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<p>In <a title="Optics" href="http://en.wikipedia.org/wiki/Optics">optics</a>, <strong>astigmatism</strong> (from <a title="Greek language" href="http://en.wikipedia.org/wiki/Greek_language">Greek</a>: α- <em>a-</em> "without" + στίγματος <em>stigmatos</em>, gen. of στίγμα <em>stigma</em> "a mark, spot, puncture"<sup class="reference" id="_ref-0"><a title="" href="http://en.wikipedia.org/wiki/Astigmatism#_note-0">[1]</a></sup>) is when an optical system has different <a title="Focus (optics)" href="http://en.wikipedia.org/wiki/Focus_%28optics%29">foci</a> for <a title="Ray (optics)" href="http://en.wikipedia.org/wiki/Ray_%28optics%29">rays</a> that propagate in two perpendicular <a title="Plane (mathematics)" href="http://en.wikipedia.org/wiki/Plane_%28mathematics%29">planes</a>. If an optical system with astigmatism is used to form an image of a <a title="Cross" href="http://en.wikipedia.org/wiki/Cross">cross</a>, the vertical and horizontal lines will be in sharp focus at two different distances.</p>
<p><table id="toc" summary="Contents" class="toc"> <tbody> <tr> <td> <div id="toctitle"> <h2>Contents</h2> <span class="toctoggle">[<a class="internal" id="togglelink" href="javascript:toggleToc()">hide</a>]</span></div> <ul> <li class="toclevel-1"><a href="http://en.wikipedia.org/wiki/Astigmatism#Forms_of_astigmatism"><span class="tocnumber">1</span> <span class="toctext">Forms of astigmatism</span></a> <ul> <li class="toclevel-2"><a href="http://en.wikipedia.org/wiki/Astigmatism#Third-order_astigmatism"><span class="tocnumber">1.1</span> <span class="toctext">Third-order astigmatism</span></a> </li> <li class="toclevel-2"><a href="http://en.wikipedia.org/wiki/Astigmatism#Astigmatism_in_systems_that_are_not_rotationally_symmetric"><span class="tocnumber">1.2</span> <span class="toctext">Astigmatism in systems that are not rotationally symmetric</span></a> <ul> <li class="toclevel-3"><a href="http://en.wikipedia.org/wiki/Astigmatism#Ophthalmic_astigmatism"><span class="tocnumber">1.2.1</span> <span class="toctext">Ophthalmic astigmatism</span></a> </li> <li class="toclevel-3"><a href="http://en.wikipedia.org/wiki/Astigmatism#Astigmatism_due_to_misaligned_or_malformed_lenses_and_mirrors"><span class="tocnumber">1.2.2</span> <span class="toctext">Astigmatism due to misaligned or malformed lenses and mirrors</span></a> </li> <li class="toclevel-3"><a href="http://en.wikipedia.org/wiki/Astigmatism#Deliberate_astigmatism_in_optical_systems"><span class="tocnumber">1.2.3</span> <span class="toctext">Deliberate astigmatism in optical systems</span></a> </li> </ul> </li> </ul> </li> <li class="toclevel-1"><a href="http://en.wikipedia.org/wiki/Astigmatism#References"><span class="tocnumber">2</span> <span class="toctext">References</span></a> </li> <li class="toclevel-1"><a href="http://en.wikipedia.org/wiki/Astigmatism#See_also"><span class="tocnumber">3</span> <span class="toctext">See also</span></a> </li> <li class="toclevel-1"><a href="http://en.wikipedia.org/wiki/Astigmatism#External_links"><span class="tocnumber">4</span> <span class="toctext">External links</span></a> </li> </ul> </td> </tr> </tbody></table><script type="text/javascript">
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</script></p><p><a id="Forms_of_astigmatism" name="Forms_of_astigmatism"></a> </p><h2><span class="editsection">[<a title="Edit section: Forms of astigmatism" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit&section=1">edit</a>]</span> <span class="mw-headline">Forms of astigmatism</span></h2>
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<div class="thumbinner" style="WIDTH: 266px"><a class="image" title="Astigmatism" href="http://en.wikipedia.org/wiki/Image:Astigmatism.svg"><img class="thumbimage" height="136" alt="Astigmatism" width="264" border="0" src="http://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/Astigmatism.svg/264px-Astigmatism.svg.png" width="264" border="0" /></a>
