my new glasses

Discussion in 'Glasses' started by quiasmox, Sep 20, 2005.

  1. quiasmox

    quiasmox Guest

    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.
     
    quiasmox, Sep 20, 2005
    #1
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  2. quiasmox

    otisbrown Guest

    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
     
    otisbrown, Sep 20, 2005
    #2
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  3. quiasmox

    Guest Guest

    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
     
    Guest, Sep 20, 2005
    #3
  4. quiasmox

    p.clarkii Guest

    "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!
     
    p.clarkii, Sep 20, 2005
    #4
  5. quiasmox

    Dom Guest

    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
     
    Dom, Sep 20, 2005
    #5
  6. quiasmox

    quiasmox Guest

    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.
     
    quiasmox, Sep 21, 2005
    #6
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