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Thread: Compensated Rx

  1. #26
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    Quote Originally Posted by drk View Post
    You want the truth?

    You can't handle the truth!


    The sad truth is, phoroptors have a very crappy means by which to measure vertex. You need like a super secret spy glass to read the vertex on most of them...
    I tell my > 8D clients to pull the phoropter snug against the face, if the prescriber/technician doesn't do so automatically. It's likely that the frame that I choose will rest at the same distance, essentially as close as possible w/o touching skin or lashes.

    WRT individualization...

    Here are just a few (but, again, not all) of the possibilities that you are likely to run into:

    Free-form lens with no individualized optimization (e.g., the original Definity lens)
    Free-form lens with a prescription "tweak" at the distance or near reference points
    Free-form lens with full prescription optimization over the entire lens based on average parameters, with or without the "tweak" at the reference points
    Free-form lens with full prescription optimization over the entire lens based on specified parameters (requires additional measurements)
    Free-form lens with design customization (i.e., the viewing zone configuration, periphery design, and/or corridor length is modified)
    Free-form lens with design customization and full prescription optimization, with or without the "tweak" at the reference points

    Note that any lens with a prescription "tweak" at the distance and/or near reference points will require a Compensated Rx in order to verify the original prescription.

    Darryl Meister/Optiboard
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  2. #27
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    If I jam the phoroptor in their faces and cut off their eyelashes, I think it's at 13.75 mm.

    I'll be darned if I didn't underminus myself a quarter...perfect refraction...perfect vertex distance...then the glasses slip 5 mm due to the horrible -7D lens weight and I can't see to drive at night...

    SPECIAL SAUCE, HOW SWEET THE FLAVOR MUST BE!

  3. #28
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    Quote Originally Posted by drk View Post
    If I jam the phoroptor in their faces and cut off their eyelashes, I think it's at 13.75 mm.

    I'll be darned if I didn't underminus myself a quarter...perfect refraction...perfect vertex distance...then the glasses slip 5 mm due to the horrible -7D lens weight and I can't see to drive at night...

    SPECIAL SAUCE, HOW SWEET THE FLAVOR MUST BE!
    That 5mm slippage may be due to the special sauce that you speak of, perhaps the addition of honey to create more friction.
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  4. #29
    What's up? drk's Avatar
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    Yes...

  5. #30
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    Quote Originally Posted by Paul Smith LDO View Post
    Curious as to what you mean. Free Form; as explained to me by the experts, are all compensated designs by definition. POW means that you are customizing the frame, Rx, to the individual as the frame is fitted to them. Average POW loses it's meaning in the translation; as you are now incorporating default or borrowed statistical data. Simply put one is either customized or not.
    To me a noncompensated free form lens would be one that creates asphericity and atoricity on the fly, or in the case of a progressive changes inset or corridor on the fly during surfacing not from a mold, but not altering the RX from its written form when read through the lensmeter.

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    Quote Originally Posted by Tallboy View Post
    To me a noncompensated free form lens would be one that creates asphericity and atoricity on the fly, or in the case of a progressive changes inset or corridor on the fly during surfacing not from a mold, but not altering the RX from its written form when read through the lensmeter.
    Altering/compensation of the Rx through Free Form can take place through POW/Customization or Default/Borrowed statistics. Seiko for instance is a Free Form Design and for years they have used their own statistical data. Shamir Element, In Touch, Spectrum, same same.
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  7. #32
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    Quote Originally Posted by Paul Smith LDO View Post
    Altering/compensation of the Rx through Free Form can take place through POW/Customization or Default/Borrowed statistics. Seiko for instance is a Free Form Design and for years they have used their own statistical data. Shamir Element, In Touch, Spectrum, same same.
    The spectrum and element don't alter the RX though right? To me that is a non compensated free form lens. Where as the Physio W3 or X design are free form compensated lenses using default POW garnered by statistical data

  8. #33
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    Quote Originally Posted by drk View Post

    So, long story short, we have to move the optical center of the lens in all sorts of directions so that the light coming into the pupil is correct.
    Thanks, doc! That helped me understand it much better!

