

Thanks for sharing your story!

Over the years I've ordered a dozen pairs from various online stores as they appear to check out the quality and inspect materials. They've all been filed in the round bin too.

John L said...

Very interesting article. Thank you for posting it

I have a question. My nose is crooked from being broken. How is that going to affect my PD? Will a single number be sufficient, or do I need a left and a right measurement? My nose is curved to the left. With my current glasses, the left pupil is closer to the inner edge of the frames. My prescription is single vision, -150 in each eye.

Thanks again.

Daniel Livingston said...

Monocular PD, i.e. separate values for each eye, is almost always more accurate. If you have reason to believe your eyewear might fit less evenly than usual, such increased accuracy will be valuable to you.

One can obtain monocular PD values via my method. However doing so introduces greater inaccuracy and relies heavily on your own measuring skills.

If you have any reason to believe you require less than routine eyewear design, online eyewear is not for you. Seek an optician to get proper eyewear for your needs.

Kevan said...

I understand Opticians' frustrations with people asking for the PD, but seriously....it's medical information. We should have a right to know our pupil distance. Also, sometimes there are options online that are easier to get,especially with RX sunglasses. G-15/Green lenses for way less than the \$300 Lenscrafters wants. Real glass lenses instead of Trivex or polycarb. And of course...many times opticians don't take a particular insurance plan. So why should I drive all around town wasting gas looking for one that takes my insurance for a discount on the glasses (or call, which I've done too), when I can buy them online for the same discounted price or cheaper?

I've been getting glasses at Optometrists for 20 years and I still go to them for adjustments. But for sunglasses...online is a better option. It just is. If you already have your frame and you know it fits and looks good, and all you want is a simple, cheaper options for the lenses, it just makes more sense.

Daniel Livingston said...

Yup that's the Walmart argument I make myself: when you don't care about quality, Walmart's fine. If they don't fit too well, you don't see clearly or comfortably, or they break with 3 months of wear, but you don't care, then why spend any more than you have to? But don't make the mistake of thinking your getting the same thing for less. You only get Walmart quality at Walmart prices.

But what if you do care about your glasses or sunglasses? Would you like them to last 2 or more years? Would you like to enjoy the clearest most comfortable vision possible for that whole period? Would you enjoy a comfortable proper fit every time you put them on? That's not going to happen online or at cheap opticals.

As far as PDs, they are medical information only if taken by a state licensed professional, and only if your state legislates them. States that don't regulate PDs (every one AFAIK) don't care, and states that don't enforce license on their opticians (lots) can't regulate it because an unlicensed professional can't create protected medical information, therefore the PDs they take us not legal medical information.

Kevan said...

Well, to be honest what I've done with my last two pairs of glasses and last two pairs of sunglasses, since probably 2004, is buy the frames on Ebay (where prices are usually lower...almost \$100 lower for my last pair of Burberry frames than what Lenscrafters was asking) and get the lenses fitted by Optician (since my last optician retired and my eye doctor's glasses shop doesn't take my insurance, Lenscrafters has been where I go).

Thanks to Youtube, I have learned how to adjust the temples and nose-pieces for a proper fit; the only thing I still have trouble with is if the bridge gets twisted. I've also replaced screws, nose-pieces etc. myself.

So the problem of course is the lenses. My last pair of Serengetis lasted 6 years, so I recently got two pairs of Ray Ban and Randolph Aviators. The Ray Bans have green lenses; Lenscrafters quoted me almost \$300 for the green polarized lenses alone, but cheaper if I bought a new frame too. Well, I already bought the frame; I didn't need a new one....

Bottom line is, it's getting ridiculous. Some online places like Zenni look sketchy, but then there are places like Hidalgo's which claim to have sold Rx eyewear since 1969, so I'm sure they have some experienced opticians on hand. Can't get them done without the PD though...so I'm either going to try what you suggest here or track down one of those little PD-specific rulers that sits on the bridge of your nose. Do those work well? I'm sure it won't let me know my PD accurate to 2 decimal places but it can't be that far off right?

In any case, my next eye appointment is in November. I will ask them to measure my PD before they dilate my pupils.

Paul Militello said...

Daniel,
Thanks for taking the time to post this blog and keep up with it. Last week I got new glasses at a well-known chain optical shop, They are progressive lens. I think the only change from 4 years ago was the add power for reading. I have had headaches and eyestrain since i got the glasses. When I look at objects at a distance or even computer distance, to see clearly out of the right lens, i need to turn my head right slightly. and turn left with the left lens. they are not both in focus at once. does this sound like they measured the PD to short? or is it a script problem?

the store used a "camera" which took a snapshot of my eyes and did the measurement. It was 61. using your method, I came up with 63 for distance. How can the camera/computer test my distance PD,when I am not focusing on a distant object?
Thanks Paul

Daniel Livingston said...

Hi Paul, and thank you!

I know the chain you mean. The digital measuring devices can be great (I use a sophisticated model myself), but they can also be poor. They are accurate but do not compensate for user error. If the operator is not well-versed with the machine they can generate poor results. Newer salespeople in the chains are trained only on these devices and may not understand exactly what is even being measured, and cannot troubleshoot the results or even recognize erroneous ones. As with all computers, they happily spit out garbage results when they get garbage user input.

Also good catch about measuring distance PD while you focus on a near object.