18 Eyeball Optics - PSL302Y: Lecture 18, by Prof. MacKay!...

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Eyeball Optics - Camera model : pupil = aperture -> lens -> retina = flm - Pin-hole pupil: only 1 light ray From every pt in visual feld enters eyeball - Entire depth oF feld perFectly Focused on retina = no blurring (no matter how Far or close an object is) = no pbm Focusing - Dilate pupil: occurs in dim conditions - Requires lens to reFocus divergent beam oF light; +light rays From every pt in visual feld - Must reFocus image on retina For every object you Focus on - ReFocus For objects at diFF depths in visual feld = no perFect Focus For everything Regulate size oF pupil with diFF muscles - Circular/sphincter muscles on pupil edge: -Contract: pupil diameter (in bright sunlight) -Max = pinhole pupil - Radial muscles on iris edge: -Contract: pupil diameter (in dim light) -Lens required For Focusing - Iris : sphincter muscles (around pupil) close pupil aperture diameter + radial muscles (on edges) open pupil - Focusing : - Pinhole pupil: requires no lens = entire depth oF feld perFectly Focused on retina - Normal pupil, near vision: lens Focuses divergent rays From one plane in depth on retina = requires more Focusing power ( thick convergent lens) - Normal pupil, distant vision: less divergence in rays = requires less Focusing power ( thin convergent lens) - Iris muscles (sphincter/circular+radial): adjust diameter oF pupil aperture - Ciliary (circular) muscles connected to lens by suspensory ligaments - Adjust lens Focusing: tight c.m = relaxed lens <-> relaxed c.m. = tight lens - Contracted : lens becomes almost spherical (relaxed) = thick lens (near vision) - Relaxed : pulls on lens and stretches it (contracted) = thin lens (distant vision) ±²³´µ¶ PSL302Y: Lecture 18, by ProF. MacKay Mon., Oct.25, 2010 1 oF 8
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Lens accommodation - Corneal refraction is 43 diopters: strong refraction but not adjustable = Fxed - Does most of the refraction of divergent light rays - Lens refraction adjustable (13-26 diopters = less refraction = Fne focusing) - Ciliary muscles pulls on lens perimeter - Ciliary muscles relaxed -> lens pulled taut (thin) = good for distance vision - Ciliary muscles contracted -> lens relaxed (thickens) = good for near vision - Summary: Relaxed muscles = lens stressed, vs. stressed muscles = lens relaxed Retina - Image is upside-down and backward, but want it to be perfectly focused on retina - Translate into neuronal impulses, but several layers of processing in retina - Retina is built backwards - Multiple layers of neurons: neuronal bodies stacked on top w/ light channels - Photoreceptors are farthest from incoming light (at back of retina, bottom layers) - Light rays must pass thru all retinal layers via light channels btwn columns of cells to get to photoreceptors - Protected by pigmented epithelium : - Prevents photoreceptors from being overexposed - Melanin loads prevent light re±ection (which would destroy focusing) - In fovea (“pit”): all layers pulled away = expose photoreceptors, crammed in density - Visual acuity is highest here: good for
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18 Eyeball Optics - PSL302Y: Lecture 18, by Prof. MacKay!...

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