Digital RAW Photography

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Digital RAW Photography IntroductionSeptember
2006

• What is RAW and should I care? 
• REVIEW - How most still camera sensors capture a
  photo.
• (Sigmas Foveon sensor works differently, but
  well ignore it in this discussion)

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REVIEW

• The sensor is capable of only seeing gray values.
  Its an analog device that records light
  (photons) as an electrical charge. The charge
  from each picture element (pixel) is amplified
  and converted to a 10, 12 or 14 bit digital
  signal via the ADC (analog to digital converter)
  with up to 1,024, 4,096 or 16,384 levels, or
  shades of gray.

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REVIEW

• The RAW file is a dump of the gray scale data
  that the CCD captures. It is a relatively compact
  file with far more color information than the
  JPG/TIFF files produced inside the camera. We
  will see why this is so a bit later
• Cameras that actually write the raw format to a
  data file use 12 or 14 bit ADC. Cameras that
  only record TIFF and/or JPG files use a 10 or 12
  bit ADC.

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REVIEW

• How does the Raw data translate into an RGB
  image?
• On top of the CCD's array of pixels (also called
  photo sites) is fixed a colored filter with
  millions of little colored cells, each aligned
  with a single CCD pixel. The filters are arranged
  in alternating rows, every other row with the
  filter colors Red and Green, and the remaining
  rows as Blue and Green. Green appears twice as
  often because our eyes are most sensitive to
  differences in green for the detail in images.
  This is the Bayer mosaic or array.

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REVIEW

• Still we don't have a RGB image at this point. To
  get this we need to interpolate the RGB values
  from the information we have in the neighboring
  pixels. This interpolation, or de-mosaic process
  creates a file with RGB values for each pixel.
  The de-mosaic logic ranges from complex to
  extremely complex and constitutes the lions
  share of the computation work done by the Raw
  converter

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• Digital Camera Imaging
• Process

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Raw Converter

• De-mosaicing, is the key role a raw converter
  plays, but its not the only one. Raw conversion
  requires the following additional steps
• Colorimetric interpretation
• White Balance
• Gamma Correction
• Anti-aliasing Sharpening
• Noise Reduction,

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Colorimetric interpretation

• Many different filter sets are in use with
  digital cameras. So the raw converter has to
  assign the correct, specific color meanings to
  the red, green, and blue pixels, usually in a
  colorimetrically defined color space such as CIE
  XYZ, which is based directly on human color
  perception.
• Filter set information must be available from the
  sensor manufacturer to support a camera

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White balance

• The white balance setting on the camera has no
  effect whatsoever on the captured pixels when you
  shoot rawits simply recorded as a metadata tag
  in the raw file. Some raw converters can read
  this tag and apply it as the default white
  balance (which the user can then override if
  desired), while others may ignore it completely
  and analyze the image to determine white balance.
  Some allow pointing at a neutral feature in
  the photo, and / or specifying color temperature.

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Gamma correction.

•   Digital raw captures have linear gamma (gamma
  1.0), a very different tonal response from that
  of either film or the human eye. So the raw
  converter applies gamma correction to
  redistribute the tonal information so that it
  corresponds more closely to the way our eyes see
  light and shade. (More discussion later)

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Anti-aliasing and Sharpening

• Problems can arise with very small details in an
  image. If the detail is only captured on a
  red-sensing pixel or a blue-sensing pixel, the
  raw converter may have a hard time figuring out
  what color that pixel should really be. Simple
  de-mosaicing methods struggle to maintain edge
  detail so raw converters employ edge-detection
  and anti-aliasing to compensate sharpening to
  restore edge acutance.

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Noise reduction

• High ISO settings increase noise when the analog
  signal is amplified before being processed by the
  ADC. Noise reduction may be applied before
  de-mosaic (color noise suppression) and / or
  after to the pixel luminance. Some converters
  also suppress hot pixels that show up in long
  exposures.
• Its worth noting that ISO setting occurs before
  the ADC step, so it cant be changed by the Raw
  converter.

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Converter Summary

• All raw converters perform these tasks, but they
  may use very different algorithms to do so, which
  is why the same image may look quite different
  when processed through different raw converters
  (or cameras).
• 3rd party converters have a UI (user interface)
  that allows a degree of control for some or all
  the functions.
• Many converters add controls for saturation,
  contrast, brightness and more.

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All digital cameras have a raw converter

• Vendors make a subjective determination of what
  the best look is, and then adjust their
  converter to produce that result. (In-camera
  process variation is limited to the cameras
  preset options). Also to save processing time,
  given the limitations in the cameras small CPU,
  the converter uses only 8 bits of the available
  10 or more bits of pixel information. The unused
  bits are discarded. When the RBG file is encoded
  and written, either as a TIFF or JPG, the
  original raw file is discarded.

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In-camera Image Quality

• Despite the limitations of the in-camera
  converter, the photo quality can be excellent.
  However, the quality may not hold up if its
  extensively edited, so getting exposure and white
  balance right are critical.

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Raw Inconveniences

• Raw files cant be directly viewed (The EXIF file
  contains a low res thumbnail for camera LCD
  display)
• Raw shots are SLOW - A 12-bit raw file contains
  5.3X more data vs. an 8 bit TIFF 20X more than a
  high quality JPG, so writing time to memory
  suffers.
• Investment of your time - Converters will do a
  default conversion, with results not much (or
  no) better than in-camera (but 12-bit file is
  retained).
• Raw file formats are proprietary - Not all
  converters can convert every format. Adobes DNG
  open format hasnt been accepted by the camera
  manufacturers

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Inconveniences Contd

• 12-bit color information is only available with
  RAW files
• Full power of a computer to perform more complex
  conversion algorithms utilizing up to 16 bit
  files yields enormous control over image tonality
• Post-capture processing flexibility - many more
  options and tools to adjust contrast, saturation,
  white balance, sharpening, and exposure.

