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)

Wow! I love the examples. Bravo! Ventura 16:12, 2004 Jul 29 (UTC) Why doesn't the perscription form have a space for iPD,mPD,bVD. According to the article, these three distances may be critical for progressive lenses.

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:

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

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)

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 flawed 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:

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)

As an engineer, I can handle complexity and jargon; however I appreciate Wikipedia's readable style of presenting complex information to it's mass readership -- and this Article is one of the best that I've encountered. Of course it should be made more accurate, but without being made more obscure -- that is the genius of Wikipedia. HalFonts (talk) 03:38, 24 July 2014 (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 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?
Shouldn't this information be added to the article? I had to come here to find the answer. 3 o'clock is the opposite depending on whether you are the patient or the optometrist.

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:
 * [[Image:Axes_for_eye_glasses_cylindrical_lenses.jpg|150px]]
 * 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&deg; 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) I concur 173.73.100.150 (talk) 17:28, 3 December 2014 (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)

Please add monocular pupillary distance notation left/right clarification. If a Doctor writes PD information, on a prescription, as 31/32, which is the distance from the patient's left eye (from patient's point of view) to the center of the patient's nose? 75.25.120.199 (talk) 17:10, 4 November 2014 (UTC) mush101@hotmail.com

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)

Comment
Loved the article. Well Done. — Preceding unsigned comment added by Maheshsinghjoshi (talk • contribs) 13:53, 15 October 2011 (UTC)

not a doctor and *therefore*
I have removed "Opticians are not eye doctors and, therefore, are not licensed to write an eyeglass prescription." Later in the paragraph optometrists are said to write eyeglass prescription. And they are not doctors. So the removed text cannot be right. Paul Beardsell (talk) 22:33, 18 September 2015 (UTC)

Removing redundant information
{|class="collapsible collapsed" cellpadding="0" cellspacing="3" style="font-size:88%;border: 1px solid #aaa; padding:none; width:100%; margin:none;" ! colspan="10" style="background-color:#ccf; text-align:center; padding:0;" | Removed Section
 * align="left" |
 * align="left" |

Lens power
The values indicated in the sphere and cylinder columns of an eyeglass prescription specify the optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the lens refracts or bends light. A diopter is the reciprocal of the focal length in meters. If a lens has a focal length of $1/3$ meters, it is a 3 diopter lens.

A +10 diopter lens, which has a focal length of 10 centimeters, would make a good magnifying glass. Eyeglass lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus. A typical human eye without refractive error has a refractive power of approximately 60 diopters.

Stacking lenses combines their power by simple addition of diopter strength, if their separation is negligible. A +1 diopter lens combined with a +2 diopter lens forms a +3 diopter system.



Lenses come in positive (plus) and negative (minus) powers. Given that a positive power lens will magnify an object and a negative power lens will make it look smaller, it is often possible to tell whether a lens is positive or negative by looking through it.

Positive lenses cause light rays to converge and negative lenses cause light rays to diverge. A −2 lens combined with a +5 lens forms a +3 diopter system.



A −3 lens stacked on top of a +3 lens looks almost like flat glass, because the combined power is 0.



In science textbooks, positive lenses are usually diagrammed as convex on both sides; negative lenses are usually diagrammed as concave on both sides. In a real optical system, the best optical quality is usually achieved where most rays of light are roughly normal (i.e., at a right angle) to the lens surface. In the case of an eyeglass lens, this means that the lens should be roughly shaped like a cup with the hollow side toward the eye, so most eyeglass lenses are menisci in shape.

The most important characteristic of a lens is its principal focal length, or its inverse which is called the lens strength or lens power. The principal focal length of a lens is determined by the index of refraction of the glass, the radii of curvature of the surfaces, and the medium in which the lens resides. For a thin double convex lens, all parallel rays will be focused to a point referred to as the principal focal point. The distance from the lens to that point is the principal focal length of the lens. For a double concave lens where the rays are diverged, the principal focal length is the distance at which the back-projected rays would come together and it is given a negative sign. For a thick lens made from spherical surfaces, the focal distance will differ for different rays, and this change is called spherical aberration. The focal length for different wavelengths will also differ slightly, and this is called chromatic aberration.

Spherical lenses and spherical correction
Usually:
 * the spherical component is the main correction
 * the cylindrical component is "fine tuning".

Depending on the optical setup, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur.

Whatever the setup, spherical lenses act equally in all meridians: they magnify, introduce blur, or correct blur the same amount in every direction.

An ordinary magnifying glass is a kind of spherical lens. In a simple spherical lens, each surface is a portion of a sphere. When a spherical lens acts as a magnifier, it magnifies equally in all meridians. Here, note that the magnified letters are magnified both in height and in width.



Similarly, when a spherical lens puts an optical system out of focus and introduces blur, it blurs equally in all meridians:



Here is how this kind of blur looks when viewing an eye chart. This kind of blur involves no astigmatism at all; it is equally blurred in all meridians.



Spherical equivalent refraction is normally used to determine soft lens power and spherical glasses power. Some jobs, such in the police or armed forces, may require holders to have eyesight below a maximum spherical equivalent.

Amount of refractive error and degree of blur


The leftmost image here shows a Snellen eye chart as it might be seen by a person who needs no correction, or by a person who is wearing eyeglasses or contacts that properly correct any refractive errors he or she has.

