Changes

When a human views the real world, vergence and focus distances typically match, and the human visual system has evolved to expect this.

When a human views the real world, vergence and focus distances typically match, and the human visual system has evolved to expect this.

Fixed-focus biscopic displays have a fixed focus distance, but the vergence distance can change widely, resulting in a mismatch. The human visual system has not evolved to view these types of artificial 3D images comfortably. Vergence-focus conflict can be a very unpleasant sensation for the viewer.<ref>{{Cite journal |title=The zone of comfort: Predicting visual discomfort with stereo displays |url=https://jov.arvojournals.org/article.aspx?articleid=2121032 |journal=Jov ArvoJournals |volume=11 |issue=8}}</ref><ref>{{Cite journal |last1=Wann |first1=John P. |last2=Mon-Williams |first2=Mark |date=May 1997 |title=Health issues with virtual reality displays: what we do know and what we don't |url=https://dl.acm.org/doi/10.1145/271283.271307 |journal=ACM SIGGRAPH Computer Graphics |language=en |volume=31 |issue=2 |pages=53–57 |doi=10.1145/271283.271307 |s2cid=17366319 |issn=0097-8930}}</ref><ref>{{Cite journal |date=March 2008 |title=Vergence–accommodation conflicts hinder visual performance and cause visual fatigue |url=https://jov.arvojournals.org/article.aspx?articleid=2122611 |journal=JOV Arvo Journals |volume=8 |issue=3}}</ref>

Fixed-focus biscopic displays have a fixed focus distance, but the vergence distance can change widely, resulting in a mismatch. The human visual system has not evolved to view these types of artificial 3D images comfortably. Focal conflict can be a very unpleasant sensation for the viewer.<ref>{{Cite journal |title=The zone of comfort: Predicting visual discomfort with stereo displays |url=https://jov.arvojournals.org/article.aspx?articleid=2121032 |journal=Jov ArvoJournals |volume=11 |issue=8}}</ref><ref>{{Cite journal |last1=Wann |first1=John P. |last2=Mon-Williams |first2=Mark |date=May 1997 |title=Health issues with virtual reality displays: what we do know and what we don't |url=https://dl.acm.org/doi/10.1145/271283.271307 |journal=ACM SIGGRAPH Computer Graphics |language=en |volume=31 |issue=2 |pages=53–57 |doi=10.1145/271283.271307 |s2cid=17366319 |issn=0097-8930}}</ref><ref>{{Cite journal |date=March 2008 |title=Vergence–accommodation conflicts hinder visual performance and cause visual fatigue |url=https://jov.arvojournals.org/article.aspx?articleid=2122611 |journal=JOV Arvo Journals |volume=8 |issue=3}}</ref>

Vergence-focus conflict is often encountered when viewing [[stereogram]]s, [[3D movie]]s, or [[virtual reality]] (VR). It can cause visual fatigue and headaches after a short period of time; It is one of the main contributors to [[virtual reality sickness]]. The phenomenon can make it impossible to focus on objects close to the eye in VR, limiting the development of VR software.<ref name=facebook>{{Citation |title=Vergence-Accommodation Conflict: Facebook Research Explains Why Varifocal Matters For Future VR |url=https://www.youtube.com/watch?v=YWA4gVibKJE |language=en |access-date=2022-09-26| others=This sequence of clips is taken from Douglas Lanman's talk 'Quality Screen Time: Leveraging Computational Displays for Spatial Computing' at the 33d annual Electronic Imaging Symposium (26-30 January 2020)}}</ref>

Focal conflict is often encountered when viewing [[stereogram]]s, [[3D movie]]s, or [[virtual reality]] (VR). It can cause visual fatigue and headaches after a short period of time; It is one of the main contributors to [[virtual reality sickness]]. The phenomenon can make it impossible to focus on objects close to the eye in VR, limiting the development of VR software.<ref name=facebook>{{Citation |title=Vergence-Accommodation Conflict: Facebook Research Explains Why Varifocal Matters For Future VR |url=https://www.youtube.com/watch?v=YWA4gVibKJE |language=en |access-date=2022-09-26| others=This sequence of clips is taken from Douglas Lanman's talk 'Quality Screen Time: Leveraging Computational Displays for Spatial Computing' at the 33d annual Electronic Imaging Symposium (26-30 January 2020)}}</ref>

