Exchangeable image file format

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This article is about a format for storing metadata in image and audio files. For information about filename and directory structures of digital cameras, see Design rule for Camera File system.
Exchangeable image file format
Filename extension .JPG, .TIF, .WAV
Developed by JEIDA, now JEITA, CIPA
Initial release 1995 (1995)
Latest release
2.3
(26 April 2010; 4 years ago (2010-04-26), revised 20 December 2012; 21 months ago (2012-12-20))
Extended from TIFF, JPEG, WAV
Extended to DCF

Exchangeable image file format (officially Exif, not EXIF according to JEIDA/JEITA/CIPA specifications) is a standard that specifies the formats for images, sound, and ancillary tags used by digital cameras (including smartphones), scanners and other systems handling image and sound files recorded by digital cameras. The specification uses the following existing file formats with the addition of specific metadata tags: JPEG discrete cosine transform (DCT)[1] for compressed image files, TIFF Rev. 6.0 (RGB or YCbCr) for uncompressed image files, and RIFF WAV for audio files (Linear PCM or ITU-T G.711 μ-Law PCM for uncompressed audio data, and IMA-ADPCM for compressed audio data).[2] It is not supported in JPEG 2000, PNG, or GIF.

This standard consists of the Exif image file specification and the Exif audio file specification.

Background[edit]

The Japan Electronic Industries Development Association (JEIDA) produced the initial definition of Exif. Version 2.1 of the specification is dated 12 June 1998. JEITA established Exif version 2.2 (aka "Exif Print"), dated 20 February 2002 and released in April 2002.[3] Version 2.21 (with Adobe RGB support) is dated 11 July 2003, but was released in September 2003 following the release of DCF 2.0. The latest, version 2.3, released on 26 April 2010 and revised in May 2013, was jointly formulated by JEITA and CIPA. Exif is supported by almost all camera manufacturers.

The metadata tags defined in the Exif standard cover a broad spectrum:

  • Date and time information. Digital cameras will record the current date and time and save this in the metadata.
  • Camera settings. This includes static information such as the camera model and make, and information that varies with each image such as orientation (rotation), aperture, shutter speed, focal length, metering mode, and ISO speed information.
  • A thumbnail for previewing the picture on the camera's LCD screen, in file managers, or in photo manipulation software.
  • Descriptions
  • Copyright information.

Technical[edit]

The Exif tag structure is borrowed from TIFF files. On several image specific properties, there is a large overlap between the tags defined in the TIFF, Exif, TIFF/EP, and DCF standards. For descriptive metadata, there is an overlap between Exif and IPTC Information Interchange Model info, which also can be embedded in a JPEG file.

When Exif is employed for JPEG files, the Exif data are stored in one of JPEG's defined utility Application Segments, the APP1 (segment marker 0xFFE1), which in effect holds an entire TIFF file within. When Exif is employed in TIFF files (also when used as "an embedded TIFF file" mentioned earlier), the TIFF Private Tag 0x8769 defines a sub-Image File Directory (IFD) that holds the Exif specified TIFF Tags. In addition, Exif also defines a Global Positioning System sub-IFD using the TIFF Private Tag 0x8825, holding location information, and an "Interoperability IFD" specified within the Exif sub-IFD, using the Exif tag 0xA005.

Formats specified in Exif standard are defined as folder structures that are based on Exif-JPEG and recording formats for memory. When these formats are used as Exif/DCF files together with the DCF specification (for better interoperability among devices of different types), their scope shall cover devices, recording media, and application software that handle them.

Geolocation[edit]

See also: Geotagging

The Exif format has standard tags for location information. As of 2014 many cameras and most mobile phones have a built-in GPS receiver that stores the location information in the Exif header when a picture is taken. Some other cameras have a separate GPS receiver that fits into the flash connector or hot shoe. Recorded GPS data can also be added to any digital photograph on a computer, either by correlating the time stamps of the photographs with a GPS record from a hand-held GPS receiver or manually by using a map or mapping software. The process of adding geographic information to a photograph is known as geotagging. Photo-sharing communities like Panoramio, locr or Flickr equally allow their users to upload geocoded pictures or to add geolocation information online.

Program support[edit]

Exif data are embedded within the image file itself. While many recent image manipulation programs recognize and preserve Exif data when writing to a modified image, this is not the case for most older programs. Many image gallery programs also recognise Exif data and optionally display it alongside the images.

