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Medical Forum / General / Vision / September 2005my 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 "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! >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. Many myopes find their prescription decreases spontaneously by a half diopter in their twenties or thirties. Your glasses have _less_ effective minus when you push them away from your face. At your last prescription, you were prescribed -325 because you told the doctor that -300 was blurry. Otis calls that "overprescribing." Doctors do not. -MT On 9/19/05 7:40 PM, in article 1127184007.459843.182110@g14g2000cwa.googlegroups.com, "quiasmox@yahoo.com" > 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 [quoted text clipped - 14 lines] > -- > 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 > On 9/19/05 7:40 PM, in article > 1127184007.459843.182110@g14g2000cwa.googlegroups.com, "quiasmox@yahoo.com" [quoted text clipped - 26 lines] > 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 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 |
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