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Thread: Prism thinning vs ordering prism. Is there a difference?

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    Prism thinning vs ordering prism. Is there a difference?

    this is like the “decentered or ground-in” debate but spicier!

    So, these days digital lenses are not so good for ordering one-offs. A patient’s Rx changed after a few years in only one eye and the doc told them “yeah just order one lens”. You get the idea. We’ve repeatedly asked our lab to match the prism thinning that we ask for, sometimes it happens, sometimes it doesn’t. So my shop has taken to the idea of just ordering a one of lens as prescribed prism to get the matching prism-thinning effect that we want. What are your opinions on this?

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    There should be no reason that you can’t do that but you should understand the root case use of your problem. The prism in prism thinning is calculated to give you the thinnest lens possible by making the top and bottom of the the lens equal in thickness so the whole lens can be made thinner. The new Rx might require a different amount of prism than the old Rx to optimize the thickness. You might be able to match the prism but might not have lens that optimized for thickness. That is why sometimes it matches what you had before and sometimes it doesn’t.

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    Quote Originally Posted by Lensman11 View Post
    There should be no reason that you can’t do that but you should understand the root case use of your problem. The prism in prism thinning is calculated to give you the thinnest lens possible by making the top and bottom of the the lens equal in thickness so the whole lens can be made thinner. The new Rx might require a different amount of prism than the old Rx to optimize the thickness. You might be able to match the prism but might not have lens that optimized for thickness. That is why sometimes it matches what you had before and sometimes it doesn’t.
    we get that a million percent, however, for all of the ways you can manipulate the Rx with POW measurements and for them saying they can surface within a hundredth of a diopter, they should be able to match the prism thinning that we request.

    consider this, if you order an Rx pair with very different powers from right to left, they will match the prism thinning and have the thickness management take a backseat.

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    You are correct about a pair with different rx’s the compromise would be made. I am just telling you why you not getting the prism you had the first time. Order what you need to match the other eye and accept the thickness as best as can be considering the circumstances. Unless the change in Rx is considerable the thickness will not be noticeable.

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    Quote Originally Posted by Lensman11 View Post
    You are correct about a pair with different rx’s the compromise would be made. I am just telling you why you not getting the prism you had the first time. Order what you need to match the other eye and accept the thickness as best as can be considering the circumstances. Unless the change in Rx is considerable the thickness will not be noticeable.
    The heart of this post is to determine if it is the same or not. I do not think that is.

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    It is absolutely the Same thing just as decentering for prism is the same as grinding prism.

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    Quote Originally Posted by Lensman11 View Post
    It is absolutely the Same thing just as decentering for prism is the same as grinding prism.
    I don’t know: if you have a lens with ground in prism (let’s say 5 base up) and if you look at it on the lensometer, and move it vertically, it will pretty much read that same 5 up at all vertical points(with a little change based on underlying power).

    with decentered, let’s say you you have a -5.00, you scooch the OC down 10mm and you get the same 5 up. For central vision this works. But If you look up 2mm, well then you’ve effectively lost a diopter of prism.

    wether or not this is objectively going to affect the patient is on a case by case basis for sure. But I think it’s more accurate to have ground in.

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    You are incorrect about prism. Ground in prism would only show no movement if there is no power in the lens. That would be a lens that you could not decenter to obtain prism. There is absolutely no difference. If you look at the end result they will look and act the same to the patient.

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    Quote Originally Posted by Prentice Pro 9000 View Post
    <snip>

    But I think it’s more accurate to have ground in.
    I typed out a reply but it vanished somehow (due to my caveman computer skills, no doubt!), and Lensman beat me to it as well as addressed my own doubts on the matter! :)

    Basically, as Lensman has confirmed of my current understanding of prism induced by grinding in vs. decentration, an explanation which I have found useful in practice:

    Grinding in prism is when the optical center of the lens is ground decoupled from the geometric center of the lens whereas prism by decentration is moving the entire lens to induce prism, this movement being necessary since the optical and geometric center are ground coincident. Therefore both would behave the same optically despite differing at the grinding stage (and you wouldn't find a fixed amount of prism across the lens for any lens with power, as Lensman has kindly highlighted).

