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Thread: Decentration of spherical lens question

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    Decentration of spherical lens question

    Hello, I was wondering if someone could help me with the following true/false question.
    My answer is wrong from what the book tells me. I believe the book has it's errors so I just want to verify.
    Thank you. I appreciate the help.

    Attachment 12601

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    Master OptiBoarder optical24/7's Avatar
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    1. Bad link, just post the question.

    2. You're an Ophthalmologist?

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    1. Regarding the prismatic power that results following decentration of a spherical lens:
    i. Prism power is directly proportional to decentration (T)
    ii. Prism power is inversely proportional to power of the spherical lens. (F)
    iii. A concave lens decentred nasally results in a base-in prism. (F)
    iv. A convex lens decentred superiorly results in a base up prism. (?) Book says it's T but wouldn't it be F since convex lenses are base in prisms.
    v. A convex lens decentred temporaly results in a base out prism. (?) Book says it's T but wouldn't base be inwards.



    2. First year resident of ophthalmology (Didn't find the option for residents). These questions are practice for ICO optics exam.


    Thanks for the help.

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    Quote Originally Posted by Alonso View Post
    1. Regarding the prismatic power that results following decentration of a spherical lens:
    i. Prism power is directly proportional to decentration (T)
    ii. Prism power is inversely proportional to power of the spherical lens. (F)
    iii. A concave lens decentred nasally results in a base-in prism. (F)
    iv. A convex lens decentred superiorly results in a base up prism. (?) Book says it's T but wouldn't it be F since convex lenses are base in prisms.
    v. A convex lens decentred temporaly results in a base out prism. (?) Book says it's T but wouldn't base be inwards.



    2. First year resident of ophthalmology (Didn't find the option for residents). These questions are practice for ICO optics exam.


    Thanks for the help.
    http://www.medrounds.org/optics-review/2006/05/18.html

    Click image for larger version. 

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    For simplifications sake, picture the pupil at the center of the plus lens where the two bases touch. If there is dioptric power in the vertical meridian, decentering the lens upwards places the bases up, and the apexes down. The opposite is true for minus lenses. Use Prentice's rule to determine the value in prism diopters.

    Hope this helps,
    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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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by Alonso View Post
    1. Regarding the prismatic power that results following decentration of a spherical lens:
    i. Prism power is directly proportional to decentration (T)
    ii. Prism power is inversely proportional to power of the spherical lens. (F)
    iii. A concave lens decentred nasally results in a base-in prism. (F)
    iv. A convex lens decentred superiorly results in a base up prism. (?) Book says it's T but wouldn't it be F since convex lenses are base in prisms.
    v. A convex lens decentred temporaly results in a base out prism. (?) Book says it's T but wouldn't base be inwards.



    2. First year resident of ophthalmology (Didn't find the option for residents). These questions are practice for ICO optics exam.


    Thanks for the help.
    Prism = Dec()cm * Power

    A proportional equation is described as

    y = k * x

    y = dependent variable
    k = constant
    x = independent variable

    If we look at the prism formula the Power is our constant, the decentration is our independent variable and the prism is our dependent variable. What that boils down to is the answer, prism is directly proportional to decentration. ; )

    It's a math question not an optics question!
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    Make optic: "It's a math question not an optics question!"
    Well, I think that in this case point is on generally understanding. Maybe visual representation will help better. This picture shows a spherical convex lens. Orientation of base of prisms are opposite than in concave lens (not represented in picture).
    In short: spherical convex lens: in the same direction of decentration there will be base of prism also. So, if we moved lens out, out will be base of prism also.
    With concave lens is opposite.
    Last edited by Wlada; 03-02-2016 at 04:15 PM.

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by Wlada View Post
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    Make optic: "It's a math question not an optics question!"
    Well, I think that in this case point is on generally understanding. Maybe visual representation will help better. This picture shows a spherical convex lens. Orientation of base of prisms are opposite than in concave lens (not represented in picture).
    In short: spherical convex lens: in the same direction of decentration there will be base of prism also. So, if we moved lens out, out will be base of prism also.
    With concave lens is opposite.
    Great illustration, proportionality has nothing to do with the sign or direction. Proportions are just ratio's. If we were to get real specific we would have to take into account the prism formula is just an approximation which would throw everything out of whack. Yes though you are right about base directions.

    We shouldn't overthink it since most of ophthalmic optics is designed with cheats and simplifications introduced to simplify optics for the masses, in this case the question looks like it is designed to build upon in further lessons, I think in this case you have a better understanding of prism. In essence you have peaked behind the curtain, the great wizard of Oz is just a charlatan.
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