Flat focus: Difference between revisions
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# To collimate or refocus the light from the display so that it appears to originate from a farther distance than its physical location. | # To collimate or refocus the light from the display so that it appears to originate from a farther distance than its physical location. | ||
The "flat focus" characteristic arises because the optical design targets a *single* virtual distance for optimal sharpness. This fixed focal distance is a design choice, often set somewhere between 1.5 meters (approx. 5 feet) and [[optical infinity]] (practically, distances beyond ~6 meters / 20 feet where accommodation change becomes negligible)<ref name="Kramida2016">Kramida, | The "flat focus" characteristic arises because the optical design targets a *single* virtual distance for optimal sharpness. This fixed focal distance is a design choice, often set somewhere between 1.5 meters (approx. 5 feet) and [[optical infinity]] (practically, distances beyond ~6 meters / 20 feet where accommodation change becomes negligible)<ref name="Kramida2016">Kramida, G. (2016). Resolving the Vergence‑Accommodation Conflict in Head‑Mounted Displays. IEEE Transactions on Visualization and Computer Graphics, 22(7), 1912‑1931. https://doi.org/10.1109/TVCG.2015.2473855</ref>. A common target distance for many consumer VR headsets is around 2 meters<ref name="RoadToVRFocus">Lang, B. (2019, May 21). Oculus Rift S Review – A Good Choice for VR Newcomers, a Difficult Choice for VR Vets. Road to VR. https://www.roadtovr.com/oculus-rift-s-review-good-choice-for-newcomers-difficult-choice-for-vr-vets/</ref>. | ||
Light rays from every pixel on the flat microdisplay pass through the lens system. Ideally, the lenses bend these rays such that they appear to the eye as parallel or slightly diverging bundles, mimicking how light arrives from an object located at the chosen fixed focal distance. The eye's [[crystalline lens]], therefore, only needs to accommodate to *that specific distance* to perceive a sharp image across the entire display, regardless of whether the content shown is a virtual object meant to be centimeters away or kilometers away. | Light rays from every pixel on the flat microdisplay pass through the lens system. Ideally, the lenses bend these rays such that they appear to the eye as parallel or slightly diverging bundles, mimicking how light arrives from an object located at the chosen fixed focal distance. The eye's [[crystalline lens]], therefore, only needs to accommodate to *that specific distance* to perceive a sharp image across the entire display, regardless of whether the content shown is a virtual object meant to be centimeters away or kilometers away. | ||
== Relevance in VR and AR == | == Relevance in VR and AR == | ||
The flat focus design is prevalent in VR/AR primarily due to its relative simplicity, cost-effectiveness, and ability to deliver wide fields of view with manageable [[aberration (optics)|optical aberrations]] using lens technologies like [[aspheric lens|aspheric]] or [[Fresnel lens|Fresnel lenses]], and more recently, [[pancake lens|pancake lenses]] (which achieve a thinner profile but typically still maintain a fixed focus)<ref name="PancakeOptics">Maimone, | The flat focus design is prevalent in VR/AR primarily due to its relative simplicity, cost-effectiveness, and ability to deliver wide fields of view with manageable [[aberration (optics)|optical aberrations]] using lens technologies like [[aspheric lens|aspheric]] or [[Fresnel lens|Fresnel lenses]], and more recently, [[pancake lens|pancake lenses]] (which achieve a thinner profile but typically still maintain a fixed focus)<ref name="PancakeOptics">Maimone, A., Georgiou, A., & Kollin, J. S. (2017). Holographic Near‑Eye Displays for Virtual and Augmented Reality. ACM Transactions on Graphics, 36(4), 85:1‑85:16. https://doi.org/10.1145/3072959.3073624</ref>. | ||
However, the primary consequence of flat focus is the introduction of the [[Vergence-Accommodation Conflict]] (VAC)<ref name="VACReview">Hoffman, | However, the primary consequence of flat focus is the introduction of the [[Vergence-Accommodation Conflict]] (VAC)<ref name="VACReview">Hoffman, D. M., Girshick, A. R., Akeley, K., & Banks, M. S. (2008). Vergence‑Accommodation Conflicts Hinder Visual Performance and Cause Visual Fatigue. Journal of Vision, 8(3):33, 1‑30. https://doi.org/10.1167/8.3.33</ref>. This conflict arises because the cues the brain receives for depth perception become inconsistent: | ||
* '''Vergence Cues:''' Based on stereoscopic rendering ([[binocular disparity]]), the user's eyes converge or diverge naturally to fuse the image of virtual objects presented at different depths. For a nearby virtual object, the eyes converge significantly. | * '''Vergence Cues:''' Based on stereoscopic rendering ([[binocular disparity]]), the user's eyes converge or diverge naturally to fuse the image of virtual objects presented at different depths. For a nearby virtual object, the eyes converge significantly. | ||