    Also, w.r.t. to prism thinning, since as I recall most labs use a fraction of the add to determine the magnitude of prism thinning (usually 1/2 to 2/3 add), wouldn't your amount of prism thinning be similar if not the same even when replacing just one lens, provided similar adds are on both eyes? Assuming of course your lab doesn't use different formulae, and that the Rx on both eyes is similar.

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    Prism thinning should not be calculated based on a percentage of the add. The best results are obtained using the add power the distance Rx the height and the shape of the frame. Without the use of a computer it would be extremely difficult to calculate.

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    Quote Originally Posted by Lensman11 View Post
    You are incorrect about prism. Ground in prism would only show no movement if there is no power in the lens. That would be a lens that you could not decenter to obtain prism. There is absolutely no difference. If you look at the end result they will look and act the same to the patient.
    So it’s a a total 1:1? I always thought ground in would be superior at least at the very margins.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    For lurkers who are not sure what prism thinning is, here is a tour de force tutorial on progressive designs in general by the late great titan and Zeiss lens designer, Darryl Meister.

    His knowledge and understanding of the differential geometry used in various designs, without the marketing mumbo jumbo (I'm talking about W.A.V.E. bull patties here), can be equaled by a few but surpassed by none.

    If you are like me you read this once. Turn on a fan to clear the room of smoke that came out of your ears, then read it again and again and it eventually starts to sink in.

    http://216.144.236.77/cecourse.php?u...essive_lenses/

    How and why prism thinning is done is at the very bottom of the article.

    Every "Course" he wrote from this Opticampus OB Forum that Steve Machol generously keeps alive is a must read if you want to be a better optician.

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    Quote Originally Posted by Prentice Pro 9000 View Post
    this is like the “decentered or ground-in” debate but spicier!

    So, these days digital lenses are not so good for ordering one-offs. A patient’s Rx changed after a few years in only one eye and the doc told them “yeah just order one lens”. You get the idea. We’ve repeatedly asked our lab to match the prism thinning that we ask for, sometimes it happens, sometimes it doesn’t. So my shop has taken to the idea of just ordering a one of lens as prescribed prism to get the matching prism-thinning effect that we want. What are your opinions on this?
    I've always taken the stance don't order just 1 lens for progressive or digital lenses. If the lenses get made together I know there is almost no chance there will be an error on the prism thinning. We have a lot of jobs and I try to have rules for myself and the office that will make less problems in the long run. I definitely agree that if I did one progressive lens sometimes our main lab would get it correct and sometimes they would not. I would probably guess it would be 50/50. Maybe that's a sign we need a different lab? Who knows but keep it simple stupid seems to work for me.

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    Quote Originally Posted by Lensman11 View Post
    Prism thinning should not be calculated based on a percentage of the add. The best results are obtained using the add power the distance Rx the height and the shape of the frame. Without the use of a computer it would be extremely difficult to calculate.
    Much appreciated for this, and it does make sense (though now I wonder what those lab staff and reps who told me the percentage of Add thing over the years actually meant, maybe they were just providing a rule of thumb for quick verification).

    On a practical aspect of the same topic, regarding ordering just one side for such PALs, wouldn't most modern lens ordering systems in the labs be able to input the Rx + fitting data for both eyes, and then tell their lab system to only produce one side as needed? It is my understanding from some of the better lab reps I've worked with (former opticians, etc) that if in doubt over possible such matching issues on both sides, to provide the Rx for both eyes for their input, but they'll do a binocular calculation and then only physically produce one lens as required.

    Background: first encountered that years ago with either a Hoya or Seiko PAL order for an antimetropia case, one lens got broken, I reordered just one side without altering any details and it went wonky... turns out that the original binocular order had meant a certain prism thinning was done for the pair, but the single side reorder meant the second order arrived with significantly different prism thinning. Problem was resolved by providing the original binocular order details + specifying only produce the side required, which matched the original prism thinning.

