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Thread: vertex distance

  1. #1
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    vertex distance

    Hey there optiboard goers,
    First, let me apologize if this has been mentioned in another thread, I did a quick search on the board but couldn't find anything specific.
    After doing some research I came across the importance of vertex distance. The Carl Zeiss Optical Training Guide has a section on the mathematics behind this measurement. Ive talked to opticians that are much smarter then me (which is the first thing I like to do when I don't understand something (and quite plainly why I like posting here <3)) how important this measurement truly is. Clearly in a higher power lens (+10D) its quite dramatic. In the garden variety Rx it just doesn't seem that important and the standard refraction distance (14mm?) seems right. Ive been doing this for some time and this never seems to come up except for the higher end progressive lenses like the Varilux S or the Hoyalux iD lifestyle or the ipseo. So, except for Rx's of 10+ diopters, is this a necessary measurement? Should I prompt my office manager to purchase a distometer? I can obtain that measurement using the Visio Office but is it necessary for every patient?

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    What's up? drk's Avatar
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    No, it's not necessary.

    Save it for +/- 8.00

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    This all depends upon the lens design you are putting the patient into. I'm a pesky -5.25 and damn particular whether I am putting myself into a compensated SV or PAL, the vertex better be taken. As to what spherical power you want to draw a line in the sand, it never hurts to do it on a regular bases. I have used a distometer for many years as part of my measurements for fabrication of eyewear, as a general rule. Keep in mind that many of us use a plethora of seemingly unnecessary measurements to ward off redos and to eliminate several steps in the investigation process of a non-adapt scenario. I, also employ the use of chicken bones to fend off bad ju-ju.

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    Rising Star Chad Sobodash's Avatar
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    I do it for all patients. I don't see why you wouldn't, regardless of the Rx. Then again, I also typically order lenses to hundredths of a diopter (if my VX40 will read it, why not?).

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    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    What good is a fitting distance (we now call it a vertex distance) without the vertex distance (also known as the distance from the patients cornea to the phoropter)?

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    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by Judy Canty View Post
    What good is a fitting distance (we now call it a vertex distance) without the vertex distance (also known as the distance from the patients cornea to the phoropter)?
    Absolutely nothing, in this context...

    If you're measuring to determine the degree of effective power change in the lens due to a difference between the doctor's vertex distance and the wearer's vertex distance, you need to know both distances. Let's call that Rule #1.

    I propose Rule #2.

    The wearer's vertex distance should be as short as possible, but not so short that the eyelashes touch the back of the lens.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    Quote Originally Posted by Robert Martellaro View Post
    The wearer's vertex distance should be as short as possible, but not so short that the eyelashes touch the back of the lens.
    Well, I love the conversation so far, and it looks like were all over the spectrum here. "Go not to the elves for counsel, for they will say both yes and no." ~ J.R.R. Tolkien
    I like the idea of using chicken bones to keep away bad Ju-Ju, but more to the point I've worked with customers that wear distance glasses at the end of their noses and reading glasses pushed up into their nasal cavities. The stronger the RX the more that measurement comes into play it seems? no?

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    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    Absolutely nothing, in this context...

    If you're measuring to determine the degree of effective power change in the lens due to a difference between the doctor's vertex distance and the wearer's vertex distance, you need to know both distances. Let's call that Rule #1.

    I propose Rule #2.

    The wearer's vertex distance should be as short as possible, but not so short that the eyelashes touch the back of the lens.
    That's my point. Pantoscopic angle--yup. Wrap angle--yup. Vertex distance--meh.

  9. #9
    What's up? drk's Avatar
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    I say, meh to all your POWs! (Except vertex in high power lenses, and faceform in wrap sunwear.)

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    Master OptiBoarder mshimp's Avatar
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    Measuring Vertex distances are over rated on run of the mill R/x's between +/- 3.00. 80% of prescriptions!

  11. #11
    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    Quote Originally Posted by chaoticneutral View Post
    Well, I love the conversation so far, and it looks like were all over the spectrum here. "Go not to the elves for counsel, for they will say both yes and no." ~ J.R.R. Tolkien
    I like the idea of using chicken bones to keep away bad Ju-Ju, but more to the point I've worked with customers that wear distance glasses at the end of their noses and reading glasses pushed up into their nasal cavities. The stronger the RX the more that measurement comes into play it seems? no?
    Only if you have a fixed starting point--that's the real vertex distance.

    System for Ophthalmic Dispensing, Third Edition, page 339
    Last edited by Judy Canty; 06-24-2014 at 04:18 PM.

  12. #12
    Master OptiBoarder rbaker's Avatar
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    Quote Originally Posted by mshimp View Post
    Measuring Vertex distances are over rated on run of the mill R/x's between +/- 3.00. 80% of prescriptions!
    What about the other 20%? Are they just SOL?

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    What's missing from this discussion is that actual "fitting distances" *are* used by a few FF designs, along with an upload frame tracing, to more properly globally map the optimization. FOV angles subtended at the lens surface change with VD. It's not just about power compensation.