  9. #34
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    Quote Originally Posted by Tallboy View Post
    The spectrum and element don't alter the RX though right? To me that is a non compensated free form lens. Where as the Physio W3 or X design are free form compensated lenses using default POW garnered by statistical data
    Compensation takes place both ways. If you are providing POW measurements you are adjusting the values for the compensation; customizing the Rx. If you are using propitiatory default mathematics or the lens manufactures values, it is still compensated just not customized. Not all Free Form lens designs can be customized.
    I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain

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    Quote Originally Posted by Paul Smith LDO View Post
    Compensation takes place both ways. If you are providing POW measurements you are adjusting the values for the compensation; customizing the Rx. If you are using propitiatory default mathematics or the lens manufactures values, it is still compensated just not customized. Not all Free Form lens designs can be customized.
    Then why do Spectrum lenses read "as prescribed" through the lensometer?

  11. #36
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    Quote Originally Posted by Tallboy View Post
    Then why do Spectrum lenses read "as prescribed" through the lensometer?
    Because the spectrum compensates for the Rx through BC's. When I opened my first digital account Free Form(full digital back surfacing) was described two ways compensation for the Rx through BC's or compensation for the frame geometry through POW. What we need is continuity in the language we use. When I discuss Free Form I define it in terms of values. Either we are providing values as POW to personalize/customize through frame geometry or we are personalizing the Rx through BC's. So call the former compensated and the latter personalized. Until we can agree on a language it will be difficult to agree on design.
    I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain

  12. #37
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    Quote Originally Posted by Paul Smith LDO View Post
    Because the spectrum compensates for the Rx through BC's. When I opened my first digital account Free Form(full digital back surfacing) was described two ways compensation for the Rx through BC's or compensation for the frame geometry through POW. What we need is continuity in the language we use. When I discuss Free Form I define it in terms of values. Either we are providing values as POW to personalize/customize through frame geometry or we are personalizing the Rx through BC's. So call the former compensated and the latter personalized. Until we can agree on a language it will be difficult to agree on design.
    Right so the Spectrum compensates for base curve caused abberations, but it doesn't use a compensated RX. Two different things, I was speaking on compensated RXs being integrated into the atoric/aspheric backside design. Essentially a Shamir Spectrum SV is just a rotationally symetrically aspheric design created on the backside instead of the frontside.

    My free form nomenclature would be:

    Optimized/Customized to RX (Base curves are optimized now that we don't have to rely on tools for cyl, progressive designs are customized to the power of the lens - not just the ground yoked prism but also the corridor and inset.)

    Personalized- (Includes the previous technologies but also includes POW variables that change the progressive design and compensate the lens RX across the surface leading to better peripheral vision and depth perception) This can include lenses that compensate the RX (not compensate for the rx) using the formulas available to all opticians as well as variable asphericity/atoricity of the design.

    Whatever the new stuff is doing (X/AutoIII etc) which I am not wholly understanding to be honest, maybe one day it will be explained better to me like I am 5.

  13. #38
    Master OptiBoarder MakeOptics's Avatar
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    All free form lenses should provide best form optimization.

    - This could mean they optimize the surfaces for a non best form base curve to reduce abberations. This can mean different things, they could optimze the lens for the sphere power, cyl power, mean power, or both.

    - They could optimize for oblique astigmatic error, power error, average of the previous, distortion. No one does or should compensate for spherical aberration or coma, they may may consider skewing one direction or the other when rounding due to sph abbe or coma but anyone who claims otherwise is wasting words.

    Vertex compensation must happen before design compensation.

    - Field of view changes as a lens moves closer or further, this effect allows the designer to soften or harden to improve the design.

    Hybrid free form lenses have default parameters built into the molded design.

    - Someone mentioned 5 dihedral 9 panto and 14 vertex, those defaults must be backed out of the compensation in hybrid designs.

    - Changes in sphere, cyl, and axis are not directly correlated between both eyes it is possible to have one eye change by 1 degree and the other by 18. In this particular case the right eye has -1.00DC and the left -0.50DC, if you look at ANSI stadards you can see the the slop in axis for a 0.25, 0.50, 0.75, 1.00, etc. isn't linear yet the effective power error produced by the slop is now constant.

    COR, center of rotation is used in every design.

    - That does not mean COR is a variable that is accessable to the ECP. A schematic eye has the COR as roughly 13.5mm from cornea to COR, refracted vertex distance is often quoted as 13.5mm if you add those you get the back vertex to center of rotation 27mm this is used in the design. Some lenses claim adjustments for myopes or hyperopes adjusting the axial length of the eye by either reducing or increasing the corneal to center of rotation of the eye when ray tracing.

    - When providing fitting vertex distance, the lens should not be vertex compensated this value should allow for the design to change, not the powers as the refracted vertex distance is unknown.

    Panto and Face Form (dihedral) are required and vary throughout the lens.

