autofocus-20apr10-150dpi-med (1)

autofocus-20apr10-150dpi-med (1) - Autofocus (AF) CS 178,...

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Autofocus (AF) Marc Levoy Computer Science Department Stanford University CS 178, Spring 2010
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! Marc Levoy Outline ! viewfnders and manual Focusing ! view cameras and tilt-shiFt lenses ! active autoFocusing time-oF-±ight triangulation ! passive autoFocusing phase detection contrast detection ! autoFocus modes ! lens actuators 2
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! Marc Levoy View camera with focusing screen ! large format: 4 ! 5” or 8 ! 10” Flm or scanned digital ! ground glass focusing screen dim hard to focus inverted image 3 Sinar 4 ! 5 (Adams)
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! Marc Levoy Twin-lens refex with Focusing screen ! medium Format: 2 " ! 2 " ±lm only, no longer manuFactured medium Format still exists, but only in SLRs ! diFFerent perspective view than main lens sees 4 (Adams)
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! Marc Levoy Manual rangefnder ! accurate ! painstaking ! diFFerent perspective view than main lens sees ! triangulation concept widely applicable 5 (Adams) ! Leica M9 (digital Full-Frame)
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! Marc Levoy Single lens refex (SLR) with autoFocus 6 Nikon ±4 (²lm camera)
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! Marc Levoy Single lens refex (SLR) with autoFocus ! with mirror down, some light is shunted to A± ! remainder refected up to Form image on diFFuse Focusing screen ! seen (upright) in view²nder, with same perspective as main sensor ! AE light meter also sees Focusing screen ! mirror rotates to expose main sensor when taking picture 7 view²nder autoFocus (A±) autoexposure (AE) Focusing screen
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! Marc Levoy ! coverage is Fraction oF sensor image covered by the viewfnder, i.e. ! magnifcation is apparent size oF objects in viewfnder relative to unaided eye, i.e. with a 50mm lens on camera ! example: Canon 5D II coverage = 98% magnifcation = 0.71 ! 8 h 1 h 2 h 1 h 2 or sometimes area 1 area 2 ! 2 ! 1 ! 1 2 " tan 1 tan 2 Beware oF crop Factor! ! example #2: Canon 7D coverage = 100% magnifcation = 1.0 ! (but 50mm lens acts like 80mm on 7D; and 7D’s 1.0 ! would be 0.62 ! on 5DII) (see http://www.neocamera.com/Feature_viewfnder_sizes.php) This topic is somewhat arcane, so I’ve recolored it orange. (You’re not responsible for it.) If you really want to read more gory details, I’ve added a URL to the text box below.
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! Marc Levoy Electronic viewfnders ! pros same view as lens without need For re±ex mirror can tone map to show eFFect oF chosen exposure ! cons poor resolution and low dynamic range relative to optical Is the display being tone mapped? Will the shot look like this? 9
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autofocus-20apr10-150dpi-med (1) - Autofocus (AF) CS 178,...

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