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Thread: prescribing prism by decentration. how to calculate?

  1. #26
    Master OptiBoarder Darryl Meister's Avatar
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    Practically speaking, for spherical-ish surfaces at least, there shouldn't be any difference between prism obtained through decentration compared to prism obtained by surfacing. Of course, this assumes that the edge and center thickness of the lens remain constant.

    Prism is produced when the front and back surfaces are at an angle with respect to each other at some point on the lens. Surfacing prism into a lens using a conventional generator and prism rings tips the back surface of the lens relative to the front surface.

    The same effect can also be achieved by moving or decentering the back surface relative to the front surface. If you wedge a lever under a log, for instance, raising the lever so that it forms an angle to the ground, the log rolls away from the lever.

    Decentering a lens with power works in much the same way. In this case, however, the angle formed by the steeper surface as it rolls forms a steeper angle than the flatter surface, resulting in a relative angle between the two (or prism).

    Nevertheless, an illustration of the relationship between the front and back surfaces produced in any of these cases would reveal a similar configuration if they are all compared with the position of either the front or back surface held constant.
    Last edited by Darryl Meister; 09-27-2009 at 11:36 PM.
    Darryl J. Meister, ABOM

  2. #27
    ABOM Wes's Avatar
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    That's what I thought.

    Quote Originally Posted by Darryl Meister View Post
    Practically speaking, for spherical-ish surfaces at least, there shouldn't be any difference between prism obtained through decentration compared to prism obtained by surfacing. Of course, this assumes that the edge and center thickness of the lens remains constant.

    Prism is produced when the front and back surfaces are at an angle with respect to each other at some point on the lens. Surfacing prism into a lens using a conventional generator and prism rings tips the back surface of the lens relative to the front surface directly.

    The same effect can also be achieved by moving or decentering the back surface relative to the front surface. If you wedge a lever under a log, for instance, raising the lever so that it forms an angle to the ground, the log rolls away the lever.

    Decentering a lens with power works in much the same way. In this case, however, the angle formed by the steeper surface as it rolls forms a steeper angle than the flatter surface, resulting in a relative angle between the two (or prism).

    Nevertheless, an illustration of the relationship between the front and back surfaces produced in any of these cases would reveal a similar configuration if they are all compared with the position of either the front or back surface held constant.
    It's the same thing.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

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  3. #28
    ATO Member HarryChiling's Avatar
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    ............
    Last edited by HarryChiling; 10-12-2009 at 09:00 PM.
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  4. #29
    Master OptiBoarder Darryl Meister's Avatar
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    Yeah but you can't just say that anymore, unless your Der Meister
    I'm glad to know that my name carries such stock. But I still provided a rather descriptive answer explaining why they are the same. ;)
    Darryl J. Meister, ABOM

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    ATO Member HarryChiling's Avatar
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    .............
    Last edited by HarryChiling; 10-12-2009 at 09:00 PM.
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  6. #31
    OptiBoard Apprentice fagin's Avatar
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    Quote Originally Posted by Darryl Meister View Post
    I'm glad to know that my name carries such stock. But I still provided a rather descriptive answer explaining why they are the same. ;)
    and very nice it was, too. I believe the problem that a lot of folk have with comprehending prism lies with the comparison of the apparent complexity of surfacing a lens at an angle to produce a prism with the very simple idea of shunting a standard lens about so that its optical centre is moved. If you refer purely to the prismatic effect of a lens, then the method by which this is induced becomes secondary to the desired final result.

  7. #32
    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Quote Originally Posted by RT View Post
    Tell me how far you should decenter a plano power lens to get 2 diopters of prism.
    All the way.

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    Prism is prism is prism. Don't matter how you get it folks it's all the same! Doesn't matter how you skin the rabbit: same rabbit underneath the skin!

  9. #34
    Underemployed Genius Jacqui's Avatar
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    Quote Originally Posted by Murray O'Brien View Post
    Prism is prism is prism. Don't matter how you get it folks it's all the same!!
    Not when I look through it. :D

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    Mmm, nearly 30 years in the business and this is the first time I've ever heard of this subject given consideration. Any dfference you feel must be due to some other issue. Base curves? Thickness? Material? BVD? MRP position? Wrap angle ? Panto? I'm assuming you were comparing specs made to exactly the same R/X. As a matter of interest what is your R/X and what lenses were you comparing?

  11. #36
    Bad address email on file leestaniforth's Avatar
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    hi all,

    As many have pointed out, prism is prism etc blah blah, and of course you only surface prism into a lens when power of the lens is not enough to induce said prism and cut out into you frame considering oc's and required blank size of a stock lens.

    But.. did anybody actually answer the question .. how?:hammer::finger:, of course prentice rule, but what if the powers are not along 90 or 180 i.e


    lee

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    Hello Lee,
    I don't understand your question. Could you expand on it a bit? If you're talking about the first post in this thread then I think that was answered early on. And it really doesn't matter if the cyl axes are oblique, the method of marking up prism in a vertometer remains the same.