The images labelled 1D, 2D, and 3D give a very rough impression of the degree of blur that might be seen by someone who has one, two, or three diopters of refractive error. For example, a nearsighted person who needs a −2.0 diopter corrective lens will see something like the 2D image when viewing a standard eye chart at the standard 20-foot distance without glasses.

A very rough rule of thumb is that there is a loss of about one line on an eye chart for each additional 0.25 to 0.5 diopters of refractive error.

The top letter on many eye charts represents 20/200 vision. This is the boundary for legal blindness; the US Social Security administration, for example, states that "we consider you to be legally blind if your vision cannot be corrected to better than 20/200 in your better eye." Note that the definition of legal blindness is based on corrected vision (vision when wearing glasses or contacts). It's not at all unusual for people to have uncorrected vision that's worse than 20/200.

Cylindrical lenses and cylindrical correction
Some kinds of magnifying glasses, made specifically for reading wide columns of print, are cylindrical lenses. For a simple cylindrical lens, the surfaces of the lens are portions of a cylinder's surface. Consider how this would refract light. When a cylindrical lens acts as a magnifier, it magnifies only in one direction. For example, the magnifier shown magnifies letters only in height, not in width.



Similarly when a cylindrical lens puts an optical system out of focus and introduces blur, it blurs only in one direction.



This is the kind of blur that results from uncorrected astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing. A cylindrical lens of the right power can correct this kind of blur. When viewing an eye chart, this is how this kind of blur might appear:



Compare it to the kind of blur that is equally blurred in all directions:



When an eye doctor measures an eye—a procedure known as refraction—usually he begins by finding the best spherical correction. If there is astigmatism, the next step is to compensate it by adding the right amount of cylindrical correction.

Axis
Spherical lenses have a single power in all meridians of the lens, such as +1.00 D, or −2.50 D.

Astigmatism, however, causes a directional blur. Below are two examples of the kind of blur you get from astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing.

A cylindrical lens of the right power and orientation can correct this kind of blur. The second example is a little bit more blurred, and needs a stronger cylindrical lens.

But notice that in addition to being smeared more, the second example is smeared out in a different direction.





A spherical lens is the same in all directions; you can turn it around, and it doesn't change the way it magnifies, or the way it blurs:





A cylindrical lens has refractive power in one direction, like a bar reading magnifier. The rotational orientation of that power is indicated in a prescription with an axis notation.





The axis in a prescription describes the orientation of the axis of the cylindrical lens. The direction of the axis is measured in degrees anticlockwise from a horizontal line drawn through the center of a pupil (the axis number can be different for each eye) when viewed from the front side of the glasses (i.e., when viewed from the point of view of the person making the measurement). It varies from 1 to 180 degrees.

In the illustration below, viewed from the point of view of the person making the measurement, the axis is 20° if written in plus notation or 110° if written in minus notation. (20° and 110° being perpendicular to each other.)



The total power of a cylindrical lens varies from zero in the axis meridian to its maximal value in the power meridian, 90° away. in the example above the axis meridian is located in the 20th meridian, and the power meridian is located in the 110th meridian.

The total power of a lens with a spherical and cylindrical correction changes accordingly: in the meridian specified by axis in the prescription, the power is equal to the value listed under "sphere". As you move around the clock face, the power in a given meridian will get steadily closer to the sum of the values given for sphere and cylinder until you reach the meridian 90° from the meridian specified by the axis, where the power is equal to the sum of sphere and cylinder. The above section has been removed. The information is better incorporated into corrective lens, as it is more a discussion of correction and optics rather than the components of an eyeglass prescription. Garvin Talk 19:40, 31 October 2017 (UTC)
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Removed section that would be better incorporated into presbyopia
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Distant vision (DV) and near vision (NV)
The DV portion of the prescription describes the corrections for seeing far away objects. The NV portion is used in prescriptions for bifocals to see very close objects. For most people under forty years of age, the NV or near-vision portion of the prescription is blank because a separate correction for near vision is not needed. In younger people, the lens of the eye is still flexible enough to accommodate over a wide range of distances. With age, the lens hardens and becomes less and less able to accommodate. This is called "presbyopia"; the presby- root means "old" or "elder". (It is the same root as in the words priest and presbyterian.)

The hardening of the lens is a continuous process, not something that suddenly happens abruptly in middle age. Though it is typically by the middle age when the process has progressed to the point where it starts to interfere with reading. Therefore, almost everybody needs glasses for reading from the age of 40–45. Because young children have a wider range of accommodation than adults, they sometimes examine objects by holding them much closer to the eye than an adult would.

This chart (which is approximate) shows that a schoolchild has over ten diopters of accommodation, while a fifty-year-old has only two. This means that a schoolchild is able to focus on an object about 10 cm from the eye, a task for which an adult needs a magnifying glass with a magnification of about 3.5.



The NV correction due to presbyopia can be predicted using the parameter age only. The accuracy of such a prediction is sufficient in many practical cases, especially when the total correction is less than 3 diopters.

When someone accommodates, they also converge their eyes. There is a measurable ratio between how much this effect takes place (AC:A ratio, CA:C ratio). Abnormalities with this can lead to many orthoptic problems. The above section deals almost exclusively with the topic of presbyopia. Garvin Talk 21:47, 31 October 2017 (UTC)
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