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	     CS 448 - Camera designs and features, April 13, 2004

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	  *** Components of a camera (besides lens and aperture) ***

Viewfinder:
	o 35mm format (24mm x 36mm)
		- separate viewfinder
			-> Ansel Adams, p. 12-13
		- single-lens reflex (SLR)
			-> mirror and pentaprism, Goldberg, p. 137
	o medium format (2 1/4 x 2 1/4 inch)
		- twin lens reflex
			-> Adams, p. 22-23
		- single lens reflex
			-> Adams, p. 24-25
	o large format (4 x 5 inch, 5 x 7, 8 x 10, 11 x 14)
		- ground glass screen
			-> Adams, p. 30-31, i.e. a view camera
Focusing aids:
	o split wedge
		- two prisms
			-> Goldberg, p. 12
		- gather light from two regions in lens
			-> Goldberg, p. 18
		- If image is well focused, the two images coincide, else not
			-> Goldberg, p. 14-15, but this figure looks wrong

Autofocus systems: (from Goldberg)
	o active methods
		- time of flight using ultrasonic chirp, or
		- triangulation using narrow beam of infrared light
		- both fail at long distances, dark or reflective objects, etc.
	o passive methods
		- these methods require a reasonably bright scene

		- two-view methods (for non-SLR cameras) compares 1D images
		  from two viewpoints using 1D CCD strip, looks for match
			-> Goldberg, p. 33

		- contrast method compares contrast of images at three depths,
		  if in focus, image will have high contrast, else not
			-> Goldberg, p. 36
			-> Canon 10D?

		- phase methods compares two parts of lens at the sensor plane,
		  if in focus, entire exit pupil sees a uniform color, else not
		- assumes object has diffuse BRDF

		- a variant directs two parts of lens through lenslets, if in
		  focus, left and right halves of lenslet images are identical
			-> Goldberg, p. 41

		- another variant directs two parts of lens through lenslets,
		  if in focus, distance between imagelets matches a reference
			(similar in spirit to split wedge focusing)
			-> Goldberg, p. 43

	o use depth info to drive zoom lens, to keep object fixed size in image
		(Goldberg, p. 47)
	o rubber focus: vary focus, hoping that sharp edges will dominate blurs
		(Goldberg, p. 50)
		- related to conventional tomography

Image stabilization:

	o optical image stabilization
		- Canon: gyro -> moves a lens group perp to optical axis

	o digital image stabilization
		- basic operation is finding corresponding features
		  in adjacent images
		- find offset dx,dy such that norm( f1(x,y) - f2(x+dx,y+dy) )
		  over a window of x,y pixels in images f1 and f2 is minimized
		- same as finding the cross-correlation between two images
		- many variants, e.g. for treating multiple images at once
		- output is called the optical flow field
			-> photo of arcade, flow field, Lim, SPIE '01
			   (already shown during intro class)

	o computational expense
		- window size: larger window is more robust, but more expensive
		- computation frequency: can be computed per-block
			-- 16 x 16 pixels in MPEG motion estimation
		  or per-block, then interpolated to pixels, or
		  or per-pixel

		- robustness increases with increased frame rate
		  because objects move a shorter distance, and
		  intensities change less per frame

		- computational expense is independent of frame rate
		  because smaller search windows suffice to find objects

Exposure:
	o exposure = intensity x time = aperture area x shutter duration
		- closing the aperture by 1 f-number = doubling shutter speed

	o trading off shutter speed (motion blur) and aperture (depth of field)

	o use image stabilization to increase longest handheld exposure

Exposure metering: (Goldberg, p. 55-60)
	o zonal systems
		- e.g. spot versus area

	o exposure systems expect (and force) a gray average
		- fails on snow or coal bins

	o special algorithms
		- recognizing and metering for faces
		- detecting backlit scenes

	o separate light meters
		- works because light arriving from an area source
		  is independent of distance
			-> Demonstration using photometer and white wall
			   (spot mode, through viewfinder)