    And yes, I paid for that remake :P

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    Quote Originally Posted by Uncle Fester View Post
    For lurkers who are not sure what prism thinning is, here is a tour de force tutorial on progressive designs in general by the late great titan and Zeiss lens designer, Darryl Meister.

    His knowledge and understanding of the differential geometry used in various designs, without the marketing mumbo jumbo (I'm talking about W.A.V.E. bull patties here), can be equaled by a few but surpassed by none.

    If you are like me you read this once. Turn on a fan to clear the room of smoke that came out of your ears, then read it again and again and it eventually starts to sink in.

    http://216.144.236.77/cecourse.php?u...essive_lenses/

    How and why prism thinning is done is at the very bottom of the article.

    Every "Course" he wrote from this Opticampus OB Forum that Steve Machol generously keeps alive is a must read if you want to be a better optician.

    Also this one is all about prism thinning. http://216.144.236.77/cecourse.php?url=prism_thinning/

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    Andy
    what happened when you ordered one lens is exactly what I explained. A single lens will produce a different amount of prism thinning than a pair if the Rx is not exactly the same. When the Rx is different you will get a compromise on the thickness as the priority is matching the prism.
    many decades ago prism thinning was done as a percentage of the add power it was better than not doing at all but not nearly as good as what we do today considering the height shape and add power.

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    Quote Originally Posted by AndyOptom View Post
    Much appreciated for this, and it does make sense (though now I wonder what those lab staff and reps who told me the percentage of Add thing over the years actually meant, maybe they were just providing a rule of thumb for quick verification).

    On a practical aspect of the same topic, regarding ordering just one side for such PALs, wouldn't most modern lens ordering systems in the labs be able to input the Rx + fitting data for both eyes, and then tell their lab system to only produce one side as needed? It is my understanding from some of the better lab reps I've worked with (former opticians, etc) that if in doubt over possible such matching issues on both sides, to provide the Rx for both eyes for their input, but they'll do a binocular calculation and then only physically produce one lens as required.

    Background: first encountered that years ago with either a Hoya or Seiko PAL order for an antimetropia case, one lens got broken, I reordered just one side without altering any details and it went wonky... turns out that the original binocular order had meant a certain prism thinning was done for the pair, but the single side reorder meant the second order arrived with significantly different prism thinning. Problem was resolved by providing the original binocular order details + specifying only produce the side required, which matched the original prism thinning.

    And yes, I paid for that remake :P
    Edit: Sorry I just read Kwill had already posted the website.

    Yes the % of add is a quick approximation I believe. I just came across it on Opticampus: http://216.144.236.77/cecourse.php?url=prism_thinning/

    "Prism-Thinning Calculation

    So how much prism-thinning should be used? A common rule-of-thumb formula provided by some manufacturers to minimize the thickness difference at the top and bottom edges of an uncut progressive lens blank is:
    Prism = 0.6 × Add
    This shows that a quantity of base down prism equal to 60% of the add power should be used to slim the lens blank down. Some manufacturers may recommend using 2/3rd the add power (about 67%). These rule-of-thumb approaches are often recommended when the combined distance and add power through the vertical meridian of the lens exceeds +1.50 D or so.For instance, a progressive lens with an add power of +3.00 D might require roughly 2/3 × 3.00 = 2.0 Δ (prism diopters) of base down prism.Note that these rule-of-thumb formulas do not consider factors like the fitting height and the distance power, but still produce satisfactory results in many cases. Ideally, the amount of prism-thinning used should be based upon the following factors:

    • Add power
    • Distance prescription
    • Fitting cross height
    • Fitting cross decentration
    • Frame shape"
    Last edited by NAICITPO; 02-17-2023 at 12:23 PM. Reason: Didn't read what Kwill had already posted

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    It’s not Fitting cross height.
    It’s PRP Height.

