Digital Camera

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Digital Camera

• Essential ElementsPart 2

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Digital Image Quality

• Image Quality - more comprehensive than
  resolution and more meaningful
• 4 Pillars of Quality
• http//www.normankoren.com/Tutorials/MTF7.HTMLPil
  lars
• Resolution- a function of lenspixel numberfile
  processing software
• Noise - analogous to grain in film photography
• Dynamic Range Maximum signal / Minimum usable
  signal- usually expressed in powers of 2
  (f-stops)
• Color Accuracy White Balance, Saturation,

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Digital Camera Resolution

• Megapixels only one of many factors that limit
  resolution
• Other factors
• Pixel size or pitch
• Lens quality
• Interpolation software
• System noise (electronic and photon)
• Image data file compression

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Resolution

• http//www.dpreview.com/learn/?/Glossary/Digital_I
  maging/Resolution_01.htm
• Measured in line pairs / mm
• Vertical (primary), Horizontal
• High contrast bar chart input
• Low ISO (low noise) exposure
• In camera processed, then printed

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Digital Noise

• http//clarkvision.com/imagedetail/does.pixel.size
  .matter/
• CCD Noise Photon Shot noise
• Photons arrive at random intervals, so there is
  some uncertainty about the true number
  collected
• Photon Noise ( Photons collected)1/2
• Total Noise ((Photon Noise)2(Electronic
  Noise)2)1/2
• Effective S/N Photons/Total Noise
• Photon noise is dominant in a usable signal

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(2) Digital Noise Contd

• Why do I care about noise?
• Because we see it as luminance variation in
  uniform areas within the photo
• ISO 12232.1998 standard
• S/N gt10 acceptable quality for 18 gray patch
  (R,B,G 46)
• S/N gt32 noise not visible
• S?N gt0.3 white patch just visible on black
  background

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Digital Noise ExampleS/N 10 _at_ 18 gray
(R,B,G46) (219 photons)
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Digital Noise ExampleS/N 4 _at_ 18 gray
(R,B,G46) (73 photons)
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(3)Digital Noise Contd

• Noise standard sets a floor on the number of
  photons that need to be collected
• Signal219, Shot Noise14.8, Read noise16,
• Total noise (475)1/2 21.8
• S/N 219/21.8 10
• CCD is a linear amplifier, so (256/46)x2191219
  photons is minimum full well at level 256 for
  an acceptable image, at least from the stand
  point of noise.

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(4)Digital Noise Contd

• Oops!
• The inherent ISO of the CCD is 50, but most
  cameras set the default ISO to 100
• Therefore we need to double the full well
  capacity of the pixel to 2438 photons in order to
  maintain the minimum acceptable noise standard

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(5)Digital Noise Contd

• High ISO aka photon multiplier
• Since fewer photons are counted to create a given
  luminance value, the S/N is also lower compared
  to a lower ISO setting

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Digital Noise Conclusions

• Larger pixels enable more photons to be
  collected gt higher S/N
• Shadows gt lower S/N
• High ISO gt lower S/N

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Dynamic Range

• Definition Maximum photon capacity / minimum
  (usable) photon count.
• Maximum photon capacity is limited by pixel size
  small pixels collect fewer photons
• Minimum (usable) photon count is noise limited

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(2)Dynamic Range Contd

• It is convenient to discuss dynamic range in
  terms of f-stops, i.e doubling the exposure gt1
  f-stop, or zone .
• Printed images are limited to 5 1/2 zones
• A good monitor is limited to 8 zones
• Real scenes may exceed 10 zones
• 8 bit/ color digital file can contain 8 zone
  exposure range (if the camera can deliver!)

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(3)Dynamic Range Contd

• Pixel photon collection requirements
• 6.02n 1.8 _at_ pixel saturation
• n digital bits /channel
• 8 bits required for full range jpg image, but a
  10 bit A-D converter is used to fill the the
  shadows when the tonal correction is applied to
  linear data collected.
• Therefore n10 in the equation above
• S/Nmin 62 or 4100 photons _at_ saturation
• vs. 2438_at_ saturation for acceptable noise

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Minimum Pixel Size

• Current CCD (or CMOS) collect 835 photons/
  micrometer2
• Minimum pixel area 4.9 micrometer2
• For square pixels pitch (4.9)1/2
• 2.2 micrometer
• Cameras with pixels this small have barely enough
  dynamic range for a JPG photo and will require
  noise reduction for any ISO greater than 100

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(4)Dynamic Range contd

• JPG is limited to 8 bits/channel but the actual
  dynamic range is less, depending on the A-D
  converter and the noise floor
• After A-D conversion, the signal from the linear
  CCD undergoes tonal correction to match our eye
  response Sout K x (Sin)1/Gamma
• Gamma 2
• K 16 for 8bit Sout