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<div class="magnify" style="FLOAT: right"><a class="internal" title="Enlarge" href="http://en.wikipedia.org/wiki/Image:Astigmatism.svg"><img height="11" alt="" width="15" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" width="15" /></a></div>
Astigmatism</div>
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<p>The second form of astigmatism occurs when the optical system is not symmetric about the optical axis. This may be by design (as in the case of a <a title="Cylinder (geometry)" href="http://en.wikipedia.org/wiki/Cylinder_%28geometry%29">cylindrical</a> lens), or due to manufacturing error in the surfaces of the components or misalignment of the components. In this case, astigmatism is observed even for rays from on-axis object points. This form of astigmatism is extremely important in <a title="Ophthalmology" href="http://en.wikipedia.org/wiki/Ophthalmology">ophthalmology</a>, since the human <a title="Eye" href="http://en.wikipedia.org/wiki/Eye">eye</a> often exhibits this aberration due to imperfections in the shape of the <a title="Cornea" href="http://en.wikipedia.org/wiki/Cornea">cornea</a> or the <a title="Lens (anatomy)" href="http://en.wikipedia.org/wiki/Lens_%28anatomy%29">lens</a>.</p>
<p><a id="Third-order_astigmatism" name="Third-order_astigmatism"></a></p>
<h3><span class="editsection">[<a title="Edit section: Third-order astigmatism" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit&section=2">edit</a>]</span> <span class="mw-headline">Third-order astigmatism</span></h3>
<div class="thumb tright">
<div class="thumbinner" style="WIDTH: 277px"><a class="image" title="Page explaining and illustrating astigmatism" href="http://en.wikipedia.org/wiki/Image:Astigmatism.png"><img class="thumbimage" height="375" alt="Page explaining and illustrating astigmatism" width="275" border="0" src="http://upload.wikimedia.org/wikipedia/en/thumb/d/da/Astigmatism.png/275px-Astigmatism.png" width="275" border="0" /></a>
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<div class="magnify" style="FLOAT: right"><a class="internal" title="Enlarge" href="http://en.wikipedia.org/wiki/Image:Astigmatism.png"><img height="11" alt="" width="15" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" width="15" /></a></div>
Page explaining and illustrating astigmatism<sup class="reference" id="_ref-1"><a title="" href="http://en.wikipedia.org/wiki/Astigmatism#_note-1">[2]</a></sup></div>
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<p>In third-order astigmatism, the sagittal and transverse rays form <a title="Focus (optics)" href="http://en.wikipedia.org/wiki/Focus_%28optics%29">foci</a> at different distances along the optic axis. These foci are called the <em>sagittal focus</em> and the <em>transverse focus</em>, respectively. In the presence of astigmatism, an off-axis point on the object is not sharply imaged by the optical system. Instead, sharp <em>lines</em> are formed at the sagittal and transverse foci. The image at the transverse focus is a short line, oriented in the direction of the <em>sagittal</em> plane; images of circles centered on the optic axis, or lines tangential to such circles, will be sharp in this plane. The image at the sagittal focus is a short line, oriented in the <em>tangential</em> direction; images of spokes radiating from the center are sharp at this focus. In between these two foci, a round but "blurry" image is formed. This is called the <em>medial focus</em> or <em>circle of least confusion</em>. This plane often represents the best compromise image location in a system with astigmatism.</p>
<p>The amount of aberration due to astigmatism is proportional to the <a title="Square (algebra)" href="http://en.wikipedia.org/wiki/Square_%28algebra%29">square</a> of the angle between the rays from the object and the optical axis of the system. With care, an optical system can be designed to reduce or eliminate astigmatism. Such systems are called <a title="Anastigmat" href="http://en.wikipedia.org/wiki/Anastigmat">anastigmats</a>.</p>
