1Lecture 13 Ch. 4 Photography –continuedCh. 5 –The Eye Feb. 23, 2010 Exams will be back on Feb. 25Homework 5 is due Feb. 25 Read all of Ch. 5. on The Eye. 2Photography (cont’d)• Polarizing and haze filters• Tripod• More 35 mm slides• Film3Polarizer selects one polarization(rejects the other)Polarization is perpendicular to the plane containing the incident and scattered rays (vertical for low sun)Polarization parallel to the water surface(probably horizontal) Walden pondSunlight is randomly polarized4UV/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 HazeYellow fog lights on cars don’t contain blue that would scatter from the fog.5TripodHolds 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 longer6Film –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 isthe negative image. • 4. A lens system projects the negative image onto a piece of coated paper, which is developed into a positive image. 7Positive and negative. The film(negative)The print8Latent imageDeveloped image9Film 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. 10More color slides11Ch. 5 The Eye1212• Anatomy: The parts of the eye• How it “works”• Night vision• Response time• Eye problems and fixes13Evolution of the eye13Wikipedia1415Comparison of eye to camera The cameraLensDiaphragm (f-stop)Focusing knobFilmThe eyeLens and corneaIrisCiliary muscleRetina16Eye parts that you seeLens changes shape to focusCiliary muscle Suspensory ligamentIris changes its opening to adjust lightthe pupilis the openingSclera the white outer wall17The hyaloid canal in the fetus has an artery, but this regresses during development. 18Focusingmostly by corneaassisted to varying degrees by eyelensciliary muscles puff up to relax the lens for close focusing19IrisWide open at night , f/2 or f/3more aberrationsless depth of fieldClosed down in daytime, f/8fewer aberrationsmore depth of fieldYou can check depth of field:Try it: Close one eye, hold up thumb, stuff behind thumb is out of focus.20Retinahas 108nerve endings to detect imagerods, for night visioncones, for color and detail, 7 millionoptic nerve = 106transmission linesfovea, region of best vision (cones)21Rods and cones• Rhodopsin, a photochemical, responds to lightIt 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 22Retina detailsChoroid, outside layer with blood supplyPhotoreceptors: rods and conesPlexiform layer, insidelayer with nervesPhotopicvision, in bright light, cones are usedScotopic, in low light, rods are usedmore rods per nerve combines signals23Some words• Aqueous humor, front fluid• Vitreous humor, back fluid• Blind spot = optic nerve attachment place2425More about the eyeNight visionResponse timeEye problemshyperopia, myopia, presbyopia, astigmatism26Dark 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.27Persistence 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.28Aberrations of the eyelens• Spherical aberrationCornea is not spherical surface (aspherical)Iris cuts out rays through the edge of the lensIndex of refraction is not uniform• Curvature of fieldretina is curve d to correct for this• Chromatic aberration:Bluest light is absorbed29Eye 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)30Eye problems continued• Myopia, see close objects clearly, onlyfixed by a negative lens• Hyperopia, see things far, onlyfixed by a positive lens• Presbyopia, stiff lens, no accommodationBifocal glasses have near and far focal