Talk:Eyeglass prescription

Comment

Note: the material in this article is derived from The Optics of an Eyeglass Prescription, which is copyright © 1998 Daniel P. B. Smith. The article in the form in which it was contributed to Wikipedia is licensed by Daniel P. B. Smith under the terms of the Wikipedia copyright, i.e. the GFDL. Dpbsmith (talk) 20:37, 4 August 2005 (UTC)

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Wow! I love the examples. Bravo! Ventura 16:12, 2004 Jul 29 (UTC)

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Also many thanks from me for this great article! There is still one thing I might have misunderstood, maybe you can help me:

My eyeglass values (given by my optometrist) are:

side	sph	cyl	A
r	-4	+2,75	10
l	-3,5	+2,75	170

If I understand the article right, this would mean the following strengths at the given directions:

side	direction	strength
r	10°	-4 +2,75 = -1,25
r	10°+90°=100°	-4
l	170°	-3,5 +2,75 = -0,75
l	170°-90°=80°	-3,5

If direction 10° is nearly horizontal (a little lower on the left), and 170° as well (a little lower on the right), this would mean that I need a much greater strength vertically than horizontally.

But if I hold my glasses horizontally, objects become much thinner horizontally than vertically. A standing bottle seems to be tallest when I hold the glasses horizontally, with the lens I look through a little higher than the other one.

Could you tell me if I don't understand the article right, or there is an error in it, or the information on my glasses don't match the actual glasses?

Thank you!

- Note: Above comment by User:RMeier.

• The cylinder power is in the meridian 90 degrees to the cylinder axis, so it appears that your eyeglasses were made correctly. Take a look at the illustration in the cylinder to see if that makes sense to you. Edwardian 20:07, 4 August 2005 (UTC)

• P.S. Here is the "clarifying" statement written by the original author of the article: The total power of a lens with a spherical and cylindrical correction changes accordingly: along the axis specified on the prescription it is equal to the value listed under "spherical", and it reaches the sum of "spherical" and "cylindrical" along the axis perpendicular to the one listed on the prescription. Edwardian 20:11, 4 August 2005 (UTC)

• • In response to an email sent to me by RMeier: Edwardian's explanation above is correct. Dpbsmith (talk) 20:36, 4 August 2005 (UTC)

• Thanks to Edwardian and Dpbsmith for the explanations, and sorry for not signing. The statement you cited explains what I see, but the example at the end of the article contradicts it: There you state that the power along the axis stated in the prescription is equal to the sum (2+1=3 at 90°), and the value unter spherical is for the perpendicular axis (and this is what confused me). Could you check this again? Thanks! RMeier 08:01, 5 August 2005 (UTC)

• Thanks again for the clarification. I corrected the example in the article, please check. Btw, what does Superm401 mean with his cleanup-tone? --RMeier 09:49, 8 August 2005 (UTC)

• Thanks for making the correction. I can't speak for Superm401, but he apparently feels that the article does not have the writing style that he thinks encyclopedia articles should have. I don't agree, but I'm a highly interested party so I am not going to take any action myself.

• Probably the right thing to do would be to leave a note on Superm401's talk page and ask him to discuss it here. Or, if you like the article as it stands, you could always be bold and simply remove the notice, preferably with some kind of edit comment like "I think it's OK; please discuss in Talk." Or, if you agree with Superm401, you could edit the article, rewriting the language to suit yourself. I certainly wouldn't object to simple wordsmithing as long as the new content is clear and concise. Or, of course, you could do nothing at all. By the way: welcome to Wikipedia. Dpbsmith (talk) 14:56, 8 August 2005 (UTC)

• I found a likely cause for the critics here (but he should have pointed there, and cleanup-tone would not be the right term either). Maybe Rvollmert is right, I also looked for this information at the places he mentions (among others) before I found it here. But since I'm not too firm in this area and find nothing to complain about the tone either, I'd rather translate this article for the german Wikipedia (where there is nothing comparable) or contribute to others. Thanks for your welcome :) --RMeier 15:50, 8 August 2005 (UTC)

• There is a difference between power "along" an axis, "at" an axis, and "in" an axis. I know that some think this article may be too long, but it may need a good description of meridians (principle meridians, power meridians, axis merdians, etc.), optical crosses, and power crosses earlier on to help clarify things. It certainly will make it longer. I can work on the text, but I'm not familiar with how to link images. Edwardian 07:53, 9 August 2005 (UTC)

