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Camera Raw Formats (Group Description)

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name Camera Raw Formats
Description

Collective description of a group of proprietary formats employed by digital cameras that use a color filter array or Fovean sensor to capture image data. Several examples are listed in File type signifiers below.

Raw files contain data captured by the sensors in the array. The data generally receives a very modest amount of processing before being output by the camera. The production of usable images requires additional processing after the data files have been transferred to a computer. Some information about this pre- and post-processing is provided in the Notes below.

Raw files permit the creator-editor to interpret the image as the various post-processing steps are applied, tailoring an outcome that suits the subject matter and intended use. This outcome may be contrasted to the production of "finished" images in a camera—typically formatted as TIFF_UNC_EXIF or JPEG_EXIF—for which algorithms built into the camera make the post-processing decisions for the photographer.

The proprietary nature of raw formats, however, means that there is a risk that any given format will not be supported for the long term, especially if the manufacturer goes out of business. In at least one instance, data elements have been encrypted within a raw format, forcing the use of the manufacturer's own software for post-processing. The proprietary and undocumented nature of raw formats has been the subject of complaint by professionals; one outlet for their ideas is the OpenRAW organization. Meanwhile, formats that normalize camera raw data, like Adobe's DNG_1_1, are intended to mitigate this risk. (DNG may also contain a copy of the raw file as created.)

Production phase Used for content in the initial state.
Relationship to other formats
    Used by DNG_1_1, Adobe Digital Negative (DNG), Version 1.1

Local use Explanation of format description terms

LC experience or existing holdings Nikon nef raw files have been created by and are held by the American Folklife Center.
LC preference None at this writing, although normalization to DNG_1_1 may emerge as a preferred practice.

Sustainability factors Explanation of format description terms

Disclosure There is very little disclosure of raw format specifications by manufacturers and this is a contentious issue for some photographers; see, for example, Michael Reichmann and Juergen Specht's The Raw Flaw (2005).
    Documentation  
Adoption Varied, depends upon the extent of use of specific cameras. Several of the camera brands listed by Adobe are very popular.
    Licensing and patents Not investigated at this time.
Transparency All raw formats require special software to convert them into usable images. Some raw formats are exported from the camera in a compressed mode.
Self-documentation Most formats include metadata, both for the interpretation of the data (e.g., about white balance) and to provide the types of additional information specified by EXIF_2_2. Professionals in the field report, however, that raw files do not support the widely used IPTC structure for descriptive metadata, or do so in a non-standard and imperfect way.
External dependencies None
Technical protection considerations Not investigated at this time.

Quality and functionality factors Explanation of format description terms

Still Image
Normal rendering Provided when files are opened in special software.
Clarity (high image resolution) High levels of image quality are latent in each raw file. The data must be processed to yield an image; the processing can adjust images for a number of desired outcomes.
Color maintenance See professional photographer Richard Anderson's comments headed Accurate Rendering in Notes below. The specification for DNG_1_1 has a brief outline for mapping camera color space to CIE XYZ color space (pp. 47-48).
Support for vector graphics, including graphic effects and typography Not relevant
Functionality beyond normal rendering Raw files are highly malleable; see Notes below.

File type signifiers Explanation of format description terms

Tag Value Note
Filename extension cr2
Produced by Canon cameras
Filename extension crw
Produced by Canon cameras
Magic numbers Hex: 49 49 1A 00 00 00 48 45 41 50 43 43 44 52 02 00 01
ASCII: II [null] HEAPCCDR
For crw; from The File Extension Source; see also The Canon RAW (CRW) File Format
Tag Value Note
Filename extension dcr
Produced by Kodak cameras
Filename extension kdc
Produced by Kodak cameras
Tag Value Note
Filename extension mrw
Produced by Minolta cameras
Magic numbers Hex: 00 4D 52 4D
ASCII: .MRM
For mrw; from The File Extension Source
Tag Value Note
Filename extension nef
Produced by Nikon cameras
Tag Value Note
Filename extension orf
Produced by Olympus cameras
Magic numbers See note.  According to Exifprobe documentation, "Olympus has taken the unusual step of marking their formats with unique 'magic numbers' in the header (a different magic number for each flavor). . . . One of the formats (magic=0x5352='RS') contains primary image data in 'packed' 12 bit CFA format. The other (magic=0x4f52='RO') provides 12 bit 'unpacked' (16 bits per sample) CFA data."
Tag Value Note
Filename extension pef
Produced by Pentax cameras
Tag Value Note
Filename extension raf
Produced by Fuji cameras
Tag Value Note
Filename extension srf
Produced by Sony cameras
Tag Value Note
Filename extension x3f
Produced by Sigma cameras
Magic numbers Hex: 46 4F 56 62
ASCII: FOVb
For x3f; from The File Extension Source

Notes Explanation of format description terms

General

Regarding processing of raw files in the camera prior to export: The Wikipedia article "Raw Image Format" (as of August 17, 2006) reports, "In general, this processing is limited to algorithms that require direct access to the camera's hardware. This includes 'long exposure noise reduction' (aka 'dark frame subtraction') and the mapping out of 'hot' (too bright) or 'dead' (too dim) pixels. It also often includes rudimentary noise reduction." For a discussion of dead pixels, see http://web.archive.org/web/20070513082749/http://www.cs.wisc.edu/~jduffy/699/.

Regarding post-processing: Most digital camera color filter arrays employ the Bayer pattern (an array with twice the number of green elements than red or blue, to produce sufficient green information to satisfy the needs of human perception). The conversion of this data to the familiar red, blue, and green channels of an RGB image is called demosaicing. Bruce Fraser's excellent white paper "Understanding Digital Raw Capture" names the other post-processing actions typically entailed in the conversion of a raw file:

  • White balance
  • Colorimetric interpretation
  • Gamma correction
  • Noise reduction, anti-aliasing, and sharpening

It is worth noting that the Fovean sensor used in Sigma, Polaroid, and HanVision cameras employs a structure that is drastically different from the Bayer array and produces raw images comprised of RGB data. Fovean raw files from Sigma cameras can be processed using Adobe software.

Regarding accurate rendering: From a paper by the professional photographer Richard Anderson for a 2006 symposium at the I3A (International Imaging Industry Association, apparently disbanded as of October 2013):

"In color management theory, completely accurate rendering from scene through camera to computer to finished print requires profiling of all devices. There is some debate as to whether cameras can and should be profiled, and how those profiles should be used by a RAW processing software. I use the solution provided by the Adobe Camera RAW calibrate tab, where a MacBeth 24 color checker can be photographed, and a script can be run that essentially creates a camera profile unique to each camera body. Others prefer treating the camera as essentially a scanner, where they shoot a color target, measure it and create a camera profile. Whether this profile works under all lighting conditions, or just the specific condition under which the target was shot is the subject of some debate. There is the issue of how the profile should be applied, before or after rendering for instance. Also, not all software has the ability to apply profiles in the rendering process."

History  

Format specifications Explanation of format description terms


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Last Updated: Thursday, 16-Jan-2014 16:33:31 EST