Exchangeable image file format

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

Contents 1 Background 2 Technical 3 Geolocation 4 Program support 5 Problems 6 Related standards 7 Viewing and editing Exif data 8 Example 9 FlashPix extensions 10 Exif audio files 11 MakerNote data 12 See also 13 References 14 External links

Background

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

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 and copyright information.

Technical

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 is 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.

Geolocation

The Exif format has standard tags for location information. Currently, only very few cameras, such as the Ricoh Caplio 500SE, Nikon Coolpix P6000 or some higher-end mobile phones, have a built-in GPS receiver and store the location information in the Exif header when the picture is taken. For other cameras, such as Nikon D300, Nikon D90, or FinePix S5 Pro, a separate GPS receiver that fits into the flash connector or hot shoe is available. 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 using a map or mapping software. The process of adding geographic information to a photograph is known as geocoding. Photo sharing communities like Panoramio, locr or Flickr equally allow their users to upload geocoded pictures or to add geolocation information online.

Most of Nokia's Nseries mobile phones (such as the N95) are equipped with a GPS receiver, and using Location Tagger^[1], a piece of software from Nokia Beta Labs, all captured images are tagged with corresponding GPS coordinates when GPS signal is available. The second generation of iPhone (known as the iPhone 3G) by Apple Inc. is also equipped with a GPS receiver, and uses the receiver to geotag photographs taken with the device. The first generation iPhone is not equipped with GPS, and uses a service provided by Skyhook to triangulate and approximate the location at which the picture was taken using nearby cellular phone towers and WiFi hot-spot signal strength data. The Skyhook service provides approximate GPS location information which is then added to the Exif data associated with the picture. Also mobile phones with the Android operating system can geotag images with the included camera application.

Program support

Exif data is 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^[2], FotoFly^[3] for .Net and Exiv2^[4] for C, or Image::ExifTool^[5] for Perl, parse Exif data from files and read/write Exif tag values.

Problems

Apart from not being a maintained standard, 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.^[6]

• 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.^[7]

• 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.^[8] 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.

• Some 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.^[9]

• 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.

• 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.

• Since the Exif tag contains information about the photo, it can pose a privacy issue. For example, a photo taken with a GPS-enabled camera can reveal the exact location it was taken, which is undesirable in some situations. By removing the Exif tag with software such as ExifTool before publishing, the photographer can avoid possible problems.

Related standards

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^[10], 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.

Viewing and editing Exif data

In the original release of Windows XP, 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 and then the Advanced button. However, this can damage certain Exif headers if changes are applied.^[11] As of the release of Service Pack 3, Windows XP still shows evidence of corrupting Exif tags when modifying JPG file properties via the file properties window.^[12]

In Windows Mobile (Pocket PC), freeware XnView Pocket can display the Exif data.

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. The Opanda IExif Viewer is a free stand-alone application for viewing Exif data and also a plug-in for Internet Explorer and Firefox on Windows platforms. EXIFeditor is another free application that allows editing of EXIF information. It allows examination of detailed Exif data online by right clicking on an image.^[13] FxIF and Exif Viewer are multi-platform extensions for Firefox that display Exif data in the image properties dialog. This feature is native in the web browser Opera, under image properties.

Retrieval of detailed Exif data not usually displayed by other programs can be accomplished using ExifTool which runs in Perl, and is available for all platforms.

For AmigaOS/MorphOS, SView5 as well as .NET covers a lot of the previously mentioned functionality.

To deal with the privacy problems that Exif may pose to those who share their photographs with others, specialized metadata removal tools which can remove Exif exist.^[14]

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 (rotation)	top - left [8 possible values[15]]
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

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.^[16] 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)

Exif audio files

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.^[17] 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

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

• OZHiker: Agfa, Canon, Casio, Epson, Fujifilm, Konica/Minolta, Kyocera/Contax, Nikon, Olympus, Panasonic, Pentax/Asahi, Ricoh, Sony ^[18]
• Kamisaka: Canon, Casio, FujiFilm, ISL, KDDI, Konica/Minolta, Mamiya, Nikon, Panasonic, Pentax, Ricoh, Sigma, Sony, WWL^[19]
• X3F Info: Sigma/Foveon^[20]
• ExifTool: Canon, Casio, FujiFilm, JVC/Victor, Kodak, Leaf, Konica-Minolta, Nikon, Olympus/Epson, Panasonic/Leica, Pentax/Asahi, Ricoh, Samsung, Sanyo, Sigma/Foveon, Sony^[21]
• Olypedia: Olympus^[22]

See also

• Comparison of image viewers (Exif view/edit functions)
• Design rule for Camera File system (DCF)
• Digital photography
• eXtensible Metadata Platform (XMP)
• Geocoded photo
• Image file formats
• Image processing
• International Press Telecommunications Council (IPTC)
• IPTC Information Interchange Model
• Point of interest
• Steganography
• Tag Image File Format / Electronic Photography (TIFF/EP)

References

External links

• Exif 2.2 Specification as PDF file or as HTML
• Exif in the TIFF Tags Directory
• Exif standard version 2.21
• Metadata working group