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Thread: Self-studying optical dispensing

  1. #1
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    Back Vertex Power and Prescriptions

    Hi all,

    I'm currently working as an optical assistant at Specsavers and have just finished my first year (of 5) studying Optometry in Australia. Unfortunately I'm not in a position to undertake a separate qualification to become a qualified dispenser, due mostly to clashes with University classes, and yet I am required to dispense spectacles in my current job role, and a qualification is not necessary. We don't delve deeply into dispensing at any point in the optometry course, and clinical optics is still a good 2 years away. Alas, dispensing without proper training doesn't sit right with me, so I'm trying to construct my own self-study course that I can do during any time I can spare in order to bring me up to a 'qualified' level (without the qualification of course).

    I've been spending a lot of time trying to study up on ophthalmic optics, and unfortunately I've hit a road block. I firmly believe that the answer is obvious, yet I can't see it. Say I was presented with the following Rx.

    +1.00 -0.75 x 180

    Now am I correct in saying that this Rx. is telling me that this lens has a nominal power of +1.00D in the 180 degree meridian and a nominal power of +0.25D in the 90 degree meridian? Furthermore, this lens can be assumed to be thin, and so the nominal power is a good approximation of the total power of the lens, and is equal to the sum of the front and back surface powers. Is my understanding correct?

    So although I'm fairly certain I've got a good grasp on the above material, the problem arises with the introduction of thick lenses and the idea of vertex powers. I understand the concept of front and back vertex powers, and I can calculate them just fine with the appropriate formulas. What I'm struggling to understand is this: suppose I was presented with another Rx.

    +8.75 -0.25 x 180

    In this case, is the Rx. still presenting the nominal power of the lens? Since the thin-lens assumption loses validity as power increases, I'm thinking that this Rx. is not simply stating the nominal power of the lens as the sum of the front and back surface powers, since that would not be a great approximation. The different texts I've been studying have introduced the idea of the back vertex power of a lens (specifically with thick lenses), and have stressed its importance in ophthalmic optics. So then, would the above Rx. be referring to the back vertex power? I'm completely lost as to where the back vertex power sits in the optical world, and what its role is.

    I feel like I'm missing something glaringly obvious; if anyone could shed some light on any of this, I would be filled with inexplicable joy .

    Kind Regards,
    Daniel
    Last edited by TheRobotious; 12-17-2014 at 03:51 AM.

  2. #2
    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by TheRobotious View Post
    Hi all,

    I'm currently working as an optical assistant at Specsavers and have just finished my first year (of 5) studying Optometry in Australia. Unfortunately I'm not in a position to undertake a separate qualification to become a qualified dispenser, due mostly to clashes with University classes, and yet I am required to dispense spectacles in my current job role, and a qualification is not necessary. We don't delve deeply into dispensing at any point in the optometry course, and clinical optics is still a good 2 years away. Alas, dispensing without proper training doesn't sit right with me, so I'm trying to construct my own self-study course that I can do during any time I can spare in order to bring me up to a 'qualified' level (without the qualification of course).

    I've been spending a lot of time trying to study up on ophthalmic optics, and unfortunately I've hit a road block. I firmly believe that the answer is obvious, yet I can't see it. Say I was presented with the following Rx.

    +1.00 -0.75 x 180

    Now am I correct in saying that this Rx. is telling me that this lens has a nominal power of +1.00D in the 180 degree meridian and a nominal power of +0.25D in the 90 degree meridian? Furthermore, this lens can be assumed to be thin, and so the nominal power is a good approximation of the total power of the lens, and is equal to the sum of the front and back surface powers. Is my understanding correct?

    So although I'm fairly certain I've got a good grasp on the above material, the problem arises with the introduction of thick lenses and the idea of vertex powers. I understand the concept of front and back vertex powers, and I can calculate them just fine with the appropriate formulas. What I'm struggling to understand is this: suppose I was presented with another Rx.

    +8.75 -0.25 x 180

    In this case, is the Rx. still presenting the nominal power of the lens? Since the thin-lens assumption loses validity as power increases, I'm thinking that this Rx. is not simply stating the nominal power of the lens as the sum of the front and back surface powers, since that would not be a great approximation. The different texts I've been studying have introduced the idea of the back vertex power of a lens (specifically with thick lenses), and have stressed its importance in ophthalmic optics. So then, would the above Rx. be referring to the back vertex power? I'm completely lost as to where the back vertex power sits in the optical world, and what its role is.

    I feel like I'm missing something glaringly obvious; if anyone could shed some light on any of this, I would be filled with inexplicable joy .

    Kind Regards,
    Daniel
    The powers you cite above are examples of back vertex power. The back vertex power is the power measured in a lensmeter through the back of the lens, easy way to remember. The nominal power i just that, nominal is defined as in name only, so your examples about of front curve power + back curve power without accounting for thickness would be the nominal power.
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  3. #3
    OptiBoard Apprentice
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    Suberb, thanks very much for the clarification, it makes more sense in the context of a lensmeter.

    Merry Christmas .

  4. #4
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    Quote Originally Posted by TheRobotious View Post
    Suberb, thanks very much for the clarification, it makes more sense in the context of a lensmeter.

    Merry Christmas .
    Robotious contact me directly I also work for the same group but over the ditch in NZ, I am more than happy to help you on your study quest. Email as below mate.

    edit: removed email.
    Last edited by Bok; 12-20-2014 at 07:11 PM.

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