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1/20/2011 Following the description in the Axis section, 'in the meridian specified by axis in the prescription, the power is equal to the value listed under "sphere". ', the example in the "Variations in prescription writing" section where Spherical=+2.0, Cylindrical=+1.0, Axis=180° would give a power of +2.00 diopters at the 180° meridian. However, the example went on to say "Both of them specify a power of +2.00 diopters at the 90th (vertical) meridian and +3.00 diopters at the 180th (horizontal) meridian." So I am a little confused which is correct. Anyone can help?

Thanks, Kenng5 Kenng5 (talk) 23:37, 20 January 2011 (UTC)

This article is still fucked when it comes to cylindrical

Added 1/24/11

As the previous commenter notes, and I here expound, this article is ambivalent about how to understand the "axis" part of your prescription. In fact there are at least three separate sentences giving distinct interpretations. Here are the three quotations:

(1) In the illustration below [...] the axis is 20° if written in minus notation or 110° (at 90 degrees to the negative axis) if written in plus notation.

(2) in the meridian specified by axis in the prescription, the power is equal to the value listed under "sphere" [...] the meridian 90° from the meridian specified by axis, where the power is equal to the sum of sphere and cylinder

(3) +2.00 +1.00 x180° vs. +3.00 −1.00 x90°: Both of them specify a power of +2.00 diopters at the 90th (vertical) meridian and +3.00 diopters at the 180th (horizontal) meridian.

As you can see, (1) would have us believe that the "axis" column of our prescription can specify either the cylindar axis or the power axis, depending on whether the plus notation or the minus notation is used. Then (2) implies that the "axis" columns *always* specifies the cylindar axis. Finally, (3) implies that the "axis" column *always* specifies the power axis. These are three of the four possibilities, and only one of them can be right in any given community. So, which is it?

I have done a small amount of research, and the book "The eye exam: a complete guide" says on pages 36--37 that (2) is correct. This book is available on google books. http://books.google.com/books?id=vNwHSXEXYmkC&lpg=PA37&ots=2QhBDGR-Yf&dq=eyeglasses%20cylindrical%20notation&pg=PA37#v=onepage&q=eyeglasses%20cylindrical%20notation&f=false . But I'm not an expert, so I don't wish to update the article.

Abbreviations

This section is a misnomer in that 5 of the 9 items discussed are not abbreviations. This needs to be changed. Edwardian 16:07, 19 August 2005 (UTC)

The "Axis" bullet in this section refers to the "9 o'clock (or east)" direction. Your clocks or maps must look different than mine. I believe this should say "9 o'clock (or west)". Also, it would be helpful to clarify whether this direction is from the point of view of the eye's owner, or of a person facing the owner. DaveBeal 16:02, 13 May 2007 (UTC)

Diopter vs. Visual acuity

Repost of my question on Talk:Visual acuity: Please include some explanation of how diopter measurments fit into this. I'm looking for a rough equivalent of the measurements used in this article and diopter measurements (which will have to be +/- becuase acuity does not determine myopia vs. hyperopia). Maybe a table with some standard acuity values (e.g. 20/20, 20/40, ... 20/400) and their equivalent diopter ranges? For example, it seems to me that 20/30 is equivalent to a +/- 1 diopter prescription. Kslays 16:10, 1 May 2006 (UTC)

Here is a table:

Dioptres	20/something
-0.5	20/25 to 20/30
-1.0	20/30 to 20/50
-3.0	20/300
-4.0	20/400
-5.0	20/600

Dioptres	20/something
+0.5	20/25
+1.0	20/40
+2.0	20/70
+3.0	20/100
+4.0	20/200

I understand this doesn't account for cylindrical (astigmatism), cataracts, or other problems, but a rough guide like this is very useful. -kslays 16:50, 19 June 2007 (UTC)

i realize that i'm about a year late here, but just want to go on the record. first, i feel like the inclusion of such a table would be outside the scope of this article. second, the inclusion of such a chart would be misleading for two reasons. first, snellen charts are not used universally. second, visually acuity and final manifest refraction (what your doctor writes on your prescription) are not necessarily correlative. Coffee joe (talk) 10:06, 13 September 2008 (UTC)

Simple English version?

