Cameras 1 Cameras

Cameras 2 Introductory Question If you’re building a camera and want to make a larger image (a telephoto lens) you should: If you’re building a camera and want to make a larger image (a telephoto lens) you should: A. increase the diameter of the lens B. decrease the diameter of the lens C. increase the curvature of the lens D. decrease the curvature of the lens

Cameras 3 Observations about Cameras They record a scene on an image sensor They record a scene on an image sensor Good cameras must focus, cheap ones don’t Good cameras must focus, cheap ones don’t They sometimes have zoom lenses They sometimes 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-number They have ratings like focal length and f-number

Cameras 4 Light from an Object An illuminated object reflects or scatters light An illuminated object reflects or scatters light You see object via reflected or scattered light You see object via reflected or scattered light The object’s light forms diffuse illumination The object’s light forms diffuse illumination You can’t tell what object looks like from this diffuse illumination You can’t tell what object looks like from this diffuse illumination

Cameras 5 Converging Lenses A converging lens bends light rays via refraction A converging lens bends light rays via refraction Light rays converge Light rays converge Rays from a common point on an object converge to a common point on far side of the lens Rays from a common point on an object converge to a common point on far side of the lens

Cameras 6 Real Images An image forms in space on far side of the lens An image forms in space on far side of the lens The image is a pattern of light in space that exactly resembles the object, except for size and orientation The image is a pattern of light in space that exactly resembles the object, except for size and orientation The image is “real” – you can put your hand in it The image is “real” – you can put your hand in it

Cameras 7 Lenses and Image Sensor Film records the pattern of light it’s exposed to Film records the pattern of light it’s exposed to If you put film in a real image, it will record a pattern of light resembling the object If you put film in a real image, it will record a pattern of light resembling the object For a good photograph, the real image should be sharply focused on the image sensor and have the right size For a good photograph, the real image should be sharply focused on the image sensor and have the right size

Cameras 8 Focusing Light reaching lens from an object is diverging Light reaching lens from an object is diverging The nearer the object, the more its light diverges The nearer the object, the more its light diverges Converging lens has trouble with diverging light Converging lens has trouble with diverging light Real image of nearby object forms farther from lens Real image of nearby object forms farther from lens Real image of distant object forms closer to lens Real image of distant object forms closer to lens

Cameras 9 Focal Length Focal length measures lens’ converging ability Focal length measures lens’ 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 Larger the object distance, then bigger image Larger the object distance, then bigger image Long focal length: big images Long focal length: big images Short focal length: small images Short focal length: small images

Cameras 10 Lens Diameter Larger lens Larger lens converges more light converges more light brighter image brighter image focus becomes more critical focus becomes more critical less depth of focus less depth of focus Smaller lens Smaller lens dimmer image dimmer image focus becomes less critical focus becomes less critical more depth of focus more depth of focus

Cameras 11 Introductory Question (revisited) If you’re building a camera and want to make a larger image (a telephoto lens) you should: If you’re building a camera and want to make a larger image (a telephoto lens) you should: A. increase the diameter of the lens B. decrease the diameter of the lens C. increase the curvature of the lens D. decrease the curvature of the lens

Cameras 12 Wide Angle vs Telephoto Wide angle lens Wide angle lens Strong to converge rays quickly and focus close Strong to converge rays quickly and focus close Bright, small image forms near lens Bright, small image forms near lens Small diameter lenses are usually adequate Small diameter lenses are usually adequate Telephoto lens Telephoto lens Weak to converge rays slowly and focus far Weak to converge rays slowly and focus far Dim, large image forms far from lens Dim, large image forms far from lens Large diameter lenses are usually necessary Large diameter lenses are usually necessary

Cameras 13 Aperture or f-number Aperture characterizes the diameter of the lens Aperture characterizes the diameter of the lens F-number is focal length over lens diameter F-number is focal length over lens diameter Large f-number Large f-number Dim image Dim image Large depth of field/focus (focus is forgiving) Large depth of field/focus (focus is forgiving) Small f-number Small f-number Bright image Bright image Small depth of field/focus (focus is critical) Small depth of field/focus (focus is critical)

Cameras 14 Lens Flaws Dispersion  different colors focus differently Dispersion  different colors focus differently Use low-dispersion glass (fluoride glasses) Use low-dispersion glass (fluoride glasses) Use multi-piece lenses or “achromats” Use multi-piece lenses or “achromats” Reflections  fog in photographic images Reflections  fog in photographic images Use antireflection coatings Use antireflection coatings

Cameras 15 Eyes Our eyes have lenses and an image sensor Our eyes have lenses and an image sensor Unlike in cameras, eyes focus by changing the focal length of the lens. Unlike in cameras, eyes focus by changing the focal length of the lens.

Cameras 16 Black and White Film Light exposure creates a latent image Light exposure creates a latent image Silver bromide grains absorb photons (a silver salt) Silver bromide grains absorb photons (a silver salt) Photon energy separates salt into silver and bromine Photon energy separates salt into silver and bromine If a 4 atom silver cluster forms  grain will develop If a 4 atom silver cluster forms  grain will develop Gold sensitization lowers threshold to 2 silver atoms Gold sensitization lowers threshold to 2 silver atoms Development turns exposed salt grains to silver Development turns exposed salt grains to silver Silver particle is misshapen and appears black Silver particle is misshapen and appears black Film forms a negative image of exposing object Film forms a negative image of exposing object

Cameras 17 Color Film Sensitizers and filters yield three latent images Sensitizers and filters yield three latent images Sensitizers and filters are built into the film Sensitizers and filters are built into the film Latent images are sandwiched together in the film Latent images are sandwiched together in the film Layers record red, green, and blue light respectively Layers record red, green, and blue light respectively During development, colored dyes are produced During development, colored dyes are produced Spent developer causes dye molecules to form Spent developer causes dye molecules to form Red : cyan, blue : yellow, green : magenta Red : cyan, blue : yellow, green : magenta Dyes form a negative image of exposing object Dyes form a negative image of exposing object

Cameras 18 Digital Cameras Instead of film, use CCD imaging chip Instead of film, use CCD imaging chip Chip is divided into tiny squares or pixels Chip is divided into tiny squares or pixels Photon causes charge transfer in pixel Photon causes charge transfer in pixel After exposure, pixels retain charge image After exposure, pixels retain charge image Charge is shifted out of pixels using MOSFETs Charge is shifted out of pixels using MOSFETs Camera obtains and saves image Camera obtains and saves image