    - A phoropter generally has the ability to adjust slight faceform and panto into the device and many docs have their devices with a certain amount set. Optimally all of this should be backed out of the compensation, or the phoropter should be set to orthogonal and all lenses compensated for fitted panto and face form.

    - These measures vary throught the lens, the provided values for panto and faceform are generally for the distance of the lens however as the eye needs to rotate towards the reading these measures for the panto and faceform at the near reference point will significantly changed and the difference in power or add may even have a cylindrical component and often does if compensated correctly. Computing this require surface curvatures so this must be done at design time with hybrid lenses haveing their default values backed out if necessary. This is required since the near vision card on the end of the phoropter is literally attached to a bar directly in front ofnthe patients face when behind a phoropter, however when in their eyewear requires the client to rotate their eyes down towards the bottom of the lens.

    Thats about the gist of compensation, interestingly enough some prior generation improvements in progressive lens design are now being marketed as innovative new features but they are just being optimized on the fly at best providing better benefits but not necessarily making things 1000x better so I don't really give them to much mind.

    This is just my opinion so take it with a grain of salt but hybrid designs require too much accuracy in production with minimal benefits currently that I generally ignore the category. The next leap in tech will hopefully allow better production of both surfaces simultaneously, by freeing both surfaces to compensate lenses can have multiple abberations to compensate for instead of picking one, compromising, or averaging.
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  14. #39
    What's up? drk's Avatar
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    Make Optics rode back into Dodge.

    Talking with ten-dollar language.

    I'm going to have to "dumbsplain'" it to the rest of us.
    Last edited by drk; 03-28-2018 at 09:24 PM.

  15. #40
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    Quote Originally Posted by MakeOptics View Post
    All free form lenses should provide best form optimization.
    "In my opinion, when lens designers make formulas to plug into the computers that drive the CNC lathes, they should be thorough."

    - This could mean they optimize the surfaces for a non best form base curve to reduce abberations.
    "The lens designers can use math to make these nice thin and light--but optically compromised--aspheric lenses less optically compromised."

    This can mean different things, they could optimze the lens for the sphere power, cyl power, mean power, or both.
    - They could optimize for oblique astigmatic error,
    power error
    average of the previous
    "There's a lot of math involved. They can't do everything, so they have to pick their battles. Every lens design company will value one thing more than the other."



    No one does or should compensate for spherical aberration or coma, they may may consider skewing one direction or the other when rounding due to sph abbe or coma but anyone who claims otherwise is wasting words.
    "There are those lens designers that will claim to compensate for higher order aberrations, but that's not really possible. Don't listen."

    Vertex compensation must happen before design compensation.
    "The only light that needs compensated is the light as it enters the eye, which is a different vergence than when it leaves the back of the lens. Drk is right, as usual."

    - Field of view changes as a lens moves closer or further, this effect allows the designer to soften or harden to improve the design.
    "If glasses sit close to the eye, the areas seem larger. So the designers like short vertex distances, and so should you. But you can't always have that. If the lens is stuck farther out from the eye, since you have narrower areas to look through, the designers will "soften" the design...make the zones larger by allowing some smoothing of the astigmatism. But the zones won't look as crisp as they would have if the lens were closer to the eye, where they could make sharper cut-offs/harder design.

    Hybrid free form lenses have default parameters built into the molded design.- Someone mentioned 5 dihedral 9 panto and 14 vertex, those defaults must be backed out of the compensation in hybrid designs.
    "Back in the old days, when all progressive lenses were 'pre-designed' and poured into a mold for a front side add, they had to assume a certain position of wear. Then you just ground the Rx on the back the old-fashioned way. Life was simpler, then.

    But now, if they use the molded front side, the designers have those assumptions built into the way they designed the front side, but if they want to get fancy on the back side and do custom vertex/panto/etc., they have to go back and mathematically erase all those assumptions and THEN put the fancy stuff on the back. They have to "compensate" for the old built-in 'compensations'."

    - Changes in sphere, cyl, and axis are not directly correlated between both eyes it is possible to have one eye change by 1 degree and the other by 18. In this particular case the right eye has -1.00DC and the left -0.50DC, if you look at ANSI stadards you can see the the slop in axis for a 0.25, 0.50, 0.75, 1.00, etc. isn't linear yet the effective power error produced by the slop is now constant.
    "Different lenses will need different compensation amounts, depending, of course on the lens powers." I guess MO is saying "the errors involved by ANSI tolerances can be more evened out so the lenses seem equally good"?