Software libraries, such as libexif[4] for C and Adobe XMP Toolkit[5] or Exiv2[6] for C++, Metadata Extractor[7] for Java, or Image::ExifTool[8] for Perl, parse Exif data from files and read/write Exif tag values.

Problems[edit]

Technical[edit]

The Exif format has a number of drawbacks, mostly relating to its use of legacy file structures.

  • The derivation of Exif from the TIFF file structure using offset pointers in the files means that data can be spread anywhere within a file, which means that software is likely to corrupt any pointers or corresponding data that it doesn't decode/encode. For this reason most image editors damage or remove the Exif metadata to some extent upon saving.[9]
  • The standard defines a MakerNote tag, which allows camera manufacturers to place any custom format metadata in the file. This is used increasingly by camera manufacturers to store myriad camera settings not listed in the Exif standard, such as shooting modes, post-processing settings, serial number, focusing modes, etc. As this tag format is proprietary and manufacturer-specific, it can be prohibitively difficult to retrieve this information from an image (or properly preserve it when rewriting an image). Some manufacturers encrypt portions of the information; for example, Nikon encrypts the detailed lens data in their newer MakerNote data versions.[10]
  • The standard only allows TIFF or JPEG files — there is no provision for a "raw" file type which would be a direct data dump from the sensor device. This has caused camera manufacturers to invent many proprietary, incompatible "raw" file formats. To solve this problem, Adobe developed the DNG format (a TIFF-based raw file format), in hopes that manufacturers would standardize on a single, raw file format.
  • The Exif standard specifically states that color depth is always 24 bits.[11] However, many modern cameras, such as the Nikon D70 which captures 36 bits of color per pixel, can capture significantly more. Since Exif/DCF files cannot represent this color depth, many manufacturers have developed proprietary, non-compatible Raw image formats.
  • Exif is very often used in images created by scanners, but the standard makes no provisions for any scanner-specific information.
  • Photo manipulation software sometimes fails to update the embedded thumbnail after an editing operation, possibly causing the user to inadvertently publish compromising information.[12]
  • Exif metadata are restricted in size to 64 kB in JPEG images because according to the specification this information must be contained within a single JPEG APP1 segment. Although the FlashPix extensions allow information to span multiple JPEG APP2 segments, these extensions are not commonly used. This has prompted some camera manufacturers to develop non-standard techniques for storing the large preview images used by some digital cameras for LCD review. These non-standard extensions are commonly lost if a user re-saves the image using image editor software, possibly rendering the image incompatible with the original camera that created it. (In 2009, CIPA released the Multi-Picture Format specification which addresses this deficiency and provides a standard way to store large previews in JPEG images.[13])
  • There is no way to record time-zone information along with the time, thus rendering the stored time ambiguous.
  • There is no field to record readouts of a camera's accelerometers or inertial navigation system. Such data could help to establish the relationship between the image sensor’s XYZ coordinate system and the gravity vector (i.e., which way is down in this image). It could also establish relative camera positions or orientations in a sequence of photos.

Privacy and security[edit]

Since the Exif tag contains metadata about the photo, it can pose a privacy issue. For example, a photo taken with a GPS-enabled camera can reveal the exact location and time it was taken, and the unique ID number of the device - this is all done by default - often without the user's knowledge. Many users may be unaware that their photos are tagged by default in this manner, or that specialist software may be required to remove the Exif tag before publishing. For example, a whistleblower, journalist or political dissident relying on the protection of anonymity to allow them to report malfeasance by a corporate entity, criminal, or government may therefore find their safety compromised by this default data collection.

In December 2012, anti-virus programmer John McAfee was arrested in Guatemala while fleeing from alleged persecution[14] in Belize, which shares a border. Vice magazine had published an exclusive interview with McAfee "on the run"[15] that included a photo of McAfee with a Vice reporter taken with a phone that had geotagged the image.[16] The photo's metadata included GPS coordinates locating McAfee in Guatemala, and he was captured two days later.[17]

According to documents leaked by Edward Snowden, the NSA is targeting Exif information under the XKeyscore program.[18]

The privacy problem of Exif data can be avoided by removing the Exif data using a metadata removal tool.

Related standards[edit]

Metadata Working Group was formed by a consortium of companies in 2006 (according to their web page) or 2007 (as stated in their own press release). It released its first document on 24 September 2008,[19] giving recommendations concerning the use of Exif, IPTC and XMP metadata in images.

Extensible Metadata Platform (XMP) was created by Adobe Systems to be a better metadata format for photography and image processing. However, it is generally unsupported in cameras.

