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Thread: Ground-in prism

  1. #26
    Master OptiBoarder Darryl Meister's Avatar
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    Still believe that the OC's are in front of the pupil with prism?
    I think he was referring to the "center" or "pole" of the aspheric surface, which should generally remain in front of the pupil, with or without prism.
    Darryl J. Meister, ABOM

  2. #27
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    Quote Originally Posted by Darryl Meister View Post
    I think he was referring to the "center" or "pole" of the aspheric surface, which should generally remain in front of the pupil, with or without prism.
    That's why we meassure OC heights in higher powered aspheric single vision lenses.

  3. #28
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by fjpod View Post
    That's why we meassure OC heights in higher powered aspheric single vision lenses.
    It is now refered to as the DRP or PRP which should be the same in this case, not the OC. Take a look at the lens description standards:
    http://www.visionsite.org/s_vision/s...ID=253&DID=384

    It helps us avoid chasing our tails over sematics.
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  4. #29
    OptiBoard Professional jrumbaug's Avatar
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    Quote Originally Posted by Neophyte Optician View Post
    If a pair of glasses is supposed to have ground-in prism (e.g. 2 prism diopters base right), when I verify it on a manual lensometer should the 2 prism diopters show up no matter which part of the lens I am looking at?

    Also, how do I grind that in?

    Answer to 1st question
    example 1 rx = plano sphere with 2 diop base out
    you will see 2 diop base out prism regardless where you measure it

    example 2 rx = -5.00 diop sphere with 2 diop base out ( rare rx )
    you will only find 2 diop prism 4mm from the optical center. which brings up the point.... to measure horizontal prism you must have the patients PD. The practical exception is low powers with high prism. Vertical prism is not PD critical ( an exception might be high cylinder at oblique angles, example rx +0.50 - 3.50 axis 045 )

    Answer to second question
    I don't know. I don't grind. But I do induce prism by decentering spherical ( not aspheric ) lens, when the rx allows it. example .. -1.00 sphere with 0.50 prism base out, move lens 5mm, but if sphere was -0.25, you can't move most blanks 20mm.

    Jim Rumbaugh

  5. #30
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    Quote Originally Posted by fjpod View Post
    Would it be acceptable, in this case, for a prescriber to write "ground-in prism" or "hyper-aspheric with ground-in prism" on this Rx?
    I thought hyper-aspheric lenses were only used in very high plus RX's (cR-39 and only come in the highest base curves, 9, 10, 12, 14) I could be wrong though....

  6. #31
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    This topic caught my attention and I was hoping someone could clear this up for me since it's on the subject. I understand that decentration will not work in some cases such as high prism and low power because the lens would not have sufficient diameter to be decentered. So, assuming there is not enough power in a lens for decentration to yield the required prism, it would have to be ground in. I get that, but my question is if that same low power lens were infinitely large (I guess that would be a complete sphere based on the base curve!!), then could the lens be decentered to yield the prism? And if you did decenter a large amount, then edge the lens appropriately, would it end up being the same as a regular sized lens with ground in prism? It just seems to me that the two lenses in this case, arrived at differently, would be identical when finished. Any insight into this would be very much appreciated.

  7. #32
    OptiBoard Professional jrumbaug's Avatar
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    Quote Originally Posted by go_hercules View Post
    This topic caught my attention and I was hoping someone could clear this up for me since it's on the subject. I understand that decentration will not work in some cases such as high prism and low power because the lens would not have sufficient diameter to be decentered. So, assuming there is not enough power in a lens for decentration to yield the required prism, it would have to be ground in. I get that, but my question is if that same low power lens were infinitely large (I guess that would be a complete sphere based on the base curve!!), then could the lens be decentered to yield the prism? And if you did decenter a large amount, then edge the lens appropriately, would it end up being the same as a regular sized lens with ground in prism? It just seems to me that the two lenses in this case, arrived at differently, would be identical when finished. Any insight into this would be very much appreciated.
    This answer to your hypothetical large lens may be a little hard to follow.
    Gee.... it may even be wrong.
    IF the calculated optical center does not fall within the lens THEN
    .......... ground in prism is thinner
    OTHERWISE
    ......... they are the same

    In the case where the O.C. (thinnest part) would not fall within the lens, the lens could be ground with a theoretrical center thickness, thinner than standard, perhaps in some cases, even a negative center thickness.

    Jim Rumbaugh

  8. #33
    ATO Member HarryChiling's Avatar
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    This topic caught my attention and I was hoping someone could clear this up for me since it's on the subject. I understand that decentration will not work in some cases such as high prism and low power because the lens would not have sufficient diameter to be decentered.

    So, assuming there is not enough power in a lens for decentration to yield the required prism, it would have to be ground in.
    YES

    I get that, but my question is if that same low power lens were infinitely large (I guess that would be a complete sphere based on the base curve!!), then could the lens be decentered to yield the prism?
    YES and NO, it is possible to eventually run into the lens circumference running into itself before you reach teh prism, but before you get to that point it would be outside the realm of reality for vision or much else.

    And if you did decenter a large amount, then edge the lens appropriately, would it end up being the same as a regular sized lens with ground in prism?
    NO, since the optical center of a stock lens is set the further off the lens it goes the thicker the edges of the lenses get, where as in a ground prism situation you could speciafy the edge thickness.

    It just seems to me that the two lenses in this case, arrived at differently, would be identical when finished.
    NO

    Any insight into this would be very much appreciated.
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