    COR, center of rotation is used in every design.

    - That does not mean COR is a variable that is accessable to the ECP. A schematic eye has the COR as roughly 13.5mm from cornea to COR, refracted vertex distance is often quoted as 13.5mm if you add those you get the back vertex to center of rotation 27mm this is used in the design. Some lenses claim adjustments for myopes or hyperopes adjusting the axial length of the eye by either reducing or increasing the corneal to center of rotation of the eye when ray tracing.
    "Yeah, center of rotation is important to the calculations. But you can never measure it, so the lens designers are going to use some default amount. Maybe using a variable center of rotation amount based on longer or shorter eyes may be useful, who knows?"

    - When providing fitting vertex distance, the lens should not be vertex compensated this value should allow for the design to change, not the powers as the refracted vertex distance is unknown.
    "When you specify the vertex distance to these lens software design computers, they actually don't use the vertex distance to change the lens powers. For example, they don't take a -10.00 and make it weaker if you specify a short vertex distance. They would like to, but opticians don't seem to know what the refracted vertex distance is. Maybe -10.00 is correct for 13 mm. Maybe it was correct for 14 mm. But since nobody knows (including the optometrist, to be frank) they don't know which way to fix it, so they just leave it alone.

    But they will harden or soften the PAL design, like we were saying above."
    Panto and Face Form (dihedral) are required and vary throughout the lens.
    "Even something as straight-forward as 'position-of-wear' is really just a generalization. If you wrap a lens, the nasal part really doesn't get wrapped all that much. But the temporal part will. Same with panto; the top part may not go out that much, but the bottom sure will go in.

    Now, if we are trying to maximize every stinking square millimeter of the lens, point-by-point, since that's how the CNC lathe works its wonders, you have to factor all the variations of panto/wrap/etc. that occur even within a lens! Mind blown, right?"

    - A phoropter generally has the ability to adjust slight faceform and panto into the device and many docs have their devices with a certain amount set. Optimally all of this should be backed out of the compensation, or the phoropter should be set to orthogonal and all lenses compensated for fitted panto and face form.
    "I don't use phoroptors that much, but drk does. He knows that you can loosen a knob and tilt the phoroptor generally in the direction of pantoscopic tilt, if the patient will sit still enough, which I doubt they ever do. So that means a decent OD's Rx will be correct with an average amount of pantoscopic tilt in the frame. Therefore, if there's too little or too much in the frame, the patient won't like it, as we all have noticed. But it's something an optician can fudge around if the patient complains at dispensing. Probably only on PALs or high power lenses.

    However, drk doesn't think there's ANY face-form/wrap on the phoroptor, so I'm wrong on that one. But I'm still smarter than he is on everything else.

    But the upshot is that we wish we knew exactly what the phoroptors were positioned like. Probably makes all this gobbledy-gook kind of angels-dancing-on-the-heads-of-pins-stuff, if you think about it.

    - These measures vary throught the lens, the provided values for panto and faceform are generally for the distance of the lens however as the eye needs to rotate towards the reading these measures for the panto and faceform at the near reference point will significantly changed and the difference in power or add may even have a cylindrical component and often does if compensated correctly.
    "Like we said, each point in the lens is effectively affected differently by tilting. We have to assign the optician-specified value for panto and faceform to something, so we may as well use the distance optical center. But when we compensate for looking through the lower, nasal part (as when reading through the PAL near zone), man do the compensations look different than at the distance optical center! I love them crazy numbers!"

    Computing this require surface curvatures so this must be done at design time with hybrid lenses haveing their default values backed out if necessary. This is required since the near vision card on the end of the phoropter is literally attached to a bar directly in front ofnthe patients face when behind a phoropter, however when in their eyewear requires the client to rotate their eyes down towards the bottom of the lens.
    "Actually, drk should chime in, here. Phoroptors have a 'converged' position for near testing, so the lenses move closer together for the near p.d., and for some perverse reason, go into a negative-face-form configuration. Plus the whole thing is tested at straight-ahead-gaze level, not tested on downgaze. Drk would probably say 'who cares?' because it's only an add power, and man, you should see how squishy those measurements are in the first place."

    Thats about the gist of compensation, interestingly enough some prior generation improvements in progressive lens design are now being marketed as innovative new features but they are just being optimized on the fly at best providing better benefits but not necessarily making things 1000x better so I don't really give them to much mind.
    "We have reached the area of diminishing returns in PAL designs. They're close to perfected."

