Flat focus: Difference between revisions - VR & AR Wiki - Virtual Reality & Augmented Reality Wiki

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== 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, A., Georgiou, A., & Kollin, J. S. (2017). Holographic near-eye displays for virtual and augmented reality. ''ACM Transactions on Graphics (TOG) - Proceedings of ACM SIGGRAPH 2017, 36''(4), Article 85, ~~1–16~~. doi:10.1145/3072959.3073624 (Discusses various HMD optics including pancake lenses)</ref>.

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 (TOG) - Proceedings of ACM SIGGRAPH 2017, 36''(4), Article 85, 1-16. doi:10.1145/3072959.3073624 (Discusses various HMD optics including pancake lenses)</ref>.

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~~. doi:10.1167/8.3.33</ref>. This conflict arises because the cues the brain receives for depth perception become inconsistent:

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. doi: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.

* '''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:

* [[Visual fatigue]] and [[eye strain]]<ref name="VACReview"/>.

* [[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~~. doi:10.1167/11.8.11</ref>.

* [[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. doi:10.1167/11.8.11</ref>.

* Difficulty focusing on real-world objects after prolonged VR/AR use.

* Inaccurate perception of [[depth]] and scale<ref name="Kramida2016"/>.

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**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. doi: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, T., Ito, Y., & Kawahito, S. (2017). Multi-focal augmented reality display using time-multiplexed focal planes. ''Optics Express, 25''(23), 28633-28645. doi: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, D., & Luebke, D. (2013). Near-eye light field displays. ''ACM Transactions on Graphics (TOG) - Proceedings of ACM SIGGRAPH Asia 2013, 32''(6), Article 220, ~~1–10~~. doi:10.1145/2508363.2508364</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, D., & Luebke, D. (2013). Near-eye light field displays. ''ACM Transactions on Graphics (TOG) - Proceedings of ACM SIGGRAPH Asia 2013, 32''(6), Article 220, 1-10. doi:10.1145/2508363.2508364</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"/>.