# Anyone an optician?



## ThePhoenix (Mar 14, 2016)

I have a question re. PD measurement.

Ta


----------



## Spandex (Feb 20, 2009)

Are you trying to measure it? The trouble with measuring your own PD is that your nose gets in the way of the ruler... So, my top tip is get a sharpie and draw a line straight up from each pupil, then simply measure between the lines across your forehead.

Simples.


----------



## Hoggy (May 8, 2002)

https://www.tescoopticians.com/help-and ... e-your-PD/

Hoggy.


----------



## ThePhoenix (Mar 14, 2016)

Thank you, but I know that stuff.

My question is a bit more subtle than that.


----------



## leopard (May 1, 2015)

Hoggy said:


> https://www.tescoopticians.com/help-and-advice/measure-your-PD/
> 
> Hoggy.


 :lol: A commercial solution is too obvious.

A pair of dividers is original ,one point on each eyeball, jobs a good 'un


----------



## Hoggy (May 8, 2002)

ThePhoenix said:


> My question is a bit more subtle than that.


Hi, Do tell.... :lol: :wink: :wink: 
Hoggy


----------



## ZephyR2 (Feb 20, 2013)

Hoggy said:


> ThePhoenix said:
> 
> 
> > My question is a bit more subtle than that.
> ...


He want's to see if people whose eyes are close together really are shifty. :lol:


----------



## bhoy78 (Oct 30, 2014)

leopard said:


> Hoggy said:
> 
> 
> > https://www.tescoopticians.com/help-and-advice/measure-your-PD/
> ...


 :lol: :lol: :lol:


----------



## PluckyDuck (Mar 29, 2017)

Hi there! A very interesting discussion!


----------



## ThePhoenix (Mar 14, 2016)

Well it doesn't sound like any of you are opticians, but this is what I am wondering:

I have for some time had an outstanding academic question regarding the PD measurement, which has recently become a practical issue for me.

When buying lenses online one of the parameters that obviously needs to be supplied to the provider is the PD measurement.

As seems to be common with opticians this value is not provided as a matter of course, so I set about measuring it myself.
The method I used I have not seen suggested anywhere, but I reckoned it to be an obvious, simple and entirely accurate one. It involves taking a piece of card and making two pin-holes in it, centred at a good estimate of the target PD then observing an object at the prescribed distance through the holes in the card, whilst alternately closing each eye. Continue to do this with pairs of pin-holes placed further, or closer together until the viewed object can be seen clearly through both, measure the inter pin-hole distance and the job is done.

The assumption that I know I have made here is that the PD figure I get relates to a distance at the plane of the lens (my card), which, when the object used for the test in the middle distance to infinity equates to the same spacing as the pupils are on the surface of the eye.

I have assumed this is what the spectacle technician is ideally needing for manufacturing alignment. However, when I had my most current eye test I specifically asked for my PD and the value I was given is 4mm shorter than that I calculate, so I set about researching another method to corroborate one of these values; the optician's vs mine.

I have found all sorts of DIY methods of measuring PD, mostly suggested by online spectacle providers and I have tried several, which luckily agree to a high level of accuracy (+ or - 0.5mm) with my pin-hole method, but I also note that some of these methods are determining the measurement at the lens plane and others at the surface of the eye.

If I have calculated the parallax issue correctly, the difference in PD between measurements at the surface of the eye and the lens plane for an observer focused on the near distance to one at infinity is probably insignificant as it is about 0.25mm overall; which is the reason I am comfortable calculating my distance glasses with my pin-hole method. But the same measurements for a reading prescription with an observer focused on a target at 300mm becomes a discrepancy of a little over 2mm and I imagine that this is significant. (Still doesn't account for the 4mm difference between my measurement and the optician!)

After all that I get to my question (phew); is PD the actual distance at the eye (as the name seems to suggest) and with the eyes viewing a distant object and consequently the spectacle manufacturer takes this into consideration depending on the type of glasses - distance or reading, the latter having the PD calculated as a little shorter?

A second question now occurs to me; for varifocal glasses, which incorporate a reading prescription, is the same PD is provided and the 'virtual' lens centres calculated and positioned progressively closer together moving down the lens to accommodate the convergence of the eyes on the closer objects?


----------



## ProjectMick (Sep 29, 2015)

I can't remember everything I learned when working in optics (not as an optometrist I might add) but in terms of your PD if you already have one given by your optician I would suggest using it as difference of 4mm may well cause adaptation issues.