* '''Accommodation Cues:''' Regardless of where the eyes are converged, the light is always coming from the fixed focal plane set by the headset optics. Therefore, the eye's accommodation reflex receives cues (primarily from [[retinal blur]]) indicating that the object is always at that fixed distance, prompting the crystalline lens to remain focused there. | * '''Accommodation Cues:''' Regardless of where the eyes are converged, the light is always coming from the fixed focal plane set by the headset optics. Therefore, the eye's accommodation reflex receives cues (primarily from [[retinal blur]]) indicating that the object is always at that fixed distance, prompting the crystalline lens to remain focused there. | ||
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This mismatch between where the eyes are pointing (vergence) and where they are focusing (accommodation) is unnatural. The human visual system is accustomed to these two mechanisms working in tandem. The conflict can lead to several negative effects: | This mismatch between where the eyes are pointing (vergence) and where they are focusing (accommodation) is unnatural. The human visual system is accustomed to these two mechanisms working in tandem. The conflict can lead to several negative effects: | ||
* [[Visual fatigue]] and [[eye strain]]<ref name="VACReview"/>. | * [[Visual fatigue]] and [[eye strain]]<ref name="VACReview"/>. | ||
* [[Headache|Headaches]]<ref name="Shibata2011">Shibata, | * [[Headache|Headaches]]<ref name="Shibata2011">Shibata, T., Kim, J., Hoffman, D. M., & Banks, M. S. (2011). The Zone of Comfort: Predicting Visual Discomfort with Stereo Displays. Journal of Vision, 11(8):11, 1‑29. https://doi.org/10.1167/11.8.11</ref>. | ||
* Difficulty focusing on real-world objects after prolonged VR/AR use. | * Difficulty focusing on real-world objects after prolonged VR/AR use. | ||
* Inaccurate perception of [[depth]] and scale<ref name="Kramida2016"/>. | * Inaccurate perception of [[depth]] and scale<ref name="Kramida2016"/>. | ||
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**Mechanically moving the lenses or displays. | **Mechanically moving the lenses or displays. | ||
**Using [[liquid lens|liquid crystal lenses]] or other electronically tunable optical elements. | **Using [[liquid lens|liquid crystal lenses]] or other electronically tunable optical elements. | ||
**Employing deformable membrane mirrors<ref name="MembraneMirrorVarifocal">Rathnayake, | **Employing deformable membrane mirrors<ref name="MembraneMirrorVarifocal">Rathnayake, A. U., Nguyen, T., & Zhan, T. (2021). Varifocal Near‑Eye Display Using a Focus‑Tunable Alvarez Lens. Optics Express, 29(19), 30935‑30947. https://doi.org/10.1364/OE.436385</ref>. | ||
* '''[[Multifocal display|Multifocal Displays]]:''' These designs present images on multiple distinct focal planes simultaneously or in rapid succession, allowing the eye to focus more naturally on the plane closest to the target object's depth<ref name="MultifocalDisplays">Mercier, | * '''[[Multifocal display|Multifocal Displays]]:''' These designs present images on multiple distinct focal planes simultaneously or in rapid succession, allowing the eye to focus more naturally on the plane closest to the target object's depth<ref name="MultifocalDisplays">Mercier, T., Ito, Y., & Kawahito, S. (2017). Multi‑Focal Augmented Reality Display Using Time‑Multiplexed Focal Planes. Optics Express, 25(23), 28633‑28645. https://doi.org/10.1364/OE.25.028633</ref>. | ||
* '''[[Light field display|Light Field Displays]]:''' These advanced displays aim to replicate the way light rays travel in the real world, providing correct focus cues by presenting slightly different information depending on the viewing angle and position of the pupil. The eye can then potentially focus naturally at different depths within the captured light field<ref name="LightfieldVR">Lanman, | * '''[[Light field display|Light Field Displays]]:''' These advanced displays aim to replicate the way light rays travel in the real world, providing correct focus cues by presenting slightly different information depending on the viewing angle and position of the pupil. The eye can then potentially focus naturally at different depths within the captured light field<ref name="LightfieldVR">Lanman, D., & Luebke, D. (2013). Near‑Eye Light Field Displays. ACM Transactions on Graphics, 32(6), 220:1‑220:10. https://research.nvidia.com/publication/2013-11_near-eye-light-field-displays</ref>. | ||
* '''[[Holographic display|Holographic Displays]]:''' True holographic displays reconstruct the [[wavefront]] of light from the virtual scene, which would inherently contain all necessary focus cues, potentially eliminating the VAC entirely. This remains a significant technical challenge for near-eye displays<ref name="PancakeOptics"/>. | * '''[[Holographic display|Holographic Displays]]:''' True holographic displays reconstruct the [[wavefront]] of light from the virtual scene, which would inherently contain all necessary focus cues, potentially eliminating the VAC entirely. This remains a significant technical challenge for near-eye displays<ref name="PancakeOptics"/>. | ||