First, I would like to commend the fact we have an expert who contributed to this topic.

Second, if we were to write a Simple English version of this article, what would we include and not include? I feel that a Simple English article would be helpful here, as some of the optometry is beyond what a Simple English reader might want to read.

Guroadrunner 06:47, 4 June 2006 (UTC)

i don't believe that the concepts being discussed can discard the technical jargon, but if there is terminology that is confusing, i think a more thorough explanation would be appropriate. if anything is still unclear, ask here and i'll work up an explanation for the layperson. Coffee joe (talk) 05:17, 13 June 2008 (UTC)

Manufacturing

What is the manufacturing process in making the lenses?

well, i make lenses so i can answer that, but i believe it's a little out of the scope of this article. also i'm not sure that the topic is 'encyclopedic' in nature. if anyone convinces me otherwise, i'd be happy to create the article.

basically liquid resin is injected into a mold and the product after hardening the resin is a lens that's ready to be cut down to the shape of the frame (a 'finished' lens), or a 'semi-finished' lens. a semi-finished is made thicker so that be back can be cut away to the shape required to make the lens the desired power. it is then put on a series of machines that rub it against laps to create the final back curve(s). one to rough it in and the second to polish it. then it is 'edged' (cut down) to the shape of the frame so it can be mounted.

i wouldn't be sure how to organize that into an article or know what info to include or exclude, and like i said i'm not sure it's appropriate for an encyclopedia.

Coffee joe (talk) 04:54, 13 June 2008 (UTC)

Editing

First, let me say that this page does an excellent job of explaining what are in fact quite complex concepts in an understandable way. I wear glasses, and I didn't know half this stuff. That said, some of the language is a bit too casual (widespread use of second person, etc.) and a good copyedit would clean it up real nice. I will try to do that in the next few days; feedback would be appreciated. 216.193.172.224 00:50, 13 September 2006 (UTC)

I would like to give major kudos to the author(s) of this article. Slight quibbles aside with the editing, I learned a tremendous amount reading this article about complex science of optics and prescriptions, explained in a very clear and approachable way. Don't change too much from the present version. Thank you! Neurodoc 05:50, 30 December 2006 (UTC)

Degrees of freedom

It should be noted, somewhere, that the human visual system actually has more degrees of freedom than an eyeglass prescription represents, which is one reason (although not the only reason) why some people's vision is considered "uncorrectable". In such a case, the vision is not really uncorrectable in principle, but the required lenses would be uneconomical or otherwise impractical to fabricate. 121a0012 (talk) 05:25, 30 April 2008 (UTC)

Can you elaborate on what you mean by "degrees of freedom"?Garvin ^Talk 03:51, 10 May 2008 (UTC)

The same thing as is normally meant (see the article). 121a0012 (talk) 05:06, 10 May 2008 (UTC)

Which other degrees of freedom do you have in mind? Obviously if a person's lens were rough, it would be uncorrectable by conventional glasses, but I don't think that's what you mean. —Ben FrantzDale (talk) 02:52, 24 May 2008 (UTC)

One could, for example, require two cylinder corrections (in different axes, obviously), rather than the one which is normally available. Or the shape of the lens or the retina could be asymmetric in some other way. (Or perhaps they could be perfectly radially symmetric, but about an axis that is not centered.) 121a0012 (talk) 23:01, 24 May 2008 (UTC)

While you could in principle make arbitrary lens geometry involving aspheric lens elements, I believe that the idea of multiple cylindrical corrections in one lens is overly complicated. Assuming the lens will have a smooth surface, then the local curvature at any point is completely defined by a spherical and cylindrical component and an orientation. That is, locally the surface of a lens will look like the surface of an ellipsoid: you can't have a smooth surface with curvatures in more than two directions. (See Curvature#Two dimensions: Curvature of surfaces and Principal curvature.) I may not be explaining this well. Do you have any references that describe what you are talking about in more detail? —Ben FrantzDale (talk) 19:12, 26 May 2008 (UTC)

The only reference I have is the demonstration given to me by my optometrist of why my vision is uncorrectable. (As an aside: You appear to be using the word "smooth" in some technical sense that isn't readily obvious to me.) 121a0012 (talk) 03:14, 27 May 2008 (UTC)

