Cameras Cameras photographs, you should: photographs, you should: - PDF document

Cameras 1 Cameras 2 Introductory Question Introductory Question If you want to alter a camera’s lens so that a If you want to alter a camera’s lens so that a � distant object appears larger in the distant object appears larger in the Cameras Cameras photographs, you should: photographs, you should: increase the diameter of the lens increase the diameter of the lens A. A. decrease the diameter of the lens decrease the diameter of the lens B. B. increase the curvature of the lens increase the curvature of the lens C. C. decrease the curvature of the lens decrease the curvature of the lens D. D. Cameras 3 Cameras 4 Observations about Cameras Observations about Cameras 6 Questions about Cameras 6 Questions about Cameras � They record a scene on an image sensor They record a scene on an image sensor � Why does a camera need a lens? Why does a camera need a lens? � � Good cameras need focusing, cheap ones don’t Good cameras need focusing, cheap ones don’t � Why do most camera lenses need focusing? Why do most camera lenses need focusing? � � Many cameras have zoom lenses Many cameras have zoom lenses � Why are lenses telephoto or wide Why are lenses telephoto or wide- -angle? angle? � � Some cameras have bigger lenses than others Some cameras have bigger lenses than others � Why do fancy lens’s have internal apertures? Why do fancy lens’s have internal apertures? � They have ratings like focal length and f They have ratings like focal length and f- -number number � Why is a good camera lens so complex inside? Why is a good camera lens so complex inside? � How does the image sensor respond to light? How does the image sensor respond to light? Cameras 5 Cameras 6 Question 1 Question 1 Light from an Object Light from an Object � Why does a camera need a lens? Why does a camera need a lens? � An illuminated object reflects or scatters light An illuminated object reflects or scatters light � � You see the object via reflected or scattered light You see the object via reflected or scattered light � The object’s light produces diffuse illumination The object’s light produces diffuse illumination � You can’t tell what the object looks like from this � Y Y You can’t tell what the object looks like from this n’t t ll n’t t ll h t th h t th bj bj t l t l k lik fr m thi k lik fr m thi diffuse illumination diffuse illumination 1

Cameras 7 Cameras 8 Clicker Question Clicker Question Converging Lenses Converging Lenses � The light rays leaving one point on a object are The light rays leaving one point on a object are � A converging lens bends light rays via refraction A converging lens bends light rays via refraction spreading when they hit a white screen. To bring spreading when they hit a white screen. To bring � Light rays that were spreading now converge Light rays that were spreading now converge them together on the screen, you can use them together on the screen, you can use � Rays from a common point on an object converge Rays from a common point on an object converge to a common point on the far side of the lens to a common point on the far side of the lens to a common point on the far side of the lens to a common point on the far side of the lens A. interference. interference. A. B. dispersion. dispersion. B. C. refraction. refraction. C. D. polarization. polarization. D. Cameras 9 Cameras 10 Real Images Real Images Clicker Question Clicker Question � An image forms in space on far side of the lens An image forms in space on far side of the lens � A magnifying glass held in sunlight forms a A magnifying glass held in sunlight forms a round spot that can burn paper. What spot(s) round spot that can burn paper. What spot(s) � The image is a pattern of light in space that exactly The image is a pattern of light in space that exactly resembles the object, except for size and orientation resembles the object, except for size and orientation will form if you hold that lens in light from the will form if you hold that lens in light from the lamps overhead? lamps overhead? lamps overhead? lamps overhead? � The image is “real” � The image is real The image is “real” – you can put your hand in it The image is real you can put your hand in it you can put your hand in it you can put your hand in it A. A round spot A round spot A. B. A square spot A square spot B. C. Spots that look exactly like the overhead lamps Spots that look exactly like the overhead lamps C. D. Round spots arranged like the overhead lamps Round spots arranged like the overhead lamps D. Cameras 11 Cameras 12 Question 2 Question 2 Lenses and Image Sensor Lenses and Image Sensor � Why do most camera lenses need focusing? Why do most camera lenses need focusing? � The sensor records the pattern of light it receives The sensor records the pattern of light it receives � If you put the sensor in a real image, it will record If you put the sensor in a real image, it will record a pattern of light that looks just like the object a pattern of light that looks just like the object � For a good photograph, the real image should be F For a good photograph, the real image should be F d h d h h h h h l i l i h h ld b ld b sharply focused on the image sensor and its size sharply focused on the image sensor and its size should match should match the image sensor. the image sensor. 2

Cameras 13 Cameras 14 Clicker Question Clicker Question Focusing Focusing � If you move an object closer to your camera, If you move an object closer to your camera, � The farther the object, The farther the object, you’ll have to refocus by moving the camera’s you’ll have to refocus by moving the camera’s � the less diverging its light as that light enters the lens the less diverging its light as that light enters the lens lens. The distance from lens to sensor must lens. The distance from lens to sensor must � the more converging that light after it leaves the lens the more converging that light after it leaves the lens � nd th n � and the nearer to the lens the real image forms. and the nearer to the lens the real image forms. nd th n r r t th l n th r r r t th l n th r l im l im f rm f rm � Different objects form real Different objects form real A. decrease so the light has less room to converge. decrease so the light has less room to converge. A. images at different distances images at different distances B. increase so the light has more room to converge. increase so the light has more room to converge. B. from the lens. from the lens. � Lens Lens- -to to- -sensor distance must sensor distance must match lens match lens- -to to- -image distance. image distance. Cameras 15 Cameras 16 Lens Diameter and Focusing Lens Diameter and Focusing Clicker Question Clicker Question � Larger lens gathers more light Larger lens gathers more light � A disposable film camera has no focus A disposable film camera has no focus adjustment. To produce sharp photographs that adjustment. To produce sharp photographs that � so the image is brighter so the image is brighter are properly exposed, it needs a are properly exposed, it needs a � but focus is more critical but focus is more critical � nd th r i l � and there is less depth of focus. and there is less depth of focus. nd th r i l d pth f f d pth f f � Smaller lens gathers less light Smaller lens gathers less light A. large diameter lens and low sensitivity film. large diameter lens and low sensitivity film. � A. � so the image is dimmer so the image is dimmer B. small diameter lens and low sensitivity film. small diameter lens and low sensitivity film. B. � but focus is less critical but focus is less critical C. large diameter lens and high sensitivity film. large diameter lens and high sensitivity film. C. � and there is more depth of focus. and there is more depth of focus. D. small diameter lens and high sensitivity film. small diameter lens and high sensitivity film. D. Cameras 17 Cameras 18 Question 3 Question 3 Focal Length Focal Length � Why are lenses telephoto or wide Why are lenses telephoto or wide- -angle? angle? � Focal length measures lens’s converging ability Focal length measures lens’s converging ability � Long focal length: weak lens, long image distance Long focal length: weak lens, long image distance � Short focal length: strong lens, short image distance Short focal length: strong lens, short image distance � B h � Behavior accurately described by the lens equation: Behavior accurately described by the lens equation: B h i r i r r t l d r t l d rib d b th l n rib d b th l n q q ti n ti n 1 1 1 + = Object distance Image distance Focal length � Larger image distance, then bigger image Larger image distance, then bigger image � Long focal length: big dim image Long focal length: big dim image � Short focal length: small bright image Short focal length: small bright image 3