Vergence-focus conflict is very difficult to overcome when designing new types of [[3D display]]s.<ref name=facebook />

Focal conflict is very difficult to overcome when designing new types of [[3D display]]s.<ref name=facebook />

== Effects ==

== Effects ==

Vergence-focus conflict is part of why VR and 3D media causes eye strain and is disorienting.<ref>{{Cite web |title=Is it normal the 3DS effect hurt my eyes? - Nintendo 3DS |url=https://gamefaqs.gamespot.com/boards/997614-nintendo-3ds/72524629 |access-date=2022-09-26 |website=gamefaqs.gamespot.com}}</ref> There is not a strong consensus on the extent of visual damage, if any, that may occur due to overexposure to vergence-focus conflict. Even though this is the case, users of classic stereoscopic devices report being unable to look at the 3D screen for a long period of time.<ref>{{Cite web |title=Is Your Nintendo 3DS Bad for Your Health? |url=https://www.pcmag.com/archive/is-your-nintendo-3ds-bad-for-your-health-262584 |access-date=2022-09-26 |website=PCMAG |language=en}}</ref>

Focal conflict is part of why VR and 3D media causes eye strain and is disorienting.<ref>{{Cite web |title=Is it normal the 3DS effect hurt my eyes? - Nintendo 3DS |url=https://gamefaqs.gamespot.com/boards/997614-nintendo-3ds/72524629 |access-date=2022-09-26 |website=gamefaqs.gamespot.com}}</ref> There is not a strong consensus on the extent of visual damage, if any, that may occur due to overexposure to vergence-focus conflict. Even though this is the case, users of classic stereoscopic devices report being unable to look at the 3D screen for a long period of time.<ref>{{Cite web |title=Is Your Nintendo 3DS Bad for Your Health? |url=https://www.pcmag.com/archive/is-your-nintendo-3ds-bad-for-your-health-262584 |access-date=2022-09-26 |website=PCMAG |language=en}}</ref>

== Measurement ==

== Measurement ==

Vergence-focus conflict can be quantified; typically, by comparing the [[optical power]] required to focus on objects at the vergence distance with the optical power required to focus on objects at the focus distance.<ref name="Shibata 11–11">{{Cite journal |last1=Shibata |first1=Takashi |last2=Kim |first2=Joohwan |last3=Hoffman |first3=David M. |last4=Banks |first4=Martin S. |date=2011-07-05 |title=The zone of comfort: Predicting visual discomfort with stereo displays |url=https://jov.arvojournals.org/article.aspx?articleid=2121032 |journal=Journal of Vision |language=en |volume=11 |issue=8 |pages=11 |doi=10.1167/11.8.11 |pmid=21778252 |pmc=3369815 |issn=1534-7362}}</ref> In this context, optical power is equal to the reciprocal of distance, with units of [[Dioptre|Diopter]] (m⁻¹). Hence the difference between the reciprocal of the vergence distance and the reciprocal of the focus distance characterizes the extent of vergence-focus conflict.

Focal conflict can be quantified; typically, by comparing the [[optical power]] required to focus on objects at the vergence distance with the optical power required to focus on objects at the focus distance.<ref name="Shibata 11–11">{{Cite journal |last1=Shibata |first1=Takashi |last2=Kim |first2=Joohwan |last3=Hoffman |first3=David M. |last4=Banks |first4=Martin S. |date=2011-07-05 |title=The zone of comfort: Predicting visual discomfort with stereo displays |url=https://jov.arvojournals.org/article.aspx?articleid=2121032 |journal=Journal of Vision |language=en |volume=11 |issue=8 |pages=11 |doi=10.1167/11.8.11 |pmid=21778252 |pmc=3369815 |issn=1534-7362}}</ref> In this context, optical power is equal to the reciprocal of distance, with units of [[Dioptre|Diopter]] (m⁻¹). Hence the difference between the reciprocal of the vergence distance and the reciprocal of the focus distance characterizes the extent of vergence-focus conflict.

In the example of a virtual reality head-mounted display, the focus distance corresponds to the distance of the virtual image plane. Often the optics is designed to place a virtual screen somewhere between 2 meters and infinity. That is, for a virtual display at a 2-meter distance, the target focus distance expressed in diopters is 0.5 D. In contrast, the vergence distance in a stereoscopic display can change freely based on the location of target content. For example, a virtual object by means of binocular disparity can be placed at a 30&nbsp;cm distance, corresponding to 3.33 Diopters. In such a case, the magnitude of the vergence-focus conflict for a person with normal vision would be 3.33-0.5=2.83 diopters.

