Let me take a stab at it...see if I understand it.
The cylinder part of a prescription has an effect on the total
magnification...at the optical center, through the lens, with a prescription
of +2.75 x +0.50, the total magnification would be +3.25. The cylinder
effect tapers off towards the edges.
To achieve the same effect (stronger in the middle, weaker on the edges) but
using a negative cylinder, you would have to start with a stronger spherical
prescription, i.e. +3.25, and then *subtract* the cylinder of -0.5, to end
up with the magnification on the edges being the original +2.75. And, for
it to have the opposite effect, the axis would have to be turned at 90
degrees from the original prescription.
This actually makes sense to me. Not sure if that should concern me, or
not! :-)
-- Chuck Knight
On Mon, Sep 22, 2008 at 1:57 PM, Bjalf <[EMAIL PROTECTED]> wrote:
>
> On Sep 22, 7:18 pm, "Chuck Knight" <[EMAIL PROTECTED]> wrote:
> > Frankly, I'm confused too. I've always been told that the combined total
> of
> > the sphere and the add equals the reading prescription.
>
> He also changed the + cylinder values to - cylinder values. When you
> do that, you apparently have to adjust the sphere values in addition
> to changing the axis values.
>
> Still, the numbers don't really add up. How did this:
> +2.75 +0.50 x 010
> +2.75 +0.25 x 005
>
> turn into this?
> +3.25 -0.50 x 100
> +2.75 -0.25 x 95
>
> >
>
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