    Barry

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    According to bit difference, Yes, there is a difference between prism thinning and ordering prism. Prism thinning refers to the process of decreasing the prism power in a lens by reducing the thickness of the prism. This can be done by grinding away some of the material on one side of the prism to make it thinner, which reduces the deviation of light passing through the lens. Prism thinning is often used to improve the visual comfort of glasses, particularly for individuals with double vision (diplopia). Ordering prism, on the other hand, refers to the process of specifying the amount and direction of prism that is needed in a lens prescription. This is typically done by an eye care professional or optometrist, who measures the amount of deviation in a person's eyes and prescribes the necessary prism correction. Ordering prism is important for individuals with certain eye conditions, such as strabismus or amblyopia, where the eyes are not properly aligned and prism correction can help improve binocular vision. So while both prism thinning and ordering prism involve the use of prism in eyeglass lenses, they refer to different aspects of the lens design process.
    Last edited by david755; 02-21-2023 at 04:15 PM.

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    David 755
    you might be on to something here unfortunately it has nothing to with prism thinning either how it is manufactured, or what the purpose of prism thinning is supposed to accomplish.

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    Quote Originally Posted by AndyOptom View Post
    Much appreciated for this, and it does make sense (though now I wonder what those lab staff and reps who told me the percentage of Add thing over the years actually meant, maybe they were just providing a rule of thumb for quick verification).

    On a practical aspect of the same topic, regarding ordering just one side for such PALs, wouldn't most modern lens ordering systems in the labs be able to input the Rx + fitting data for both eyes, and then tell their lab system to only produce one side as needed? It is my understanding from some of the better lab reps I've worked with (former opticians, etc) that if in doubt over possible such matching issues on both sides, to provide the Rx for both eyes for their input, but they'll do a binocular calculation and then only physically produce one lens as required.

    Background: first encountered that years ago with either a Hoya or Seiko PAL order for an antimetropia case, one lens got broken, I reordered just one side without altering any details and it went wonky... turns out that the original binocular order had meant a certain prism thinning was done for the pair, but the single side reorder meant the second order arrived with significantly different prism thinning. Problem was resolved by providing the original binocular order details + specifying only produce the side required, which matched the original prism thinning.

    And yes, I paid for that remake :P
    oof. In general getting ahold of someone at these companies that even understands what’s going on is a big ask. These days at least.

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    Having just caught up with the thread after a busy weekend... I figured a summary would be handy for anyone interested:

    - Optically there isn't a difference between prism obtained by decentralization, compared to prism ground in.

    - Prism thinning nowadays needs complex calculations related to the ordered Rx and also fitting data (as opposed to ye olden days when it was a fraction of the Add).

    - Prism thinning is also possibly affected when calculations are done binocularly versus monocularly. Best to consult your lab on to how best match prism thinning if you only need to order one lens.

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    Quote Originally Posted by AndyOptom View Post
    Having just caught up with the thread after a busy weekend... I figured a summary would be handy for anyone interested:

    - Optically there isn't a difference between prism obtained by decentralization, compared to prism ground in.

    - Prism thinning nowadays needs complex calculations related to the ordered Rx and also fitting data (as opposed to ye olden days when it was a fraction of the Add).

    - Prism thinning is also possibly affected when calculations are done binocularly versus monocularly. Best to consult your lab on to how best match prism thinning if you only need to order one lens.
    +1

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    Quote Originally Posted by david755 View Post
    Yes, there is a difference between prism thinning and ordering prism. Prism thinning refers to the process of decreasing the prism power in a lens by reducing the thickness of the prism. This can be done by grinding away some of the material on one side of the prism to make it thinner, which reduces the deviation of light passing through the lens. Prism thinning is often used to improve the visual comfort of glasses, particularly for individuals with double vision (diplopia).
    This sounds like something that came out of an AI bot. Where on earth did you get this information?

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    Quote Originally Posted by Kwill212 View Post
    This sounds like something that came out of an AI bot. Where on earth did you get this information?
    Ja.

    David,

    Some of what you wrote about prism is not quite right. Check out the links below, in addition to the D. Meister course on prisms and prism thinning that Kwill and Uncle Fester posted above.

    https://www.asbopticianry.com/upload...s_-_part_1.pdf

    https://www.asbopticianry.com/upload...s_-_part_2.pdf

    https://www.asbopticianry.com/upload...s_-_part_3.pdf

    All the best,

    Robert Martellaro
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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