Exchangeable image file format

From Wikipedia, the free encyclopedia

(Redirected from EXIF)

Exchangeable image file format (official abbreviation Exif, not EXIF) is a specification for the image file format used by digital cameras. It was created by the Japan Electronic Industry Development Association (JEIDA). 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 or PNG.

Version 2.1 of the specification is dated June 12, 1998 and version 2.2 is dated April 2002.

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 film speed information.
A thumbnail for previewing the picture on the camera's LCD screen, in file managers or photo manipulation software.
Location information, which could come from a GPS receiver connected to the camera. As of 2004 only a few cameras support this, though. Some people therefore use a normal receiver to track their movements, and then post-process the images by matching the timestamps in the images with the log from the receiver and can so add the missing information to images.
Descriptions and copyright information. Again this is something which is most often done when post-processing the images, as only high-end camera models let the user choose a text for these fields. (Although this feature is becoming more and more apparent even in lower end compact digital cameras).

1 Program support
2 Problems
3 Capitalization
4 Status
5 Viewing Exif
6 Example
7 See also
8 External links

[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 display it alongside the images.

[edit] Problems

The Exif standard 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 maker notes) 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).
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 colour depth is always 24 bits.<ref>JEITA CP-3451 Exif 2.2 section 4.4.3 (Pixel Composition and Sampling)</ref> Many modern cameras can capture significantly more data than this (e.g. the Nikon D70 captures 36 bits of colour per pixel). Since Exif/DCF files cannot represent this colour 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 inadvertendly publish compromising information.[1]
Although there is no specified limit on the size of the embedded thumbnail, it is not possible to have Exif metadata larger than 64 kB in a JPEG image due to the restrictions of the JPEG file format. This has forced some camera manufacturers to create a non-standard way of placing a second (larger) thumbnail in the image for display on the camera's LCD. Since it is a non standard extension, if a user re-saves the image using image editor software, then the second thumbnail is typically lost, and the picture may no longer be compatible with the camera that took it.

[edit] Capitalization

While Exif is an acronym, and would normally be displayed in upper case, the official specification defines it as "Exif".

[edit] Status

The Exif specification is currently not officially maintained because there is no public entity or people officially behind Exif. However, it remains in almost universal use by camera manufacturers.

[edit] Viewing Exif

Image:Eog EXIF.png

In 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. However, this may damage the Exif header.

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. Most UNIX image viewers would 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 plug-in for MSIE and Firefox on Windows platforms, you can right-click an image and read out detailed Exif data online. FxIF is a multiplatform extension for Firefox that shows 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 it 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 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
Maker Note	432 bytes unknown data
FlashPixVersion	FlashPix Version 1.0
Color Space	sRGB
PixelXDimension	2240
PixelYDimension	1680
File Source	DSC
InteroperabilityIndex	R98
InteroperabilityVersion	(null)

[edit] See also

[edit] External links

[edit] Applications for Displaying Exif

[edit] Applications for Editing Exif

Opanda PowerExif A powerful GUI editor for editing Exif & GPS data in images.

[edit] Exif Libraries for Software Developers

libexif is a C library for reading and writing Exif metadata (LGPL licensed)
Exiv2 is a C++ library for reading and writing Exif (and other) metadata (GPL licensed)
Metadata Extractor is a free Java library for decoding Exif metadata
Metadata Extractor is a .NET port of the above Java library
exifdump.py reads Exif data in Python
exif-read-data.php reads Exif data in PHP
JpegDate reads Exif data in Delphi

[edit] References

EXIF.org is an unofficial site dedicated to EXIF
Exif 2.2 Specification as PDF file or as HTML
Exif in the TIFF Tags Directory
Digital Image File Formats

[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: