Lecture 13 Ch. 4 Photography continued Ch. 5 The Eye Feb. 23, 2010 Exams will be back on Feb. 25 Homework 5 is due Feb. 25 Read all of Ch. 5. on The Eye. 1 Photography (cont d) Polarizing and haze filters Tripod More 35 mm slides Film 2
Polarizer selects one polarization (rejects the other) Sunlight is randomly polarized Polarization is perpendicular to the plane containing the incident and scattered rays (vertical for low sun) Walden pond Polarization parallel to the water surface (probably horizontal) 3 UV/Haze filter Scattered sunlight from tiny particles (including atoms) contains more blue and UV. Hint: That is why the sky is blue, ocean is blue. Haze is sunlight scattered from tiny particles. UV filter does not pass the scattered UV so the scene is less hazy. Haze Haze Haze Haze Yellow fog lights on cars don t contain blue that would scatter from the fog. 4
Tripod Holds the camera steady for long exposures Astronomy, indoors (museums, weddings) Low light, any exposure 1/5 sec or longer Telephoto lenses, 1/100 sec or longer 5 Film how it works 1. Plastic film is coated with emulsion containing a silver compound (silver bromide). The coated side faces the lens. 2. Light breaks chemical bonds with silver. This is the latent image. 3. Chemical developer releases silver into black grains. This is the negative image. 4. A lens system projects the negative image onto a piece of coated paper, which is developed into a positive image. 6
Positive and negative. The print The film (negative) 7 Latent image Developed image 8
Film speed and ASA number Faster film (ASA400) needs less light. Trade off: Faster film is grainier, (bigger grains) meaning less detail in the image. Film speed has a number: ASA 25 is slow with fine grain. Use outside in sun. ASA 100 is medium speed, good for inside shots. ASA 400 is good for low lighting. 9 More color slides 10
Ch. 5 The Eye 11 Anatomy: The parts of the eye How it works Night vision Response time Eye problems and fixes 12
Evolution of the eye Wikipedia 13 14
Comparison of eye to camera The camera Lens Diaphragm (f-stop) Focusing knob Film The eye Lens and cornea Iris Ciliary muscle Retina 15 Eye parts that you see Lens changes shape to focus Ciliary muscle Suspensory ligament Iris changes its opening to adjust light the pupil is the opening Sclera the white outer wall 16
The hyaloid canal in the fetus has an artery, but this regresses during development. 17 Focusing mostly by cornea assisted to varying degrees by eyelens ciliary muscles puff up to relax the lens for close focusing 18
Iris Wide open at night, f/2 or f/3 more aberrations less depth of field Closed down in daytime, f/8 fewer aberrations more depth of field You can check depth of field: Try it: Close one eye, hold up thumb, stuff behind thumb is out of focus. 19 Retina has 10 8 nerve endings to detect image rods, for night vision cones, for color and detail, 7 million optic nerve = 10 6 transmission lines fovea, region of best vision (cones) 20
Rods and cones Rhodopsin, a photochemical, responds to light It is destroyed and reformed. Signal goes to a synapse, a gap between nerve cells There are 3 kinds of cones for 3 colors red, green, blue 21 Retina details Choroid, outside layer with blood supply Photoreceptors: rods and cones Plexiform layer, inside layer with nerves Photopic vision, in bright light, cones are used Scotopic, in low light, rods are used more rods per nerve combines signals 22
Some words Aqueous humor, front fluid Vitreous humor, back fluid Blind spot = optic nerve attachment place 23 24
More about the eye Night vision Response time Eye problems hyperopia, myopia, presbyopia, astigmatism 25 Dark adaptation (night vision) Time to adapt to dark: ~30 minutes Rods: more sensitive to blue (Bl.&Wh. vision) Cones: more sensitive to red (color vision) Night adaptation: shifts from rods to cones Purkinje shift: you see blue better in the dark Red lights at the observatory help preserve night vision. 26
Persistence of vision Images remain on receptors for 1/25 second in low light 1/50 second in bright light Movies do 24 frames per second in a darkened room. TVs do 60 frames per second, ok in lighted room. 27 Aberrations of the eyelens Spherical aberration Cornea is not spherical surface (aspherical) Iris cuts out rays through the edge of the lens Index of refraction is not uniform Curvature of field retina is curved to correct for this Chromatic aberration: Bluest light is absorbed 28
Eye problems Loss of accomodation: ability to focus from 10 inches to infinity Cataracts = cloudy eyelens, replacement lens does not accommodate Floaters = dead cells floating in vitreous humor (seen against a clear sky) 29 Eye problems continued Myopia, see close objects clearly, only fixed by a negative lens Hyperopia, see things far, only fixed by a positive lens Presbyopia, stiff lens, no accommodation Bifocal glasses have near and far focal lengths 30