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So why shoot Raw?

• 12-bit color information is only available with
  Raw files
• Full power of a computer to use more complex
  algorithms utilizing up to 16 bit files yields
  enormous control over image tonality
• Post-capture processing flexibility - many more
  options and more precise tools to adjust
  contrast, saturation, white balance, sharpening,
  and exposure
• The Raw file can be processed again and again as
  better converters becomes available

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Our eyes and film photography

• Our eyes response to light is fundamentally
  different compared to the digital cameras sensor
• Its non-linear ½ the light looks about 30
  dimmer, 10X the light looks about 2X as bright.
  The human visual system easily comprehends
  10,0001 illumination range.
• Coincidentally, silver halide photography
  responds in a manner very similar to the human
  eye. We measure exposure in zones or f-stops
  i.e. doubling (or halving) exposure yields an
  exposure shift of one zone or f-stop.

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Digital Camera Sensor

• A linear device -1/2 the light is 1/2 as bright,
  1/10 the light is 1/10 as bright
• A linear device is said to have a Gamma 1
• An  RBG image, at Gamma 1, will look very dark
  (The sensor will only capture a brightness range
  about 1001 to 2501, depending on the sensor
  noise pixel size, but the scene illumination
  may vary by 10,0001 or more)

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Gamma Correction

• Gamma correction applied by the converter
  redistributes the pixel luminance values so that
  limited brightness range captured by the sensor
  is mapped to match our eyes sensitivity.
  Gamma 2.2 is a good match to distribute
  relative brightness in a print or in a video
  display.

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Gamma 1 vs. Gamma 2.2
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Tonality Dynamic Range the 12-bit advantage
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Conclusions from Table

• Levels are lost when converting from a 12-bit RAW
  format (4096 levels total) to an 8-bit file
  (8-bit BW or 24-bit color). A print has an
  effective contrast range of about 1001 (6.6
  f-stops), so image quality in an 8 bit converted
  file will be adequate after gamma transform if
  the exposure is correct and little editing is
  required.
• This is achievable in studio environments, but
  may be a problem when using "natural" (i.e.,
  uncontrolled) light. The result will be blocked
  out shadows and/or blown highlights, which
  have no tonal detail. 12-bit files have plenty
  of levels - 4096 total. (Files are processed as
  16-bit in the computer, but 4 bits are unused)

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Conclusions Contd

• You can edit to your heart's content without fear
  of banding or other artifacts arising from
  limitations of 8-bit files. More important, the
  extra levels allow exposure adjustment after the
  fact.
• White Balance adjustment is more accurate because
  the extra levels allow smaller, more accurate
  shifts in the RBG channels.

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What is NOT evident!

• Gamma transformed files are Non-linear
• Adjusting any of the converter functions
  discussed earlier should be done before changing
  gamma to avoid unexpected RBG color shifts and/or
  color artifacts. 
• Not all converters operate in this manner, nor
  does the software documentation disclose it. The
  best converters make adjustments in Gamma1
  space, but display the results in Gamma2.2 space
  after adjusting. 

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NOT evident - Contd

• Photo Editors operate on Gamma transformed files,
  so even though similar actions are available in
  the editor and the raw converter, the results may
  not be the same, especially for colorimetric
  adjustments (levels, saturation, white balance).
  Geometric operations (resize, crop, perspective,
  etc) are generally not affected by gamma.

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Guidelines for Raw workflow

• Save the original raw file. Its a digital
  negative that can be converted again as new and
  more sophisticated raw converters are developed.
  
• Use a converter that doesnt change the raw file.
  Instead, it creates a transform instruction set
  that outputs a standard format RBG. The
  converter software retains the instruction set,
  so it can be used again.

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Guidelines Contd

• Use the converter for all colorimetric
  adjustments. If local adjustments are needed,
  create additional converted files with
  appropriate adjustments, then use selections to
  build a composite via layers in the photo editor.
• Limit sharpening to the minimum required to
  compensate for anti-alias filter effects during
  the de-mosaic process.
• Selective sharpening cant be done in the raw
  converter anyway and output specific sharpening
  (web or print) should be the last step in the
  photo editor.

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Guidelines Contd

• Apply noise reduction cautiously- it comes at the
  cost of detail, which cant be recovered later. 
• Avoid cropping in the raw converter to reduce the
  file size Many of the geometric adjustments
  available in the photo editor are lens-centric.
  Perform them first, then crop.
• Some photo editors allow editing in 16-bit files.
  However, if rule 1 is followed, there is little
  advantage unless very extensive geometric editing
  is required, where accumulated 8 bit quantizing
  errors might be visible. In the end, an 8-bit
  file is all that can be viewed or printed with
  todays technology anyway.

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Raw Conversion Software

• New 3rd party software seems to appear monthly
  and revisions to existing are nearly as frequent.
  In addition every camera manufacturer who offers
  a raw format also offers a converter software
  package(s), both free and/or purchased. At last
  count there are 40 converters available on the
  web. However, many of them use the same raw
  converter engine, DC RAW, supplied by David
  Coffin http//www.cybercom.net/dcoffin/