The PD will be taken based on distance work usually and varifocal lenses will be made based on that. The lenses will then work just fine based on the PD given.

It's a difficult subject to give a straight answer on though as there are so many variables - frame size, distance from the face, tilt (just to give you three basic examples) which can alter results.

Most people will be ok with a sight variation (1 or 2mm) in PD anyway - when taken by hand results can vary by a mm or two between the people (or machine) taking them.

Sorry I can't answer in depth but thought I'd try and help a little.


----------



## Spandex (Feb 20, 2009)

I think the problem you have is that what you really need is for an optician to say, "your measurement is close enough", but no optician will ever say that even if your measurement *is* close enough.

Not telling you your PD is just a cynical ploy to make it harder for you to shop elsewhere, and given how brazen most opticians are about that, they're unlikely to help you to measure your own PDs. In fact, they will almost certainly tell you how hard it is to measure and how dangerous it is to get it wrong.

It's kind of like taking your kid into Clarkes and getting their foot measured, but the assistant refusing to tell you the size until you buy some shoes. Or paying a garage to run diagnostics on your car, only for them to refuse to give you the read-out so you have to get them to do the repairs.


----------



## Hev (Feb 26, 2005)

ThePhoenix said:


> After all that I get to my question (phew); is PD the actual distance at the eye (as the name seems to suggest) and with the eyes viewing a distant object and consequently the spectacle manufacturer takes this into consideration depending on the type of glasses - distance or reading, the latter having the PD calculated as a little shorter?
> 
> A second question now occurs to me; for varifocal glasses, which incorporate a reading prescription, is the same PD is provided and the 'virtual' lens centres calculated and positioned progressively closer together moving down the lens to accommodate the convergence of the eyes on the closer objects?


You have thought way too hard about this......Im guessing you are an engineer :lol:

PD stand for pupillary distance and you are correct that it is the measurement between the pupils while the patient is looking into the distance (at infinity). I measure this with my ruler flat against the forehead (not quite in the spectacle plane but allows the ruler to be stable while I take the measurement). It does not matter if the measurement is taken against the forehead or in the spectacle plain and the eyes are still in the same position, looking into infinity. The manufacturer will not alter this measurement as it is down to the optician to determine the patient needs.

As you read, typically your eyes turn inwards slightly but everyone has a different amount of 'inset' and depending the distance you read at, depends on your near PD. The majority of people read at approximately 40cm but if for work or other reasons, you read at a different distance, the PD needs to be altered accordingly - I would rather measure this PD but I know others who will use average figures.

Another point to keep in mind is that most people do not have a symetrical head. It is very common for a PD to be split equally between the eyes but often this is not accurate. Today for instance, I measure a man who had a PD of R: 32.5 L: 35.5. Combined this is obviously 68 and split 34/34 but on higher prescriptions, this could cause problems with induced prism, poor vision and inducing eye strain/headaches.

Even though I see the same person every few years for their specs, I will always take fresh PDs. It is not common for a PD to change massivelhy in an adult but head injury for instance can alter positioning.

Pds can also be incorrect if there is a 'squint' involved. Children especially are more likely to have a squint and being able to take an accurate PD is essential (so as not to cause further issue in later years). Hence it is illegal for online suppliers to supply to kids (the risks are too great) - if things are so tightly controlled for kids, why should adults be so different??

On the subject of online suppliers, when a job comes back from the lab, I ensure that it meets all of my specifications and in turn, I ensure that they fit you correctly and vision is as it should be. Now as much as I'd like my lab to be perfect, they are not! I send stuff back to them and I am very particular about accuracy.....when you buy online, who is doing that for you? My lab send stuff out that has passed their quality control and still errors creep in......who does a double check for online?

As for vairfocals....historically they were set with an automatic 'inset' to allow for convergence when reading but now free-form technology allows us to select the most accurate inset depending on patient/presription (yes, your convergence can be different with a higher prescription compared to smaller ones). This allows for the lens to be more precise to the wearer and minimise distortions that are experienced with varifocals.

If you need/want any more info, just shout 

Hev x
(dispensing optician for the last 23 years!)


----------



## John-H (Jul 13, 2005)

Interesting read. The only constant as your eyes swivel sound would be the distance between the centre of each eye ball presuming they are spherical. For infinite distance, pupils and retinas will be the same distance apart but for close focusing the pupils get closer together and the retinas further apart.

I know of someone (an artist) who could swivel his eyes and focus independently (one on the subject and one on the canvas simultaneously) - not sure how his glasses are set.


----------