In the example of a virtual reality head-mounted display, the focus distance corresponds to the distance of the virtual image plane. Often the optics is designed to place a virtual screen somewhere between 2 meters and infinity. That is, for a virtual display at a 2-meter distance, the target focus distance expressed in diopters is 0.5 D. In contrast, the vergence distance in a stereoscopic display can change freely based on the location of target content. For example, a virtual object by means of binocular disparity can be placed at a 30&nbsp;cm distance, corresponding to 3.33 Diopters. In such a case, the magnitude of the vergence-focus conflict for a person with normal vision would be 3.33-0.5=2.83 diopters.

== Physiology ==

== Physiology ==

A vergence-focus conflict is caused due to factors in human physiology like the [[accommodation reflex]]. Vergence-focus conflict occurs when the human brain receives mismatching cues between [[vergence]] and [[Accommodation (eye)|accommodation]].<ref name="kramida">{{Cite web |date=2022-09-22 |title=Resolving the Vergence-Accommodation Conflict in Head-Mounted Displays |url=https://3dvar.com/Kramida2016Resolving.pdf |access-date=2022-09-22 |archive-url=https://web.archive.org/web/20220922032435/https://3dvar.com/Kramida2016Resolving.pdf |archive-date=2022-09-22 |url-status=live}}</ref><ref>{{Cite journal |last1=Zhou |first1=Yao |last2=Zhang |first2=Jufan |last3=Fang |first3=Fengzhou |date=2021-12-01 |title=Vergence-accommodation conflict in optical see-through display: review and prospect |journal=Results in Optics |language=en |volume=5 |pages=100160 |doi=10.1016/j.rio.2021.100160 |s2cid=241361232 |issn=2666-9501|doi-access=free }}</ref><ref>{{Cite journal |last=Konrad |first=Robert |date=2015-11-06 |title=What is the vergence-accommodation conflict and how do we fix it? |url=https://doi.org/10.1145/2810048 |journal=XRDS: Crossroads, the ACM Magazine for Students |volume=22 |issue=1 |pages=52–55 |doi=10.1145/2810048 |s2cid=9330080 |issn=1528-4972}}</ref><ref name=anses>{{Cite web |date=2014-11-06 |title=3D technologies and eyesight: use not recommended for children under the age of six, use in moderation for those under the age of 13 |url=https://www.anses.fr/en/content/3d-technologies-and-eyesight-use-not-recommended-children-under-age-six-use-moderation-0 |access-date=2022-09-22 |website=Anses |language=en}}</ref> It often causes headaches and visual fatigue.<ref>{{Cite journal |last1=Hoffman |first1=David M. |last2=Girshick |first2=Ahna R. |last3=Akeley |first3=Kurt |last4=Banks |first4=Martin S. |date=2008-03-01 |title=Vergence–accommodation conflicts hinder visual performance and cause visual fatigue |url=https://jov.arvojournals.org/article.aspx?articleid=2122611 |journal=Journal of Vision |language=en |volume=8 |issue=3 |pages=33.1–30 |doi=10.1167/8.3.33 |issn=1534-7362 |pmc=2879326 |pmid=18484839}}</ref> The vergence-accommodation conflict is one of the main causes of [[virtual reality sickness]].<ref>{{Cite journal |last1=Lawson |first1=Ben D. |last2=Stanney |first2=Kay M. |date=2021 |title=Editorial: Cybersickness in Virtual Reality and Augmented Reality |journal=Frontiers in Virtual Reality |volume=2 |doi=10.3389/frvir.2021.759682 |issn=2673-4192|doi-access=free }}</ref>

