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Medical Forum / General / Vision / March 2004Differences between my prescriptions?
I recently had an eye test (at a major chain) to check my suitabiltiy for contact lenses (for occasional sports use), but while there, I was also given a new glasses perscription. I was in no hurry to get new glasses (as I consider my sight ok with the current ones) so just took the contacts. But when I have checked the new prescription against one I got only 6 months ago, I notice it looks quite different. The prescriptios are as follows: Original Eye Test: RIGHT EYE sph: -1.75 cyl: -0.50 axis: 30 LEFT EYE sph: -1.25 cyl: -0.50 axis: 160 Second test, 6 months later: RIGHT EYE sph: -2.50 cyl: +0.50 axis: 120 LEFT EYE sph: -2.25 cyl: +0.50 axis: 70 Soft Contact Lenses Provided: RIGHT pwr: -2.25 bc: 8.6 dia: 14.2 LEFT: pwr: -2.00 bc: 8.6 dia: 14.2 What I notice is the axis value seems completely different on each perscription, and cyl values go from a -'ve value to a +'ve value. Is this normal, or should I get retested? Thanks for your help. >I recently had an eye test (at a major chain) to check my suitabiltiy for >contact lenses (for occasional sports use), but while there, I was also [quoted text clipped - 49 lines] >Checked by AVG anti-virus system (http://www.grisoft.com). >Version: 6.0.638 / Virus Database: 409 - Release Date: 21/03/2004 From the sci.med.vision faq- http://www.faqs.org/faqs/vision-faq/part2/ 2.2 Why the difference between the way Optometrists and Ophthalmologists write the prescription ? ----------------------------------------------------------------- There are two ways of writing the prescription; referred to as "plus-cyl" form or "minus-cyl" form. The plus cyl form, sometimes used by Ophthalmologists is written +1.50 D --------------- +0.50 x 35 The equivalent minus cyl form, generally used by optometrists is written +2.00 D --------------- -0.50 x 125 2.3 How to convert between the two forms ----------------------------------------- To convert from "plus-cyl" form to "minus-cyl" form use the following steps: 1. Add the cylinder power to the sphere 2. Change the sign of the cylinder from + to - 3. Add 90 degrees to the axis if the original axis is less than 90, or 4. Subtract 90 degrees from the axis if the original is greater than 90. To convert from "minus-cyl" to "plus-cyl" form the steps are reversed, i.e 1. Add the cylinder power to the sphere power 2. Change the sign of the cylinder from - to + 3. Add 90 degrees to the axis of the cylinder if axis < 90 4. Subtract 90 degress from the axis if axis > 90 Here are both Rxs in minus cylinder form- Old Rt -1.75 -.50 x 30 New Rt -2.00 -.50 x30 Lt -1.25 -.50 x 160 Lt -1.75 -.50 x 160 Hope this helps Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman Thank you very much, that is an absolutely superb answer. You should do this for a living ;) I apologise for asking a question answered in the FAQ, I did a quick search of the group headers and found no mention of one, so I assumed there was'nt one, oops. Thanks again for clearing that up for me, It has put my mind at ease :) > From the sci.med.vision faq- > [quoted text clipped - 48 lines] > "Science is a way of trying not to fool yourself." > - Richard Feynman >l.pipex.com> > >Thank you very much, that is an absolutely superb answer. You should do >this for a living ;) And thank you for understanding it. I once had a patient who refused to believe that the minus cyl Rx I had written him was the same as the plus cyl one he had had before...even though I showed him the math. He had a fit in my office and demanded his money back because I had written a "wrong" prescription. I always tell people that its the same as saying 2+2=4, or 4-2=2. The equations look different, but describe the same exact relationship. In the prescription, you can add the astigmatism (cylinder value) to a certain sphere value, or you can subtract it. The prescriptions look different, but they describe the same exact lens power. >Thank you very much, that is an absolutely superb answer. You should do >this for a living ;) [quoted text clipped - 4 lines] > >Thanks again for clearing that up for me, It has put my mind at ease :) No reason to apologize, all I did was cut and paste from the FAQ. Now you know about transposition, and have two more points of reference pertaining to ophthalmic optics (the FAQ and sci.med.vision). Enjoy, Robert Dear Robert, I als will thank you for the conversion between these two methods of writing an astigmatism prescription. I also liked your quote: > "Science is a way of trying not to fool yourself." > - Richard Feynman Since I have the same point-of-view as Richard Feynman. Best, Otis Engineer > >I recently had an eye test (at a major chain) to check my suitabiltiy for > >contact lenses (for occasional sports use), but while there, I was also [quoted text clipped - 102 lines] > "Science is a way of trying not to fool yourself." > - Richard Feynman otisbrown@pa.net (Otis Brown) wrote in news:6dbddb9.0403252054.14395767 @posting.google.com: >> "Science is a way of trying not to fool yourself." >> - Richard Feynman [quoted text clipped - 5 lines] > Otis > Engineer Feynman understood what a control is. Scott > otisbrown@pa.net (Otis Brown) wrote in news:6dbddb9.0403252054.14395767 > @posting.google.com: [quoted text clipped - 12 lines] > > Scott Feynmann understood how to ask the "right" question and get the "right" answer -- as did Galileo. Best, Otis >> otisbrown@pa.net (Otis Brown) wrote in >> news:6dbddb9.0403252054.14395767 @posting.google.com: [quoted text clipped - 20 lines] > > Otis Feynman didn't need to remind everyone that he thought similarly to the world's greatest thinkers. Scott >Dear Robert, > [quoted text clipped - 13 lines] >Otis >Engineer Otis, http://www.physics.brocku.ca/etc/cargo_cult_science.html "The first principle is that you must not fool yourself--and you are the easiest person to fool. So you have to be very careful about that. After you've not fooled yourself, it's easy not to fool other scientists. You just have to be honest in a conventional way after that." Robert Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman |
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