<div class="boilerplate metadata plainlinks" id="stub"><table style="BACKGROUND-COLOR: transparent" cellspacing="0" cellpadding="0"> <tbody> <tr> <td><a class="image" title="Wiki letter w.svg" href="http://en.wikipedia.org/wiki/Image:Wiki_letter_w.svg"><img height="17" alt="" src="http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Wiki_letter_w.svg/17px-Wiki_letter_w.svg.png" width="17" border="0" /></a></tddiv> <td><emp> <sup>This short section requires <a class="external text" title="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit" rel="nofollow">expansion</a>.</sup></em></td> </tr> </tbody></table></div><p><a id="Astigmatism_in_systems_that_are_not_rotationally_symmetric" name="Astigmatism_in_systems_that_are_not_rotationally_symmetric"></a></p><h3><span class="editsection">[<a title="Edit section: Astigmatism in systems that are not rotationally symmetric" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit&section=3">edit</a>]</span> <span class="mw-headline">Astigmatism in systems that are not rotationally symmetric</span></h3>
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<div class="thumbinner" style="WIDTH: 177px"><a class="image" title="Blur from astigmatic lens at different distances." href="http://en.wikipedia.org/wiki/Image:Astigmatism_text_blur.png"><img class="thumbimage" height="131" alt="Blur from astigmatic lens at different distances." width="175" border="0" src="http://upload.wikimedia.org/wikipedia/commons/thumb/4/42/Astigmatism_text_blur.png/175px-Astigmatism_text_blur.png" width="175" border="0" /></a>
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<div class="magnify" style="FLOAT: right"><a class="internal" title="Enlarge" href="http://en.wikipedia.org/wiki/Image:Astigmatism_text_blur.png"><img height="11" alt="" width="15" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" width="15" /></a></div>
Blur from astigmatic lens at different distances.</div>
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<p>If an optical system is not axisymmetric, either due to an error in the shape of the optical surfaces or due to misalignment of the components, astigmatism can occur even for on-axis object points. This effect is often used deliberately in complex optical systems, especially certain types of <a title="Telescope" href="http://en.wikipedia.org/wiki/Telescope">telescope</a>.</p>
<p>In the analysis of these systems, it is common to consider tangential rays (as defined above), and rays in a meridional plane (a plane containing the optic axis) perpendicular to the tangential plane. This plane is called either the <em>sagittal meridional plane</em> or, confusingly, just the <em>sagittal plane</em>.</p>
<p><a id="Ophthalmic_astigmatism" name="Ophthalmic_astigmatism"></a> </p><h4><span class="editsection">[<a title="Edit section: Ophthalmic astigmatism" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit&section=4">edit</a>]</span> <span class="mw-headline">Ophthalmic astigmatism</span></h4>
<dl><dd>
<div class="noprint relarticle mainarticle"><em>Main article: <a title="Astigmatism (eye)" href="http://en.wikipedia.org/wiki/Astigmatism_%28eye%29">Astigmatism (eye)</a></em></div>
<p>In <a title="Ophthalmology" href="http://en.wikipedia.org/wiki/Ophthalmology">ophthalmology</a>, the vertical and horizontal planes are identified as <em>tangential</em> and <em>sagittal</em> meridians, respectively. Ophthalmic astigmatism is a <a title="Refraction error" href="http://en.wikipedia.org/wiki/Refraction_error">refraction error</a> of the <a title="Eye" href="http://en.wikipedia.org/wiki/Eye">eye</a> in which there is a difference in degree of <a title="Refraction" href="http://en.wikipedia.org/wiki/Refraction">refraction</a> in different meridians. It is typically characterized by an aspherical, non-figure of revolution <a title="Cornea" href="http://en.wikipedia.org/wiki/Cornea">cornea</a> in which the corneal profile <a title="Slope" href="http://en.wikipedia.org/wiki/Slope">slope</a> and refractive power in one meridian is greater than that of the perpendicular axis.</p>
