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Digital Camera

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Cameras with pixels this small have barely enough dynamic range for a JPG photo ... White Balance How well does the camera 'gauge' the luminance distribution ... – PowerPoint PPT presentation

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Title: Digital Camera


1
Digital Camera
  • Essential ElementsPart 2

2
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,

3
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

4
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

5
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

6
(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 10 acceptable quality for 18 gray patch
    (R,B,G 46)
  • S/N 32 noise not visible
  • S?N 0.3 white patch just visible on black
    background

7
Digital Noise ExampleS/N 10 _at_ 18 gray
(R,B,G46) (219 photons)
8
Digital Noise ExampleS/N 4 _at_ 18 gray
(R,B,G46) (73 photons)
9
(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.

10
(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

11
(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

12
Digital Noise Conclusions
  • Larger pixels enable more photons to be
    collected higher S/N
  • Shadows lower S/N
  • High ISO lower S/N

13
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

14
(2)Dynamic Range Contd
  • It is convenient to discuss dynamic range in
    terms of f-stops, i.e doubling the exposure 1
    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!)

15
(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

16
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

17
(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

18
(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

19
Tone Correction
20
Tone Correction from 8 Bit A-D Converter
21
Tone Correction from 10 Bit A-D Converter
22
Noise Dynamic Range (before tone
conversion)3.4 micrometer pixel pitch
23
Noise Dynamic Range (before tone conversion)
5.8 micrometer pixel pitch
24
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

25
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 camera
    computer is limited vs. PC

26
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.

27
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

28
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

29
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

30
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

31
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)

32
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.

33
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

34
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 98
  • Relatively low resolution, both color and detail
  • Supplementary data (histograms, shooting data)

35
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

36
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)

37
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 photo captured
  • Output Format
  • JPG High quality?
  • TIF?
  • RAW?

38
Summary Contd
  • Big Sensor Big lens computer power Highest
    image quality
  • Big camera, Big
  • Small Sensor Small lens limited computer
    power Pocket size convenience , Adequate image
    quality,
  • Less ,
  • Less control over camera output

39
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
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