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    opti-tipster harry a saake's Avatar
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    prism

    Jacqui, your just a classic overthinker, because you know there different, you have that mindset, and your looking for it, and telling yourself there is something different

  14. #39
    Underemployed Genius Jacqui's Avatar
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    I've had others tell me that thsy see it too, Harry. What I think is happening is that as we move from the OC to the edges we pick up some distortion and this is why we're seeing a difference. Yes, it may be minor, but it's still there. On a surfaced prism we're seeing out of the area that has the least distortion since it's made for the use. This distortion will happen with anyone's lenses, yours, mine, Essilor's or el cheap-o imports.

  15. #40
    Bad address email on file leestaniforth's Avatar
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    Quote Originally Posted by Win C View Post
    Hi

    I have the following prescription :

    R-3.00/-1.75x63
    L-4.50/-2.00x15
    PD63
    Prism 2 baseout R and L

    can anyone help me with how much to decentre to get the required 2 prism base out?

    I used the prism calculator from opticampus for calculation and there is vertical prism of about 0.2 to 0.3 base up after decentration presumably from the cylinder. Do i just ignore the vertical induced prism since they are minimal and about the same in both eyes?

    Sorry, i'll try again.

    The above R Lens RX has an axis of 63. If the axis was
    180 you would -3.00 to place into p=c/f, however as the axis is 63.. what power do you have to work with?:cheers:

  16. #41
    ABO-AC, NCLE-AC, LDO-NV bob_f_aboc's Avatar
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    Quote Originally Posted by leestaniforth View Post
    Sorry, i'll try again.

    The above R Lens RX has an axis of 63. If the axis was
    180 you would -3.00 to place into p=c/f, however as the axis is 63.. what power do you have to work with?:cheers:
    The power at the 180 is ~-4.44 and the power at the 90 is ~-3.32. This is obtained by using the Oblique Meridian Formula.

    DT=DS+DC(sinα)^2

    Power in desired meridian = Sphere Power + (Cylinder Power * (sine of difference between prescribed meridian and desired meridian)^2)
    A lack of planning on your part DOES NOT constitute an emergency on mine!

  17. #42
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    So, let see if i understood what has been said. On a single vision lens the prism can be achived by descentration if the power and size of the uncut lens is enought.
    The power of the lens we are talking about is the power at 180 for in or out prism and 90 for up and down prism.
    On low power lenseses the prism produced by descentration may not be enought and i will need to grind it.
    I should not try to produce a prism by descentration on aspheric lenses.
    Since bifocals has been mentioned, on non single vision lenses the prescribed prism should be grinded.

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    Underemployed Genius Jacqui's Avatar
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    Quote Originally Posted by MIOPE View Post
    So, let see if i understood what has been said. On a single vision lens the prism can be achived by descentration if the power and size of the uncut lens is enought.
    The power of the lens we are talking about is the power at 180 for in or out prism and 90 for up and down prism.
    On low power lenseses the prism produced by descentration may not be enought and i will need to grind it.
    I should not try to produce a prism by descentration on aspheric lenses.
    Since bifocals has been mentioned, on non single vision lenses the prescribed prism should be grinded.
    Yes, you are quite right. :D Although I still think all prism should be surfaced for the best performance.

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    3 ways

    hi, you have three ways to do it:
    First Ask for the lab to surface the prism
    second you can dot up with the lensometer just putting the center in the second circle depnds if it is BO or BI and R or L.
    The last one is usin this formula to find how much milimiters to descenter.
    d=sph+cil(senaxis)2/Prism

  20. #45
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    Quote Originally Posted by Jacqui View Post
    Although I still think all prism should be surfaced for the best performance.
    I really don't understand why this keeps being mentioned, there is no evidence to support that other than anecdotal. More than a few professionals have mentioned that there is no difference and yet it keeps being brought up. On the off chance someone might use this resource as the end all be all on the subject it would be nice to see this myth dropped.

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    Underemployed Genius Jacqui's Avatar
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    It's not a myth, try wearing a prismatic lens for 45+ years and then you will know.

  22. #47
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    prism by decentration

    I do it with this application I have for our lab. Image attached.



    Quote Originally Posted by Win C View Post
    Hi

    I have the following prescription :

    R-3.00/-1.75x63
    L-4.50/-2.00x15
    PD63
    Prism 2 baseout R and L

    can anyone help me with how much to decentre to get the required 2 prism base out?

    I used the prism calculator from opticampus for calculation and there is vertical prism of about 0.2 to 0.3 base up after decentration presumably from the cylinder. Do i just ignore the vertical induced prism since they are minimal and about the same in both eyes?
    Attached Thumbnails Attached Thumbnails win c.jpg  

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