    This is just my opinion so take it with a grain of salt but hybrid designs require too much accuracy in production with minimal benefits currently that I generally ignore the category. The next leap in tech will hopefully allow better production of both surfaces simultaneously, by freeing both surfaces to compensate lenses can have multiple abberations to compensate for instead of picking one, compromising, or averaging.
    "If the lens has a split front/back add, or the lens has an all-front add, despite whatever claims of theoretical superiority, molding is too variable to be dependable to do this hair-splitting. Better to just go with the designs that are digitally surfaced all on the back side.

    Some day, we may be able to have lens that's getting a complex surface cut on both the front AND the back so we can refine the lens from near-perfection to perfection. We won't have to pick and choose which aberrations to correct, since we will be able to correct almost all.

    I wonder how many more goofy lens names the marketing departments can come up with, and how expensive the accounting department can make them? And will it make a difference to the average Joe, who never cleans their glasses in the first place?

    I promise I'll post more often, too. Sorry for the absence. I've been busy, as you can see."
    Last edited by drk; 03-28-2018 at 09:16 PM.

  16. #41
    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by drk View Post
    "In my opinion, when lens designers make formulas to plug into the computers that drive the CNC lathes, they should be thorough"

    - This could mean they optimize the surfaces for a non best form base curve to reduce abberations. This can mean different things, they could optimze the lens for the sphere power, cyl power, mean power, or both.

    ,, distortion.
    Best form = the optimal surface to acheive design goals. Interestingly enough this can mean many different things. Thinnest profile could mean flatest lens, minimum astigmatic error could mean a specific surface, etc. Even traditional molded SV stock lenses vary from manufacturer to manufacturer because best form = optimized design goals. Unfortunately, that means best form is open to interpretation due to the lenses missing key information, the design goal. It was much easier when they were called Punktal (point focal), Orthogon (minimum distortion), etc. Now they're naked lenses.

    Drk, I tried not to jazz it up I was hoping dumbslaining wouldn't be necessary. Just wanted to cover a few points i consistently hear misconstrued, COR, defaults, order of operations, changes in power/axis and their inconsistent look.
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  17. #42
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    Regarding COR, how can it be measured and why are we not measuring it. The Visioffice says it measures it, does it? There are reports like this one from the 60's talking about the importance of COR with regards to lens design and the variability it can have in different patients. This references even older reports from the 40's and 50's. How were they measuring COR back then and we can't do it now? Is it a case of default values being close enough and refining it doesn't make a significant enough of improvement for the amount of work it takes to measure it?

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    Quote Originally Posted by Kwill212 View Post
    Regarding COR, how can it be measured and why are we not measuring it. The Visioffice says it measures it, does it? There are reports like this one from the 60's talking about the importance of COR with regards to lens design and the variability it can have in different patients. This references even older reports from the 40's and 50's. How were they measuring COR back then and we can't do it now? Is it a case of default values being close enough and refining it doesn't make a significant enough of improvement for the amount of work it takes to measure it?
    I think it is that for an overwhelming majority of people the effect of vertex distance on COR is by far the most significant value, so by incorporating that variable in the lens design something very beneficial is achieved. It would be interesting to see hard data on how much better a lens is performs if it accounts for COR variables not related to only the vertex distance.

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    Quote Originally Posted by Kwill212 View Post
    Regarding COR, how can it be measured and why are we not measuring it. The Visioffice says it measures it, does it? There are reports like this one from the 60's talking about the importance of COR with regards to lens design and the variability it can have in different patients. This references even older reports from the 40's and 50's. How were they measuring COR back then and we can't do it now? Is it a case of default values being close enough and refining it doesn't make a significant enough of improvement for the amount of work it takes to measure it?
    From what I can make out he Visi Office has the patient stare straight forward and then rotate their eyes to look at a fixation point a set distance to one side or the other. The measurements are known for the images by the clip that is placed on the frame when measuring using the system. The clip provides a ration of mm to px in the image, that ratio along with the known camera parameters allow the Visi Office to determine the distance the patient is standing from the machine, then the fixation point is a set distance from the camera and Visi Office machine body. With that data the angle can be measures that the eye makes to rotate from staring into the camera to looking at the fixation point. Since the mm to px ratio is known and the machine has the images from camera fixation and rotation fixation, using similar triangles the COR can be measured. At least this is my best interpretation of how the machine works. The actual clip that goes on the frame when using the machine is where all the magic happens everything else is just commodity hardware when it comes to the actual physical machine.
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    Last Post: 10-15-2012, 03:46 PM

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