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
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