<p>Astigmatism causes difficulties in seeing fine detail. In some cases vertical lines and objects such as walls may appear to the patient to be leaning over like the <a title="Leaning Tower of Pisa" href="http://en.wikipedia.org/wiki/Leaning_Tower_of_Pisa">Tower of Pisa</a>. Astigmatism can be often corrected by <a title="Glasses" href="http://en.wikipedia.org/wiki/Glasses">glasses</a> with a <a title="Lens (optics)" href="http://en.wikipedia.org/wiki/Lens_%28optics%29">lens</a> that has different <a title="Radius of curvature (optics)" href="http://en.wikipedia.org/wiki/Radius_of_curvature_%28optics%29">radii of curvature</a> in different planes (a <em>cylindrical</em> lens), <a title="Contact lens" href="http://en.wikipedia.org/wiki/Contact_lens">contact lenses</a>, or <a title="Refractive surgery" href="http://en.wikipedia.org/wiki/Refractive_surgery">refractive surgery</a>.</p>
<p>Astigmatism is quite common. Studies have shown that about one in three people suffers from it <a class="external autonumber" title="http://archopht.ama-assn.org/cgi/content/abstract/121/8/1141" rel="nofollow" href="http://archopht.ama-assn.org/cgi/content/abstract/121/8/1141" rel="nofollow">[1]</a><a class="external autonumber" title="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16059562&query_hl=7" rel="nofollow" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16059562&query_hl=7" rel="nofollow">[2]</a><a class="external autonumber" title="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15177965&query_hl=20" rel="nofollow" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15177965&query_hl=20" rel="nofollow">[3]</a>. The prevalence of astigmatism increases with age <a class="external autonumber" title="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15838729&query_hl=26" rel="nofollow" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15838729&query_hl=26" rel="nofollow">[4]</a>. Although a person may not notice mild astigmatism, higher amounts of astigmatism may cause blurry vision, <a class="extiw" title="wiktionary:squint" href="http://en.wiktionary.org/wiki/squint">squinting</a>, <a title="Asthenopia" href="http://en.wikipedia.org/wiki/Asthenopia">asthenopia</a>, <a title="Fatigue (physical)" href="http://en.wikipedia.org/wiki/Fatigue_%28physical%29">fatigue</a>, or <a title="Headaches" href="http://en.wikipedia.org/wiki/Headaches">headaches</a> <a class="external autonumber" title="http://www.eyetopics.com/articles/45/1/Astigmatism" rel="nofollow" href="http://www.eyetopics.com/articles/45/1/Astigmatism" rel="nofollow">[5]</a> <a class="external autonumber" title="http://www.medicinenet.com/astigmatism/article.htm" rel="nofollow" href="http://www.medicinenet.com/astigmatism/article.htm" rel="nofollow">[6]</a> <a class="external autonumber" title="http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20symptoms.htm" rel="nofollow" href="http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20symptoms.htm" rel="nofollow">[7]</a>.</p><p>There are a number of tests used by <a title="Ophthalmologist" href="http://en.wikipedia.org/wiki/Ophthalmologist">ophthalmologists</a> and <a title="Optometrist" href="http://en.wikipedia.org/wiki/Optometrist">optometrists</a> during <a title="Eye examination" href="http://en.wikipedia.org/wiki/Eye_examination">eye examinations</a> to determine the presence of astigmatism and to quantify the amount and axis of the astigmatism<a class="external autonumber" title="http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20treatment.htm" rel="nofollow" href="http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20treatment.htm" rel="nofollow">[8]</a>. A <a title="Snellen chart" href="http://en.wikipedia.org/wiki/Snellen_chart">Snellen chart</a> or other <a title="Eye chart" href="http://en.wikipedia.org/wiki/Eye_chart">eye chart</a> may initially reveal reduced <a title="Visual acuity" href="http://en.wikipedia.org/wiki/Visual_acuity">visual acuity</a>. A <a