Exchangeable image file format

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Exchangeable image file format (official abbreviation Exif, not EXIF[1]) is a specification for the image file format used by digital cameras. The specification uses the existing JPEG, TIFF Rev. 6.0, and RIFF WAVE file formats, with the addition of specific metadata tags. It is not supported in JPEG 2000, PNG, or GIF.

the Exif Print Logo
the Exif Print Logo

Exif was created by the Japan Electronic Industries Development Association (JEIDA). Version 2.1 of the specification is dated June 12, 1998, and version 2.2 is dated April 2002 and is also known as Exif Print. The specification is not currently maintained by any industry or standards organisation, but it remains in almost universal use by camera manufacturers.

The Exif tag structure is taken from that of TIFF files. There is a large overlap between the tags defined in the TIFF, Exif, TIFF/EP and DCF standards.

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

  • 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, 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 and copyright information.

Contents

[edit] Geolocation

Location information can be included in the metadata, which could come from a GPS receiver connected to the camera or to a picture. It uses the GPX format. This information can be added by running a GPS receiver that logs data, and then combining the data later on using programs such as GPSPhotoLinker.[1] [2]

[edit] Program support

Exif data is embedded within the image file itself. While many recent image manipulation programs recognize Exif data and will maintain most of it 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.

[edit] Problems

Apart from not being a maintained standard, the Exif format has a number of drawbacks, mostly relating to its use of legacy file structure, including:

  • 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. This is why most image editors damage or remove the Exif metadata (particularly the MakerNote) to some extent upon saving.
  • 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 a myriad of 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.[3]
  • 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.[2] Many modern cameras can capture significantly more data than this (e.g. the Nikon D70 captures 36 bits of color per pixel). Since Exif/DCF files cannot represent this color depth, many manufacturers have developed proprietary, non-compatible RAW image formats.
  • Most digital cameras can also capture video — the Exif standard has no provision for video files.
  • Exif is very often used in images created by scanners, however 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.[4]
  • Exif metadata is 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.

[edit] Competitors

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

[edit] Viewing Exif data

Eye of Gnome Screenshot showing Exif data of an image on a Linux system
Eye of Gnome Screenshot showing Exif data of an image on a Linux system
Konqueror screenshot showing Exif data
Konqueror screenshot showing Exif data

In Windows XP and later Microsoft operating systems, a subset of the Exif information may be viewed by right clicking on an image file and clicking properties; from the properties dialog click the Summary tab. However, this will damage certain Exif headers if changes are applied. Other Windows software like Exif Harvester may be more reliable.

On Mac OS X 10.4 and above, this information may be viewed in the Finder by doing Get Info on a file and expanding the More Info section.

On Unix systems using the GNOME desktop environment, a subset of Exif data can be seen by right clicking the file in the Nautilus file manager and selecting properties. In KDE, it can be seen by right clicking, selecting "Properties" and then "Meta info". Most Unix image viewers give the full set of Exif data.

There are many software tools available which allow both viewing and editing of Exif data. Opanda IExif Viewer is a free stand-alone application, and also a plug-in for MSIE and Firefox on Windows platforms, where you can right click an image and read detailed Exif data online. FxIF and Exif Viewer are multiplatform extensions for Firefox that show Exif data in the image properties dialog. The Opera browser already includes this data under Image Properties.

To retrieve detailed Exif data that is not usually displayed by other programs, you can use ExifTool. ExifTool is free software that uses Perl, and is available for all platforms. After installing on unix and unix-like systems, you can get the Exif info from the command line. Switch to the directory where the image is located then type, e.g. : "exiftool imagename.jpg". To get specific information you use the Tag name. For example, "exiftool -imagewidth -iso imagename.jpg" displays the image width and ISO setting if your camera recorded it.

[edit] Example

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.

Tag Value
Manufacturer CASIO
Model QV-4000
Orientation top - left
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)

[edit] FlashPix Extensions

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.[3] 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. 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)

[edit] Exif Audio Files

The Exif specification describes the RIFF file format used for WAV audio files,[4] and defines a number of tags for storing meta information such as artist, copyright, creation date, and more in these files. 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)

[edit] References

  1. ^ JEITA CP-3451 Exif 2.2, throughout
  2. ^ JEITA CP-3451 Exif 2.2 section 4.4.3 (Pixel Composition and Sampling)
  3. ^ JEITA CP-3451 Exif 2.2 section 4.7.2 (Interoperability Structure of APP2 in Compressed Data)
  4. ^ JEITA CP-3451 Exif 2.2 section 5 (Exif Audio File Specification)

[edit] See also

[edit] External links

[edit] Applications for Displaying Exif

[edit] Applications for Editing Exif

  • Exifer for Windows A freeware GUI Exif metadata viewer/editor in images.
  • jhead is a public domain command line program for FreeBSD, Linux, Unix, and Windows. (review)
  • Opanda PowerExif A GUI editor for editing Exif & GPS data in images.
  • ExifTool is a Perl library and command line tool for reading and writing Exif Metadata
  • PhotoMapper is a freeware that can insert/edit or join GPS coordinates with pictures, and export a file to Google Earth

[edit] Exif Libraries for Software Developers

[edit] MakerNote Information

The 'MakerNote' tag often contains interesting image information. It is normally in a proprietary binary format, but some formats have been (at least partially) decoded:

[edit] Other

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