Cameras Cameras Good cameras need focusing, cheap ones dont Good - PDF document

Cameras 1 Cameras 2 Observations about Cameras Observations about Cameras � They record a scene on an image sensor They record a scene on an image sensor Cameras Cameras � Good cameras need focusing, cheap ones don’t Good cameras need focusing, cheap ones don’t � Many cameras have zoom lenses Many cameras have zoom lenses � Some cameras have bigger lenses than others Some cameras have bigger lenses than others � They have ratings like focal length and f They have ratings like focal length and f- -number number Cameras 3 Cameras 4 6 Questions about Cameras 6 Questions about Cameras Question 1 Question 1 � Why does a camera need a lens? Why does a camera need a lens? � Why does a camera need a lens? Why does a camera need a lens? � � Why do most camera lenses need focusing? Why do most camera lenses need focusing? � � Why are lenses telephoto or wide Why are lenses telephoto or wide- -angle? angle? � � Why do fancy lens’s have internal apertures? Why do fancy lens’s have internal apertures? � 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 Light from an Object Light from an Object Converging Lenses Converging Lenses � An illuminated object reflects or scatters light An illuminated object reflects or scatters light � A converging lens bends light rays via refraction A converging lens bends light rays via refraction � You see the object via reflected or scattered light You see the object via reflected or scattered light � Light rays that were spreading now converge Light rays that were spreading now converge � The object’s light produces diffuse illumination The object’s light produces diffuse illumination � 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 � You can’t tell what the object looks like from this � Y You can’t tell what the object looks like from this Y 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 Real Images Real Images Question 2 Question 2 � An image forms in space on far side of the lens An image forms in space on far side of the lens � Why do most camera lenses need focusing? Why do most camera lenses need focusing? � 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 � 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 Cameras 9 Cameras 10 Lenses and Image Sensor Lenses and Image Sensor Focusing Focusing � The sensor records the pattern of light it receives The sensor records the pattern of light it receives � The farther the object, The farther the object, � � If you put the sensor in a real image, it will record If you put the sensor in a real image, it will record � the less diverging its light as that light enters the lens the less diverging its light as that light enters the lens � a pattern of light that looks just like the object a pattern of light that looks just like the object � 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 � For a good photograph, the real image should be For a good photograph, the real image should be F F d h d h h h h h l i l i h h ld b ld b � Different objects form real Different objects form real sharply focused on the image sensor and its size sharply focused on the image sensor and its size images at different distances images at different distances should match should match from the lens. from the lens. the image sensor. the image sensor. � Lens Lens- -to to- -sensor distance must sensor distance must match lens- match lens -to to- -image distance. image distance. Cameras 11 Cameras 12 Lens Diameter and Focusing Lens Diameter and Focusing Question 3 Question 3 � Larger lens gathers more light Larger lens gathers more light � Why are lenses telephoto or wide Why are lenses telephoto or wide- -angle? angle? � so the image is brighter so the image is brighter � but focus is more critical but focus is more critical � and there is less depth of focus. � nd th r i l 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 � � so the image is dimmer so the image is dimmer � but focus is less critical but focus is less critical � and there is more depth of focus. and there is more depth of focus. 2

Cameras 13 Cameras 14 Focal Length Focal Length Wide Angle vs Telephoto Wide Angle vs Telephoto � Focal length measures lens’s converging ability Focal length measures lens’s converging ability � Wide Wide- -angle lens converges rays strongly, angle lens converges rays strongly, � Long focal length: weak lens, long image distance Long focal length: weak lens, long image distance � so they focus close to the lens so they focus close to the lens � Short focal length: strong lens, short image distance Short focal length: strong lens, short image distance � and form a bright, small image near the lens. and form a bright, small image near the lens. � Behavior accurately described by the lens equation: � B h 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 � Small diameter lenses are usually adequate. � Sm ll di m t r l n Small diameter lenses are usually adequate. Sm ll di m t r l n r r ll ll d q d q t t 1 1 1 � Telephoto lens converges rays weakly, Telephoto lens converges rays weakly, + = Object distance Image distance Focal length � Larger image distance, then bigger image Larger image distance, then bigger image � so they focus far from the lens so they focus far from the lens � Long focal length: big dim image Long focal length: big dim image � and form a dim, large image far from lens. and form a dim, large image far from lens. � Short focal length: small bright image Short focal length: small bright image � Large diameter lenses are usually necessary. Large diameter lenses are usually necessary. Cameras 15 Cameras 16 Question 4 Question 4 Aperture or f- Aperture or f -number number � Why do fancy lenses have internal apertures? Why do fancy lenses have internal apertures? � f- -number is focal length over lens diameter number is focal length over lens diameter � and characterizes the brightness of the image. and characterizes the brightness of the image. � A large f A large f- -number lens produces a dim image number lens produces a dim image � with a large depth of field/focus (focus is forgiving). with a large depth of field/focus (focus is forgiving). i h i h l l d d h f fi ld/f h f fi ld/f (f (f i f i f i i i i ) ) � A small f A small f- -number lens produces a bright image number lens produces a bright image � with a small depth of field/focus (focus is critical). with a small depth of field/focus (focus is critical). � Fancy lenses have adjustable f Fancy lenses have adjustable f- -numbers, numbers, � to control both image brightness and depth of focus. to control both image brightness and depth of focus. Cameras 17 Cameras 18 Question 5 Question 5 Lens Flaws Lens Flaws Dispersion → different colors focus differently � Why is a good camera lens so complex inside? Why is a good camera lens so complex inside? � Dispersion different colors focus differently � Use low Use low- -dispersion glass (fluoride glasses) dispersion glass (fluoride glasses) � Use multi Use multi- -piece lenses or “achromats” piece lenses or “achromats” Reflections → fog in photographic images � Reflections fog in photographic images g g p p g p g p g g � Use antireflection coatings Use antireflection coatings Spherical aberration → imperfect focus � Spherical aberration imperfect focus Poor focusing off axis → coma distortions � Poor focusing off axis coma distortions Spherical focus projected on flat film → Astigmatism � Spherical focus projected on flat film Astigmatism � Use aspheric lenses Use aspheric lenses 3