title="Keratometer" href="http://en.wikipedia.org/wiki/Keratometer">keratometer</a> may be used to measure the curvature of the steepest and flattest meridians in the cornea's front surface<a class="external autonumber" title="http://www.stlukeseye.com/eyeq/Keratometry.asp" rel="nofollow" href="http://www.stlukeseye.com/eyeq/Keratometry.asp" rel="nofollow">[9]</a>. A <a class="new" title="Corneal topographer" href="http://en.wikipedia.org/w/index.php?title=Corneal_topographer&action=edit">corneal topographer</a> may also be used to obtain a more accurate representation of the cornea's shape <a class="external autonumber" title="http://www.emedicine.com/OPH/topic711.htm" rel="nofollow" href="http://www.emedicine.com/OPH/topic711.htm" rel="nofollow">[10]</a>. An <a title="Autorefractor" href="http://en.wikipedia.org/wiki/Autorefractor">autorefractor</a> or <a title="Retinoscopy" href="http://en.wikipedia.org/wiki/Retinoscopy">retinoscopy</a> may provide an objective estimate of the eye's refractive error and the use of <a class="new" title="Jackson cross cylinder" href="http://en.wikipedia.org/w/index.php?title=Jackson_cross_cylinder&action=edit">Jackson cross cylinders</a> in a <a title="Phoropter" href="http://en.wikipedia.org/wiki/Phoropter">phoropter</a> may be used to subjectively refine those measurements<a class="external autonumber" title="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=13900989&dopt=Abstract" rel="nofollow" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=13900989&dopt=Abstract" rel="nofollow">[11]</a> <a class="external autonumber" title="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3808608&dopt=Abstract" rel="nofollow" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3808608&dopt=Abstract" rel="nofollow">[12]</a> <a class="external autonumber" title="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8320415&dopt=Abstract" rel="nofollow" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8320415&dopt=Abstract" rel="nofollow">[13]</a>. An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the astigmatic axis and power<a class="external autonumber" title="http://www.quantumoptical.com/onlinecourses/nysso/brp/slide1.asp?courses=19" rel="nofollow" href="http://www.quantumoptical.com/onlinecourses/nysso/brp/slide1.asp?courses=19" rel="nofollow">[14]</a><a class="external autonumber" title="http://www.nova.edu/hpd/otm/nbeo/refract1.htm" rel="nofollow" href="http://www.nova.edu/hpd/otm/nbeo/refract1.htm" rel="nofollow">[15]</a>.</p><p><a id="Astigmatism_due_to_misaligned_or_malformed_lenses_and_mirrors" name="Astigmatism_due_to_misaligned_or_malformed_lenses_and_mirrors"></a> </p><h4><span class="editsection">[<a title="Edit section: Astigmatism due to misaligned or malformed lenses and mirrors" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit&section=5">edit</a>]</span> <span class="mw-headline">Astigmatism due to misaligned or malformed lenses and mirrors</span></h4>
<p>Grinding and polishing of precision optical parts, either by hand or machine, typically employs significant downward pressure, which in turn creates significant frictional side pressures during polishing strokes that can combine to locally flex and distort the parts. These distortions generally do not possess figure-of-revolution symmetry and are thus astigmatic, and slowly become permanently polished into the surface if the problems causing the distortion are not corrected. Astigmatic, distorted surfaces potentially introduce serious degradations in optical system performance.</p>
<p>Surface distortion due to grinding or polishing increases with the <a title="Aspect ratio" href="http://en.wikipedia.org/wiki/Aspect_ratio">aspect ratio</a> of the part (diameter to thickness ratio). To a first order, glass strength increases as the <a title="Cube (arithmetic)" href="http://en.wikipedia.org/wiki/Cube_%28arithmetic%29">cube</a> of the thickness. Thick lenses at 4:1 to 6:1 aspect ratios will flex much less than high aspect ratio parts, such as optical windows, which can have aspect ratios of 15:1 or higher. The combination of surface or wavefront error precision requirements and part aspect ratio drives the degree of back support uniformity required, especially during the higher down pressures and side forces during polishing. Optical working typically involves a degree of randomness that helps greatly in preserving figure-of-revolution surfaces, provided the part is not flexing during the grind/polish process.</p>