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(5)Dynamic Range contd

• A-D converter Tone conversion have a big effect
  on dynamic range
• 10 bit A-D after tone conversion allows only 5
  stops, because the first non-zero level is 8.
• 256/832 25
• 12 bit A-D allows 6 stops
• 14 bit A-D allows 7 stops

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Tone Correction
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Tone Correction from 8 Bit A-D Converter
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Tone Correction from 10 Bit A-D Converter
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Noise Dynamic Range (before tone
conversion)3.4 micrometer pixel pitch
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Noise Dynamic Range (before tone conversion)
5.8 micrometer pixel pitch
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Color Quality

• How accurately does the camera interpret the
  color in the image?
• RBG filter cut-off
• Bayer Interpolation accuracy
• White Balance
• Over flow artifacts
• Tone Curve

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Color Quality Contd

• RBG filter cut-off
• Bayer filters well controlled
• Foveon filter - technical challenge since the
  filter/sensors are stacked
• Bayer interpolation How many surrounding pixels
  are read to determine RBG channel luminance?
• Circle of confusion /or diffraction means the
  image information spans several pixels.
• Interpolation is computation intensive gt camera
  computer is limited vs. PC

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Color Quality Contd

• http//www.dpreview.com/learn/?/Glossary/Digital_I
  maging/White_Balance_01.htm
• White Balance How well does the camera gauge
  the luminance distribution
• Camera software is remarkably ingenious, but not
  perfect and there may be significant differences
  between cameras manufacturers and across the
  spectrum of luminance distribution.

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Color Quality Contd

• Overflow Artifacts
• When a pixel is over-exposed, excess electrons
  may overflow to adjacent pixels causing color
  fringing, especially in high contrast areas
• Inherent to CCD construction

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Color Quality Contd

• Chromatic Aberrations
• A function of lens design red, blue and green
  light may not focus at same point in the image.
• After interpolation, the effect is very similar
  to color fringing and most obvious in high
  contrast areas farthest from the lens axis

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Color Quality Contd

• http//www.normankoren.com/digital_tonality.htmlR
  aw_conversion
• CCD is a linear device, but we see as a log
  function
• CCD luminance proportional to exposure
• Visual Log10 luminance proportional to
• Log10 exposure
• Tone Curve converts from CCD to visual

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Power Source

• Batteries
• Alkaline - not rechargeable, readily available
• Ni-MH Rechargeable available to replace
  alkaline
• NI-MH custom form factor unique to camera
• Lithium Ion custom form factor unique to camera
  

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Illuminator

• Electronic Flash is universal
• Fixed location (entry level)
• Elevated operating position improved red eye
• Hot shoe to accept more powerful, more flexible
  flash ( articulated, power modulated)

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Focus

• Auto focus is standard on all but the most basic
  (fixed focus) cameras
• Most cameras have auxiliary lamp to aid focus
  operation in low light ambient
• More expensive cameras offer multiple focus
  programs (macro, portrait, landscape) manual
  focus.

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Exposure Control

• Digital Camera exposure latitude is far less than
  print film (and about equal to slide film). Auto
  exposure adjust is universal
• More expensive cameras offer aperture dominant
  shutter speed dominant exposure modes
• More expensive cameras offer multiple scene
  averaging programs

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Aiming Devices

• LCD (fixed or articulated)
• Auxiliary optics (point shoot)
• Parallax error 80 true field of view
• Electronic view finder (LCD lens)
• Accurate field of view gt98
• Relatively low resolution, both color and detail
• Supplementary data (histograms, shooting data)

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Aiming Devices Contd

• Reflex (SLR)
• High end cameras with multiple lens options
• Accurate field of view
• Excellent color and detail
• LCD view mode not available until after the
  shot

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Data Storage

• Fixed in-camera Entry level
• Removable flash memory
• Type 1, Type 2 compact flash (CF) cards
• SD, XD Memory stick (all smaller than CF cards)

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Summary

• Camera Properties
• Ergonomics- Does it fit your hand/eye?
• Controls- Intuitive vs. Learning Required
• Adjustments for special needs
• Features adequate for intended use
• Image Quality vs. compact size trade-off
• Pocket size gtgt photo captured
• Output Format
• JPG High quality?
• TIF?
• RAW?

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Summary Contd

• Big Sensor Big lens computer power gt Highest
  image quality
• Big camera, Big
• Small Sensor Small lens limited computer
  power gt Pocket size convenience , Adequate image
  quality,
• Less ,
• Less control over camera output

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Summary Continued

• Do research camera choices
• Phil Askey extensive image quality review
• Steves Digicam less extensive image quality,
  but concise summary with comparison to other
  camera options. Many more cameras reviewed
• Google Camera model review
• Specific camera forums users own them, reveal
  the warts along with the good