Cameras and Digital Imaging
(Some of this you can actually use in
everyday life)
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
F-ratio
Small f-ratio
Large f-ratio
Image
Brightness
Bright
Dim
Exposure Time
Short
Longer
Depth of Field
Shallow
Deep
Diffraction
Least
Most
Depth of Field
Depth of Field: f/2.7
Depth of Field: f/8.0
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
Diffraction
Wide Aperture
Lessens
Diffraction
Short Focal
Length Lessens
Diffraction
Diffraction
Creates
Interference
All Images Are Blurry
The Airy Disk
Why Bright Stars Look Bigger
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?
Bayer RGB Filter
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
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
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
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
Luna 3, October,. 1959
Lunar Orbiter, 1966
Lunar Orbiter, 1967
Direct
Film
Imaging
First
Weather
Satellite
Image
(Television
Imaging)
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
Landsat Sensors
Sensor Sweep
Tornado Track and Bad Pixels