<p><a id="Deliberate_astigmatism_in_optical_systems" name="Deliberate_astigmatism_in_optical_systems"></a></p>
<h4><span class="editsection">[<a title="Edit section: Deliberate astigmatism in optical systems" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit&section=6">edit</a>]</span> <span class="mw-headline">Deliberate astigmatism in optical systems</span></h4>
<p><a title="Compact disc" href="http://en.wikipedia.org/wiki/Compact_disc">Compact disc</a> players use an astigmatic lens for focusing. When one axis is more in focus than the other, dot-like features on the disc project to oval shapes. The orientation of the oval indicates which axis is more in focus, and thus which direction the lens needs to move. A square arrangement of only four sensors can observe this bias and use it to bring the read lens to best focus, without being fooled by oblong pits or other features on the disc surface.</p>
<p>Some <a title="Telescope" href="http://en.wikipedia.org/wiki/Telescope">telescopes</a> use deliberately astigmatic optics.</p>
<div class="boilerplate metadata plainlinks" id="stub"><table style="BACKGROUND-COLOR: transparent" cellspacing="0" cellpadding="0"> <tbody> <tr> <td><a class="image" title="Wiki letter w.svg" href="http://en.wikipedia.org/wiki/Image:Wiki_letter_w.svg"><img height="17" alt="" src="http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Wiki_letter_w.svg/17px-Wiki_letter_w.svg.png" width="17" border="0" /></a></tddiv> <td><emp> <sup>This short section requires <a class="external text" title="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit" rel="nofollow">expansion</a>.</sup></em></td> </tr> </tbody></table></div><p><a id="References" name="References"></a></p><h2><span class="editsection">[<a title="Edit section: References" href="http://en.wikipedia.org/w/index.php?title=Astigmatism&action=edit&section=7">edit</a>]</span> <span class="mw-headline">References</span></h2>
<ol class="references">
<li id="_note-0"><strong><a title="" href="http://en.wikipedia.org/wiki/Astigmatism#_ref-0">^</a></strong> <a class="external free" title="http://www.etymonline.com/index.php?term=astigmatism" rel="nofollow" href="http://www.etymonline.com/index.php?term=astigmatism" rel="nofollow">http://www.etymonline.com/index.php?term=astigmatism</a> </li>
<li id="_note-1"><strong><a title="" href="http://en.wikipedia.org/wiki/Astigmatism#_ref-1">^</a></strong> Frederic Eugene Wright, <em>The Methods of Petrographic-microscopic Research, Their Relative Accuracy and Range of Application</em>, Carnegie institution of Washington, 1911. </li>
</ol>
<li><cite class="book" id="Reference-Hecht-1987" style="FONT-STYLE: normal">Hecht, Eugene (1987). <em>Optics</em>, 2nd ed., Addison Wesley. <a class="internal" href="http://en.wikipedia.org/w/index.php?title=Special:Booksources&isbn=020111609X">ISBN 0-201-11609-X</a>.</cite><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Optics&rft.aulast=Hecht&rft.aufirst=Eugene&rft.edition=2nd+ed.&rft.pub=Addison+Wesley"> </span> </li>
</ul>
<p><a id="See_also" name="See_also"></a> </p>
<h2><span class="mw-headline">See also</span></h2>
<ul>
<li><a title="Anastigmat" href="http://en.wikipedia.org/wiki/Anastigmat">Anastigmat</a> (lens type) </li>
</ul>
<p><a id="External_links" name="External_links"></a> </p>
<h2><span class="mw-headline">External links</span></h2>
<ul>
<li><a class="external text" title="http://www.hfhut.com/category/eye-care/astigmatism/" rel="nofollow" href="http://www.hfhut.com/category/eye-care/astigmatism/" rel="nofollow">Astigmatism Articles</a> </li>
</ul>
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