Example[edit]

Konqueror screenshot showing Exif data

The following table shows Exif data for a photo made with a typical digital camera. Notice that authorship and copyright information is generally not provided in the camera's output, so it must be filled in during later stages of processing. Some programs, such as Canon's Digital Photo Professional, allow the name of the owner to be added to the camera itself.

Tag Value
Manufacturer CASIO
Model QV-4000
Orientation (rotation) top - left [8 possible values[20]]
Software Ver1.01
Date and Time 2003:08:11 16:45:32
YCbCr Positioning centered
Compression JPEG compression
x-Resolution 72.00
y-Resolution 72.00
Resolution Unit Inch
Exposure Time 1/659 sec.
FNumber f/4.0
ExposureProgram Normal program
Exif Version Exif Version 2.1
Date and Time (original) 2003:08:11 16:45:32
Date and Time (digitized) 2003:08:11 16:45:32
ComponentsConfiguration Y Cb Cr -
Compressed Bits per Pixel 4.01
Exposure Bias 0.0
MaxApertureValue 2.00
Metering Mode Pattern
Flash Flash did not fire.
Focal Length 20.1 mm
MakerNote 432 bytes unknown data
FlashPixVersion FlashPix Version 1.0
Color Space sRGB
PixelXDimension 2240
PixelYDimension 1680
File Source DSC
InteroperabilityIndex R98
InteroperabilityVersion (null)

FlashPix extensions[edit]

The Exif specification also includes a description of FPXR (FlashPix-Ready) information which may be stored in APP2 of JPEG images using a structure similar to that of a FlashPix file.[21] These FlashPix extensions allow meta information to be preserved when converting between FPXR JPEG images and FlashPix images. FPXR information may be found in images from some models of digital cameras by Kodak and Hewlett-Packard.[22] Below is an example of the FPXR information found in a JPEG image from a Kodak EasyShare V570 digital camera:

Tag Value
Code Page 1200
Used Extension Numbers 1
Extension Name Screen nail
Extension Class ID 10000230-6FC0-11D0-BD01-00609719A180
Extension Persistence Invalidated By Modification
Extension Create Date 2003:03:29 17:47:50
Extension Modify Date 2003:03:29 17:47:50
Creating Application Picoss
Extension Description Presized image for LCD display
Storage-Stream Pathname /.Screen Nail_bd0100609719a180
Screen Nail (124498 bytes of data containing 640x480 JPEG preview image)

Exif audio files[edit]

The Exif specification describes the RIFF file format used for WAV audio files, and defines a number of tags for storing meta information such as artist, copyright, creation date, and more in these files.[23] The following table gives an example of Exif information found in a WAV file written by the Pentax Optio WP digital camera:

Tag Value
Encoding Microsoft PCM
Num Channels 1
Sample Rate 7872
Avg Bytes Per Sec 7872
Bits Per Sample 8
Date Created 2005:08:08
Exif Version 0220
Related Image File IMGP1149.JPG
Time Created 16:23:35
Make PENTAX Corporation
Model PENTAX Optio WP
MakerNote (2064 bytes of data)

MakerNote data[edit]

The 'MakerNote' tag contains image information normally in a proprietary binary format. Some of these manufacturer-specific formats have been decoded:

  • OZHiker (not updated since 2008): Agfa, Canon, Casio, Epson, Fujifilm, Konica/Minolta, Kyocera/Contax, Nikon, Olympus, Panasonic, Pentax/Asahi, Ricoh, Sony[24]
  • Kamisaka (not updated since 2007): Canon, Casio, FujiFilm, ISL, KDDI, Konica/Minolta, Mamiya, Nikon, Panasonic, Pentax, Ricoh, Sigma, Sony, WWL[25]
  • X3F Info: Sigma/Foveon[26]
  • ExifTool: Canon, Casio, FujiFilm, GE, HP, JVC/Victor, Kodak, Leaf, Minolta/Konica-Minolta, Nikon, Olympus/Epson, Panasonic/Leica, Pentax/Asahi, Reconyx, Ricoh, Samsung, Sanyo, Sigma/Foveon, Sony[27]
  • Olypedia: Olympus[28]

Unfortunately, the proprietary formats used by many manufacturers break if the MakerNote tag is moved - i.e. by inserting or editing a tag which precedes it. The reason to edit to the Exif data could be as simple as to add copyright information, an Exif comment, etc. In some cases, camera vendors also store important information only in proprietary makernote fields, instead of using available Exif standard tags. An example for this is Nikon's ISO settings tag.[29]