    B

  14. #14
    What's up? drk's Avatar
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    Good point.

    Essentially, the design will harden or soften (to use incorrect terminology) based on vertex.

    Is this right?

    Edit: larger lens = softer, and smaller lens = harder. Right there, too?
    Last edited by drk; 06-25-2014 at 08:13 AM.

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    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by Barry Santini View Post
    What's missing from this discussion is that actual "fitting distances" *are* used by a few FF designs, along with an upload frame tracing, to more properly globally map the optimization.
    Barry,

    Surface a pl sph add +2.50 PAL, and a +2.00 -4.00 x 45 add +2.50 PAL, from the top six POW optimized PAL manufacturers. Map the lenses and compare the astigmatism plots for both lens powers, looking for deviations from the intended PAL design. I know that the lens manufacturers have done this, but wouldn't it be nice to see the results? If we did this on our own, using a local lab, sending the lenses to Rotlex for mapping, then publishing the results on Optiboard or an independent site, how many cease and desist letters do you think we would get?
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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  16. #16
    What's up? drk's Avatar
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    Robert, are you saying that the emperor has only a thong?

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    Rising Star Chad Sobodash's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    ... how many cease and desist letters do you think we would get?
    I've always wondered this. If anyone has input on it, please tell me.

  18. #18
    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by drk View Post
    Robert, are you saying that the emperor has only a thong?


    Quote Originally Posted by Chad Sobodash View Post
    I've always wondered this. If anyone has input on it, please tell me.
    Some companies have white papers available online, others have patented technologies incorporated into their designs that can be retrieved by patent search.

    This will get you started...

    http://www.luzerneoptical.com/pdf/ze...hite-paper.pdf

    Apologies to the OP for going slightly off-topic.

    Another reason to at least be aware of the vertex distance is the apparent fitting height with segmented multifocals. This happened recently- Prodesign frame with pads, sitting a bit further from the eyes than I would like, but really needed the pads and there was no other red frame available in my shop. WVtx distance was about 17mm, seg style was a ST28. Sunglasses were at about 13mm, and the segs needed to be set about 2mm higher to compensate (she would read frequently outdoors on the deck).

    Not as much of an issue with PALs, although the progressive optics will ramp up quicker with a longer WVtx distance, and slower with a shorter distance.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Master OptiBoarder OptiBoard Silver Supporter Java99's Avatar
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    Quote Originally Posted by drk View Post
    Robert, are you saying that the emperor has only a thong?
    I think he is, and I think he's right. I'm starting to think that the most important part of fitting POW lenses is just getting the same basic things right we've always tried to do.

  20. #20
    Master OptiBoarder Darryl Meister's Avatar
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    Quote Originally Posted by Robert
    I know that the lens manufacturers have done this, but wouldn't it be nice to see the results?
    Ask and ye shall receive.

    Click image for larger version. 

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    Quote Originally Posted by Barry
    What's missing from this discussion is that actual "fitting distances" *are* used by a few FF designs, along with an upload frame tracing, to more properly globally map the optimization.
    As Barry noted, there are really two applications of vertex distances:

    1. Compensating the prescription powers for the difference between the fitted and refracted distances. Errors in power as a result of differences in vertex distance are small until you get into higher powers, over 6 to 8 diopters. This influences vision mainly through the center of the lens. This can be done by an eyecare professional before ordering the lens.

    2. Optically optimizing the lens design using ray tracing for the fitted vertex distance. The more advanced free-form lenses (certainly not all of them) are optically optimized for an assumed vertex distance, which determines the obliquity of the line of sight during off-axis vision. This influences vision mainly through the periphery of the lens and the near zone.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    It should be noted by some following this thread that many compensated lenses use a predefined vertex distance, based on their average (about 12mm) when making calculations. So some lenses use a simple default even if they don't have the option of choosing it individually.

    So your +1.00 pt who with a vertex of 12mm will still receive those compensations in many lenses even if they don't have the option to enter that value specifically.

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    i concur. My only addition is that I occasionally find it necessary to tell the patient that wearing them at the end of their nose when they are minus lenses is counterproductive and explain why. I have also had patients that like them sucked on to their eyeballs so tight they look like oversized contact lenses and then they complain their lenses are always dirty and patients that have worn glasses so long with extreme to the max i mean touching the outer edge of their lids wrap that it took me SIX MONTHS adjusting the front tiny bits at a time to get them out of the fishbowl into a normal facial wrap... so tell me how a vertex distometer is get in THERE, and what good it will do..... (teenager ... very resistant, had to explain to mom and dad numerous times that it had to change if he was ever going to see properly.)

    all in all, vertex yes, over plus or minus 8, and you all really should be fitting those progressives in tight and tilted anyway... so vertex should not really be an issue unless you aren't paying attention. and keep up the chicken bone bad Ju-Ju repellant.

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