my new glasses

I am myopic. For a couple of years, I've noticed that my present
glasses don't work very well; specifically, to focus on anything from
the tv set to infinity, I have to hold my glasses two to four inches
away from my face. I recently had my eyes examined, and got a
prescription for glasses that was a couple of diopters less in absolute
value than my previous prescription (the one my present glasses are
based on). For example, my current glasses have -3.25 on one eye, and
the new script says -2.75.
My question- since I'm myopic, I should have a diverging lens
(according to what I've read on the internet). Wouldn't moving my
glasses away from my face increase the total divergence, and wouldn't
that indicate that I need a stronger, not a weaker lens? Or am I
thinking about something backwards?
I'm very reluctant to spend a couple of hundred dollars on glasses that
might be worse than what I have, so I'd like to be sure.
Thanks for any insights.
--
john

Dear John,

I am myopic. For a couple of years, I've noticed that my present
glasses don't work very well; specifically, to focus on anything from
the tv set to infinity, I have to hold my glasses two to four inches
away from my face.

Otis> That is strong evidence
that you were "over-prescribed"
by one or two diotpers.
The effect of moving a minus
lens AWAY from the face,
is to reduce the "effective power"
of the minus lens.

Otis> Conversly, pushing
a minus lens CLOSER to the
face will sharpen vision with
and "under-prescribed" minus.
Tilting the lens at an angle
will also increase the "apparent
powere" slightly.

Best,

Otis

On 9/19/05 7:40 PM, in article
(E-Mail Removed). com, "(E-Mail Removed)"
<(E-Mail Removed)> wrote:

> I am myopic. For a couple of years, I've noticed that my present
> glasses don't work very well; specifically, to focus on anything from
> the tv set to infinity, I have to hold my glasses two to four inches
> away from my face. I recently had my eyes examined, and got a
> prescription for glasses that was a couple of diopters less in absolute
> value than my previous prescription (the one my present glasses are
> based on). For example, my current glasses have -3.25 on one eye, and
> the new script says -2.75.
> My question- since I'm myopic, I should have a diverging lens
> (according to what I've read on the internet). Wouldn't moving my
> glasses away from my face increase the total divergence, and wouldn't
> that indicate that I need a stronger, not a weaker lens? Or am I
> thinking about something backwards?
> I'm very reluctant to spend a couple of hundred dollars on glasses that
> might be worse than what I have, so I'd like to be sure.
> Thanks for any insights.
> --
> john
>
Look at it this way. Your current prescription -3.75D forms a virtual image
of an object of infinity at a distance 0.308 meters in front of the lens. If
you hold the lens 0.075 meters (3 inches) in front of your eyes, that moves
the virtual image a distance 0.383 meters in front of your eyes. To do that
with a lens next to your eye require a 1/-0.383 = -2.613D. As they say, that
is same as -2.75D as far as government work is concerned. Also, my
calculation is an approximation, that does not take into account spacing of
the lens from the eye.

Bill

"Otis> That is strong evidence
that you were "over-prescribed"
by one or two diotpers."

wrong-- that is suggestive evidence that the prescription is now too
strong by -0.25 to -0.50 diopters.

why do you assume that the patient was overprescribed? perhaps he has
had an age-related decrease in his myopia?

oh-- I guess it is the conspiracy theory. the eyedoc overminused him
purposely by 1-2 diopters so he would become "hooked" on the "wretched
minus" and become a lifelong eyeglass addict thus enriching the eyedocs
bank account. of course!

The further the lens from your eye, the less 'curved' the wavefront
coming from the lens is, and therefore the less 'minus' power that
wavefront has.

Dom



> My question- since I'm myopic, I should have a diverging lens
> (according to what I've read on the internet). Wouldn't moving my
> glasses away from my face increase the total divergence, and wouldn't
> that indicate that I need a stronger, not a weaker lens? Or am I
> thinking about something backwards?


> --
> john
>

(E-Mail Removed) wrote:
> On 9/19/05 7:40 PM, in article
> (E-Mail Removed). com, "(E-Mail Removed)"
> <(E-Mail Removed)> wrote:
>
> > I am myopic. For a couple of years, I've noticed that my present
> > glasses don't work very well; specifically, to focus on anything from
> > the tv set to infinity, I have to hold my glasses two to four inches
> > away from my face. I recently had my eyes examined, and got a
> > prescription for glasses that was a couple of diopters less in absolute
> > value than my previous prescription (the one my present glasses are
> > based on). For example, my current glasses have -3.25 on one eye, and
> > the new script says -2.75.
> > My question- since I'm myopic, I should have a diverging lens
> > (according to what I've read on the internet). Wouldn't moving my
> > glasses away from my face increase the total divergence, and wouldn't
> > that indicate that I need a stronger, not a weaker lens? Or am I
> > thinking about something backwards?
> > I'm very reluctant to spend a couple of hundred dollars on glasses that
> > might be worse than what I have, so I'd like to be sure.
> > Thanks for any insights.
> > --
> > john
> >
> Look at it this way. Your current prescription -3.75D forms a virtual image
> of an object of infinity at a distance 0.308 meters in front of the lens. If
> you hold the lens 0.075 meters (3 inches) in front of your eyes, that moves
> the virtual image a distance 0.383 meters in front of your eyes. To do that
> with a lens next to your eye require a 1/-0.383 = -2.613D. As they say, that
> is same as -2.75D as far as government work is concerned. Also, my
> calculation is an approximation, that does not take into account spacing of
> the lens from the eye.

Thanks to you and everyone who replied. Apart from philosophical
differences, everyone seems unanimous that I need weaker glasses, and I
can trust my last prescription.
--
john