In land-vertebrate eyes, flexible lens focuses {accommodation, vision} image by changing surface curvature using eye ciliary muscles. In fish, an inflexible lens moves backwards and forwards, as in cameras. Vision can focus image on fovea, by making thinnest contour line and highest image-edge gradient [Macphail, 1999].
process
To accommodate, lens muscles start relaxed, with no accommodation. Brain tightens lens muscles and stops at highest spatial-frequency response.
distance
Far objects require no eye focusing. Objects within four feet require eye focusing to reduce blur. Brain can judge distance by muscle tension, so one eye can measure distance. See Figure 1.
Pinhole camera can focus scene, but eye is not pinhole camera. See Figure 2.
far focus
If accommodation is for point beyond object, magnification is too low, edges are blurry, and spatial-frequency response is lower, because scene-point light rays land on different retina locations, before they meet at focal point. Focal point is past retina.
near focus
If accommodation is for point nearer than object, magnification is too high, edges are blurry, and spatial-frequency response is lower, because scene-point light rays meet at focal point and then land on different retina locations. Focal point is in eye middle.
Right and left retinas see different images {retinal disparity} {binocular disparity}| [Dacey et al., 2003] [DeVries and Baylor, 1997] [Kaplan, 1991] [Leventhal, 1991] [MacNeil and Masland, 1998] [Masland, 2001] [Polyak, 1941] [Ramón y Cajal, 1991] [Rodieck et al., 1985] [Rodieck, 1998] [Zrenner, 1983].
correlation
Brain can correlate retinal images to pair scene retinal points and then find distances and angles.
fixation
Assume eye fixates on a point straight-ahead. Light ray from scene point forms horizontal azimuthal angle and vertical elevation angle with straight-ahead direction. With no eye convergence, eye azimuthal and elevation angles from scene point differ {absolute disparity}. Different scene points have different absolute disparities {relative disparity}.
When both eyes fixate on same scene point, eye convergence places scene point on both eye foveas at corresponding retinal points, azimuthal and elevation angles are the same, and absolute disparity is zero. See Figure 1. After scene-point fixation, azimuth and elevation angles differ for all other scene points. Brain uses scene-point absolute-disparity differences to find relative disparities to estimate relative depth.
horopter
Points from horopter land on both retinas with same azimuthal and elevation angles and same absolute disparities. These scene points have no relative disparity and so have single vision. Points not close to horopter have different absolute disparities, have relative disparity, and so have double vision. See Figure 2.
location
With eye fixation on far point between eyes and with eye convergence, if scene point is straight-ahead, between eyes, and nearer than fixation distance, point lands outside fovea, for both eyes. See Figure 3. For object closer than fixation plane, focal point is after retina {crossed disparity}.
With eye fixation on close point between eyes and eye convergence, if scene point is straight-ahead, between eyes, and farther than fixation distance, point lands inside fovea, for both eyes. For object farther than fixation plane, focal point is before retina {uncrossed disparity}.
Two eyes can measure relative distance to point by retinal disparity. See Figure 4.
motion
Retinal disparity and motion change are equivalent perceptual problems, so finding distance from retinal disparity and finding lengths and shape from motion changes use similar techniques.
Eye focuses at a distance, through which passes a vertical plane {fixation plane} {plane of fixation}, perpendicular to sightline. From that plane's points, eye convergence can make right and left eye images almost correspond, with almost no disparity. From points in a circle {Vieth-Müller circle} in that plane, eye convergence can make right and left eye images have zero disparity.
After eye fixation on scene point and eye convergence, an imaginary sphere {horopter} passes through both eye lenses and fixation point. Points from horopter land on both retinas with same azimuthal and elevation angles and same absolute disparities. These scene points have no relative disparity and so have single vision.
Brain fuses scene features that are inside distance from horopter {Panum's fusion area} {Panum fusion area} {Panum's fusional area}, into one feature. Brain does not fuse scene features outside Panum's fusional area, but features still register in both eyes, so feature appears double.
1-Consciousness-Sense-Vision-Physiology
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Date Modified: 2022.0225