Cameras and Digital Imaging
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Transcript of Cameras and Digital Imaging
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Cameras and Digital Imaging
(Some of this you can actually use in everyday life)
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An Important Number• The wider a camera lens opening (aperture),
the more light enters.• The greater the distance from lens to sensor
(focal length), the more light is spread out and the fainter the image
• If (focal length)/(aperture) is constant, the image is always the same brightness regardless of the size of the camera
• (focal length)/(aperture) = f-ratio
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F-ratio
Small f-ratio Large f-ratio
Image Brightness Bright Dim
Exposure Time Short Longer
Depth of Field Shallow Deep
Diffraction Least Most
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Depth of Field
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Depth of Field: f/2.7
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Depth of Field: f/8.0
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Diffraction
• Any time light encounters an edge (lens, mirror, opening of any kind), diffraction occurs
• Diffraction limits the resolution of optical instruments
• Relatively unimportant for film but much more important for digital imaging– Film is a continuous recording medium– Digital imaging involves discrete pixels
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Diffraction
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Wide Aperture Lessens
Diffraction
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Short Focal Length Lessens
Diffraction
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Diffraction Creates
Interference
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All Images Are Blurry
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The Airy Disk
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Why Bright Stars Look Bigger
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Image Resolution
• Two objects will not appear distinct unless their Airy disks are separate
• Airy disk size = 2.4 x wavelength x f-ratio– 500 nm and f/4 = 5280 nm = 5.3 microns– About the size of retinal cells
• Didn’t matter much for film• Does it pay to have pixels smaller than the
Airy disk?
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Bayer RGB Filter
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What is a Pixel?
• Digital cameras use Bayer RGB filter for color rendition
• ¼ of receptors are red sensitive, ¼ are blue sensitive and ½ are green sensitive
• Matches color sensitivity of eye• Four receptors (1R 2G 1B) = a pixel
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Super-Mega-Pixels• Pixels smaller than the Airy disk ( a few
microns) contribute no resolution• Downside of mega-pixel cameras– Fewer photons per pixel = more noise– Bloated file sizes– Probably no harm
• Biggest problem with tiny cameras is inferior lenses
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More on Megapixels
• HDTV = 2 megapixels• James Cameron filmed Avatar with 2.2
megapixel cameras• Anything over 5 megapixels probably
unnecessary• More pixels don’t help, but don’t hurt either
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Satellite Imaging
• Old Old School– Shoot on film– Develop on board– Scan with oscilloscope and photocell– Reconstruct on ground
• Examples– Luna III 1959– Lunar Orbiter
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Luna 3, October,. 1959
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Lunar Orbiter, 1966
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Lunar Orbiter, 1967
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Direct Film
Imaging
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First Weather Satellite Image
(Television Imaging)
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Spacecraft Imaging
• Photomultiplier tubes are extremely sensitive and reliable
• Television-like technology used on spacecraft well into 1980’s
• Galileo (launched 1989) was the first mission to use solid state imaging– 800 x 800 pixels
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Landsat Sensors
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Sensor Sweep
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Tornado Track and Bad Pixels