See also[edit]

References[edit]

  1. ^ Ahmed, N.; Natarajan, T.; Rao, K. R. (January 1974), Discrete Cosine Transform, IEEE Transactions on Computers C–23 (1): 90–93, doi:10.1109/T-C.1974.223784 
  2. ^ "Standard of the Camera & Imaging Products Association, CIPA DC-008-Translation-2012, Exchangeable image file format for digital still cameras: Exif Version 2.3". Retrieved 2014-04-08.  Check date values in: |accessdate= (help)
  3. ^ Technical Standardization Committee on AV & IT Storage Systems and Equipment (April 2002). "Exchangeable Image File Format for Digital Still Cameras". Version 2.2. Japan Electronics and Information Technology Industries Association. JEITA CP-3451. Retrieved 2008-01-28. 
  4. ^ "The libexif C EXIF for C". Retrieved 2009-11-08. 
  5. ^ "Adobe XMP Toolkit SDK". Adobe Inc. 
  6. ^ "Exiv2 Image Metadata Library". Andreas Huggel. Retrieved 2009-02-12. 
  7. ^ "Metadata Extractor". Drew Noakes. Retrieved 2011-02-18. 
  8. ^ "Image::ExifTool Perl library". Phil Harvey. Retrieved 2009-02-12. 
  9. ^ "TIFF Revision 6.0". Adobe. 1992-06-03. Retrieved 2009-04-07. 
  10. ^ "Nikon Tags: Nikon LensData01 Tags". Phil Harvey. 2008-01-25. Retrieved 2008-01-28. 
  11. ^ (JEITA CP-3451) Section 4.4.3: Pixel Composition and Sampling
  12. ^ Maximillian Dornseif (2004-12-17). "EXIF Thumbnail in JPEG images". disLEXia 3000 blog. Retrieved 2008-01-28. [dead link]
  13. ^ "Multi-Picture Format". CIPA. 2009-02-04. Retrieved 2014-04-29. 
  14. ^ "McAfee wins stay of deportation from Guatemala". Cnn.com. Retrieved 2012-12-26. 
  15. ^ We Are with John McAfee Right Now, Suckers, Vice, December 3, 2012, retrieved 7 December 2012 
  16. ^ Vice leaves metadata in photo of John McAfee, pinpointing him to a location in Guatemala, The Next Web, December 3, 2012, retrieved 7 December 2012 
  17. ^ John McAfee arrested in Guatemala for illegal entry, CBS News, December 5, 2012, retrieved 7 December 2012 
  18. ^ Staff (July 31, 2013). "XKeyscore Presentation from 2008 – Read in Full – Training Materials for the XKeyscore Program Detail How Analysts Can Use It and Other Systems to Mine Enormous Agency Databases and Develop Intelligence from the Web – Revealed: NSA Program That Collects 'Nearly Everything a User Does on the Internet'". The Guardian. Retrieved August 6, 2013. 
  19. ^ "Guidelines for Handling Image Metadata". Metadata Working group. 2008-09-24. Retrieved 2008-12-28. 
  20. ^ "JPEG Rotation and EXIF Orientation / Digital Cameras with Orientation Sensors etc". Impulseadventure.com. Retrieved 2012-12-26. 
  21. ^ (JEITA CP-3451) Section 4.7.2: Interoperability Structure of APP2 in Compressed Data
  22. ^ Phil Harvey (18 March 2011). "FlashPix Tags". Retrieved 29 March 2011. 
  23. ^ (JEITA CP-3451) Section 5: Exif Audio File Specification
  24. ^ Evan Hunter. "EXIF Makernotes - Reference Information". OZHiker. Retrieved 2008-01-29. 
  25. ^ "Exif MakerNote 解析カイセキ情報" (in Japanese). Kamisaka. Retrieved 2008-01-29. 
  26. ^ "SIGMA and FOVEON EXIF MakerNote Documentation". x3f.info. Archived from the original on 2007-08-05. Retrieved 2008-03-26. 
  27. ^ "ExifTool Tag Names". Phil Harvey. 2008-01-18. Retrieved 2011-01-24. 
  28. ^ "Olympus Makernotes" (in German). Olypedia. Retrieved 2008-01-29. 
  29. ^ Andreas Huggel (2012-04-25). "Makernote formats and specifications". Retrieved 2012-09-09. 

External links[edit]