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