Interesting. I'm sorry to hear that. I won't question your optometrist's professional opinion, but from the optics perspective, I don't quite understand your explanation; I would be fascinated to see a detailed explanation if you can find one. (When I said "smooth", I was sloppy; meant what mathematicians call $C^{2}$ continuous which means that the radius curvature of a surface changes smoothly as you move along it. The result of that is that at every point, such a surface can be described by a curvature in two directions, but not more than two directions. This is the kind of smoothness you'd get if you grab the ends of a stick and bend it; to contrast, a strait line leading into a half circle is only $C^{1}$ continuous—the curvature changes suddenly but the line is "smooth" in that it makes no sudden change in direction.) —Ben FrantzDale (talk) 11:19, 27 May 2008 (UTC)

The optometrist has a handheld device that fits over the bridge of the nose, which is similar to the larger device used for ordinary examinations but with the distinction that the cylinder element freely rotates on a handle the user can adjust. (I don't recall whether the power of the cylinder correction could be so adjusted -- presumably by moving the lens normal to the axis of rotation). It was quite clear after a minute or so that no rotation of the cylinder would bring the entire field of view into focus. The optometrist selects the (cylinder power, axis) pair that bring the center of the field as close to focus as possible, but in my case that leaves most of my FoV out of focus (in that eye, anyway -- the miracle is that the brain can still make use of both eyes to generate depth perception even when one is mostly out of focus. "Degrees of freedom" would be the obvious mechanical analogy for me to express that the dimensionality of my prescription is not sufficient to fully correct my vision. 121a0012 (talk) 02:53, 28 May 2008 (UTC)

astigmatism is only one kind of optical aberration, some others being foil and coma, all of which can be compound. there are some new lenses on the market that are ordered custom directly from the manufacture that can correct for these kinds of aberrations, but only in the center of the lens, creating a 'sweat-spot' in front of the eye. the doctor perscribing these lenses has to have an instrument from the manufacurer that will analize the aberations in your eye so they can have a 'map' of the aberrations to be corrected. it isn't widely available though because not many doctors want to bother with a new piece of equipment with such a narrow use. Coffee joe (talk) 05:09, 13 June 2008 (UTC)

It is the nature of progress that advances are seen as obvious once they arrive. Future technologies only seem difficult when they haven't yet been invented, developed, and refined. In only a few years we will probably have inexpensive machines that can generate high-resolution correction maps for an individual's eyes, and machines that can quickly carve a lens from a blank, following such a correction map. (We already have computers, data storage, and data communications, and knowledge of mechanics, materials handling, and optics; the rest is just R&D.) When they are here, at some point everyone will want one. They will quickly become available in many countries. At that point the current eyeglass and contact lens technologies will be mostly obsolete in those countries. Then we will look back at the days of the great "spherical/cylinder/prism" simplification in wonder; how and why did people accept such a primitive system, which only produces good correction when the wearer looks straight ahead? David spector (talk) 03:23, 30 January 2010 (UTC)

Axis sense?

Which sense is the axis angle? The person or someone facing the person. For example, if I have a 45-degree axis, does that mean it runs lower-left to upper-right of my field of view? —Ben FrantzDale (talk) 17:22, 24 May 2008 (UTC)

I asked an optometrist. The angle, as described by this diagram:

is as seen by the optometrist facing you. So 45 degrees means the cylindrical axis runs from my lower right to my upper left. —Ben FrantzDale (talk) 17:22, 24 May 2008 (UTC)

I also asked an optometrist and this seems to be correct. As seen by the optometrist facing the patient, the axis is measured in degrees counter-clockwise... so from the perspective of the patient the axis increases clockwise. Dstroma (talk) 04:50, 24 July 2008 (UTC)

If you look at Image:Geraet_beim_Optiker.jpg (from Phoropter), it looks like 0° is to the right from the view of the optometrist, or to the left from the perspective of the patient. 72.87.188.108 (talk) 06:49, 7 July 2008 (UTC)

0 is on the left or the right; it makes no difference and is the same as 180. for that reason 0 is typically avoided when writing prescriptions and the range is considered to be 1 - 180. Coffee joe (talk) 09:25, 9 July 2008 (UTC)