Shutters:

	o leaf shutter
			-> Adams, p. 82
		- speeds down to 1/500 second
		- shutter located at aperture stop
		- center of lens (not image!) gets light for longer than edges

	o focal-plane shutter
			-> Adams, p. 83, or Goldberg, p. 70
			-> My Canon has vertical-travel focal-plane shutter
		- capable of faster speeds, but
		- distorts fast-moving objects
			-> Lartigue, Grand Prix auto race, in Adams, p. 85
			   (already shown by Bennett)

	o rotary shutters for movie cameras (Goldberg, p. 86)

	o synchronization with (electronic) flash (optional)
		- electronic flash is brief, 1/500 to 1/50,000 sec
		- if focal-plane shutter, must be fully open during flash,
		  which typically limits speed to 1/90 sec (p. 86)

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			*** High speed photography ***

The speed of brief events:
	o Harold Edgerton, Stopping Time
		-> water streaming from a faucet (1932), p. 45
		-> flattened tennis ball, p. 54
		-> vortices from spinning blades, p. 58-59
		-> tennis serve, p. 83
		-> bullet through apple, p. 126
		-> milk drop, p. 127
		-> atomic explosion, 1/100,000,000 second exposure, p. 145
	o other brief events
		- airbag inflation:	0.035 sec
	o motion capture for computer animation
		- Vicon 8i captures at 250 fps
		- Bregler says 500 fps is necessary to avoid hand-editing

Visual perception:
	o How brief an event can we see?
	o critical fusion frequency (CFF)
		- the flicker rate above which the stimulus appears continuous
		- depends on brightness, contrast, and size of source
		- 50-60 fps
	o persistence of vision
		- an ember rotating on a wheel appears continuous at high speed
		  from which Chevalier d'Arcy (1765) estimated persistence at
		  0.1 second
	o tachistoscopic imagery
		- recognition after brief presentation varies depending on
		  duration and stimulus complexity, typical is 50 - 500 ms
		- "erased" by bright flash, so might be a "frame buffer"?!
		- use in subliminal advertising

Historical development of high-speed photography:
	o Muybridge (1870s) - 1/1000th second shutter
		-> galloping horse, Ray, p. 14
	o Ernst Mach (1880s) - spark photographs of bullets in flight
		-> drawing of apparatus, bullet in flight, Ray, p. 10
	o Lucien Bull (1904) - rotating film drum, 2000 fps x 50 frames
		-> bees in flight, Ray, p. 13
	o F.J. Tuttle (1930s) - rotating prism to aim light at moving film,
	  acheived up to 8000 fps, required very bright illumination
		-> diagram, Ray, p. 20
	o drum cameras - fixed film on rotating drum, single lens, 250,000 fps,
	  but limited duration, required even brighter illumination
	o rotating mirror - rotating mirror, fixed film, multiple lenses,
	  20 million fps, limited duration, only useful for explosions?!
	o image converter - single image with exposure of 1/600 millionth sec.
	o video systems - 1,000 fps at 1024 x 1024 pixels, can trade off
	  frame rate and spatial resolution

Unusual high-speed techniques:
	o stroboscopic illumination
		- strobe lights - down to 1 microsecond (Ray, p. 160)
		- pulsed laser - 15 ns (Ray, p. 165)
	o streak photography using a cross-slit camera
			-> Ray, p. 126
		- used at the racetrack
	o shadowgraphy
		- silhouette of object directly onto film (point light source)
		- shock wave refracts the air, making them visible as caustics
			-> Ray, p. 257
	o Schlieren photography (optional)
		- knife edge at object space focal distance
			-> Ray, p. 261
	o synchronization
		- microphones are often used

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			     *** Sources cited ***

Camera technology:

	Norman Goldberg,
	Camera technology: the dark side of the lens,
	Academic Press, 1992.

	Ansel Adams
	The Camera,
	Little, Brown, and Co., 1976.

High-speed photography:

	Edgerton, H.,
	Stopping Time,
	Abrams, 1987.

	Sidney F. Ray ed.,
	High Speed Photography,
	Focal Press, 1997.

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