A vergence-focus conflict is caused due to factors in human physiology like the [[accommodation reflex]]. Focal conflict occurs when the human brain receives mismatching cues between [[vergence]] and [[Accommodation (eye)|accommodation]].<ref name="kramida">{{Cite web |date=2022-09-22 |title=Resolving the Vergence-Accommodation Conflict in Head-Mounted Displays |url=https://3dvar.com/Kramida2016Resolving.pdf |access-date=2022-09-22 |archive-url=https://web.archive.org/web/20220922032435/https://3dvar.com/Kramida2016Resolving.pdf |archive-date=2022-09-22 |url-status=live}}</ref><ref>{{Cite journal |last1=Zhou |first1=Yao |last2=Zhang |first2=Jufan |last3=Fang |first3=Fengzhou |date=2021-12-01 |title=Vergence-accommodation conflict in optical see-through display: review and prospect |journal=Results in Optics |language=en |volume=5 |pages=100160 |doi=10.1016/j.rio.2021.100160 |s2cid=241361232 |issn=2666-9501|doi-access=free }}</ref><ref>{{Cite journal |last=Konrad |first=Robert |date=2015-11-06 |title=What is the vergence-accommodation conflict and how do we fix it? |url=https://doi.org/10.1145/2810048 |journal=XRDS: Crossroads, the ACM Magazine for Students |volume=22 |issue=1 |pages=52–55 |doi=10.1145/2810048 |s2cid=9330080 |issn=1528-4972}}</ref><ref name=anses>{{Cite web |date=2014-11-06 |title=3D technologies and eyesight: use not recommended for children under the age of six, use in moderation for those under the age of 13 |url=https://www.anses.fr/en/content/3d-technologies-and-eyesight-use-not-recommended-children-under-age-six-use-moderation-0 |access-date=2022-09-22 |website=Anses |language=en}}</ref> It often causes headaches and visual fatigue.<ref>{{Cite journal |last1=Hoffman |first1=David M. |last2=Girshick |first2=Ahna R. |last3=Akeley |first3=Kurt |last4=Banks |first4=Martin S. |date=2008-03-01 |title=Vergence–accommodation conflicts hinder visual performance and cause visual fatigue |url=https://jov.arvojournals.org/article.aspx?articleid=2122611 |journal=Journal of Vision |language=en |volume=8 |issue=3 |pages=33.1–30 |doi=10.1167/8.3.33 |issn=1534-7362 |pmc=2879326 |pmid=18484839}}</ref> The vergence-accommodation conflict is one of the main causes of [[virtual reality sickness]].<ref>{{Cite journal |last1=Lawson |first1=Ben D. |last2=Stanney |first2=Kay M. |date=2021 |title=Editorial: Cybersickness in Virtual Reality and Augmented Reality |journal=Frontiers in Virtual Reality |volume=2 |doi=10.3389/frvir.2021.759682 |issn=2673-4192|doi-access=free }}</ref>

Two ocular responses exist: vergence of eyes, and focus, also known as accommodation. Both of these mechanisms are crucial in stereoscopic vision. Vergence or independent inward/outward rotation of eyes is engaged to fixate on objects and perceive them as single. Incorrect vergence response can cause double vision. Accommodation is the eye’s focusing mechanism and it is engaged to produce a sharp image on a retina. Both of these mechanisms are neurally linked forming the accommodation-convergence reflex<ref>{{Cite web |title=Reflex action {{!}} Definition, Types and Mechanism and Important solved questions |date=11 November 2020 |url=https://www.crackyourtarget.com/reflex-action.html}}</ref> of eyes. One can distinguish vergence {{nowrap|distance{{px2}}{{mdash}}{{px2}}}}a distance of a point towards which both eyes are converging, and an accommodation {{nowrap|distance{{px2}}{{mdash}}{{px2}}}}a distance of a region in space towards which the focus or refractive power of the crystalline lens has been adjusted to produce a sharp image on the retina.

Two ocular responses exist: vergence of eyes, and focus, also known as accommodation. Both of these mechanisms are crucial in stereoscopic vision. Vergence or independent inward/outward rotation of eyes is engaged to fixate on objects and perceive them as single. Incorrect vergence response can cause double vision. Accommodation is the eye’s focusing mechanism and it is engaged to produce a sharp image on a retina. Both of these mechanisms are neurally linked forming the accommodation-convergence reflex<ref>{{Cite web |title=Reflex action {{!}} Definition, Types and Mechanism and Important solved questions |date=11 November 2020 |url=https://www.crackyourtarget.com/reflex-action.html}}</ref> of eyes. One can distinguish vergence {{nowrap|distance{{px2}}{{mdash}}{{px2}}}}a distance of a point towards which both eyes are converging, and an accommodation {{nowrap|distance{{px2}}{{mdash}}{{px2}}}}a distance of a region in space towards which the focus or refractive power of the crystalline lens has been adjusted to produce a sharp image on the retina.