Hello, First I would like to say thanks to the authors of this page for explaining the mysteries of the eyeglass prescription. I did, however, find one part confusing related to the axis description. I believe I understand the concepts, but one point seems to disagree with the rest of the article. In the diagram shown, there is a cylinder shown, and its axis lies on the 20 degree meridian. The text states that in minus notation, the axis would be 20, and in positive notation, the axis would be 110. I think this is reversed. Since the cylinder shown is a "positive cylinder", the axis would be 20 in positive notation, and 110 in negative notation. Loudhvx (talk) 17:40, 20 October 2010 (UTC)

Importance of Expiration Date

I believe that, in the US at least, a contact lens prescription expires after about a year. If someone has a cite for this, I think this would be useful to add, along with an explanation. --Mdwyer (talk) 06:08, 22 June 2008 (UTC)

Difference between Glasses, Contacts and IOL

The distance between the retina and the lens would seem to have some impact on the diopter numbers. That is, I've had prescriptions for glasses, contact lenses, and finally an implanted IOL, and their diopter numbers are significantly different, ranging from -9 to -13. How is this handled in a prescription? --Mdwyer (talk) 06:08, 22 June 2008 (UTC)

I don't know. I would imagine that one prescription would describe what lens you need no matter if it is in the eye, on the eye, or near the eye. Making the thin lens approximation, and assuming the corrective lens is near the eye (for some definition of near), the distance to the eye shouldn't matter: the lens just needs to have the power so that it's power in diopters plus your eye's lens power in diopters brings the rays into focus on the retina. That said, I am not an optometrist. —Ben FrantzDale (talk) 06:27, 22 June 2008 (UTC)

well, you're right and your wrong. the prescription that most people are familiar with is their spectacle prescription. this prescription typically assumes a vertex distance of 14mm. that is the distance from the front of your cornea to the back surface of the lens. 14mm is an approximate average for glasses and is the distance most refractionists operate the phoropter from. however if you were your glasses significantly closer or farther from your face the actual power of the lens used may be converted to the correct effective power. in this way that one prescription can "describe" the correction that you need. for contact lenses the vertex distance is obviously 0 and must be converted to have the same effective power. in the case of an i.o.l. the vertex distance is negative and must be further compensate. in the case of i.o.l.s used to replace the eyes original crystallin lens. the power of the original lens itself must be factored into the final power of the lens to be implanted. btw, that article about vertex distance is awful and not entirely correct. i just found that and may have to work it over but i link to it for at least some reference. Coffee joe (talk) 09:59, 9 July 2008 (UTC)

Merge articles on prism correction

There seem to be a few stub articles that deal with different aspects of prism correction. I propose that the following articles should be merged: Prism dioptre, Prentice's rule, and Prentice position. Perhaps there are more that I am missing? I'm thinking that Prism correction would be a good title for a merged article, but editors of this article may have other ideas, or perhaps it would be better to merge the articles here. I'll forward all discussion to this section, so it can be all in one place.--Srleffler (talk) 17:30, 8 March 2009 (UTC)

Who's left/right?

The vieweing direction is specified for the cylinder correction. However, the definition of LEFT and RIGHT for the eyes is not clear. The standard medical definition of Left and Right is from the patients perspective; following this principle, the Left eye is on the same side as my Left hand. If, however, we apply the same principle as for the cylinder axis, my Left eye in the prescription is actually my Right eye because then Left is the left of the optometrist, and not my Left. Is it please possible to clarify this right in the beginning? —Preceding unsigned comment added by 88.193.112.185 (talk) 18:25, 7 June 2010 (UTC)

Эту проблему пытался решить еще Edgar Poe в своем рассказе Golden Bug. —Preceding unsigned comment added by 85.198.140.215 (talk) 20:45, 28 February 2011 (UTC)

Variations in prescription writing

Did this get munged in editing?

It says "Both of them show the same information, namely a power of +2.00 diopters at the 90th (vertical) meridian and +3.00 diopters at the 180th (horizontal) meridian:"

and "The result in both cases is +2.00 diopters at the 150th meridian and +3.00 diopters at the 60th meridian."

Isn't the second statement the correct one, or do they refer to something different?

Archangle0 (talk) 19:57, 10 March 2011 (UTC)

Edits by Hertz1888 resolve my concerns. Archangle0 (talk) 16:23, 18 March 2011 (UTC)