Exchangeable image file format

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.
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).

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.

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 doesnt decode/encode. This is why most image editors damage or remove the Exif metadata to some extent upon saving.
The tags available under the standard do not take into account modern digital cameras - Digital SLR's are very popular now, but the standard has no provision for adding metadata about the make, model or settings of lens, flash, filters or macro lenses fitted, nor dimensions of the sensor, details of the type of shutter or apeture,
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, incompatable 'raw' file formats.
The JPEG and standard TIFF file formats supported by the standard are both limited to 24 bit colour. Many modern cameras can capture significantly more data than this but can only make use of it in a proprietory 'raw' format, hence the Exif standard helps reduce the motivation for camera manufacturers to provide higher colour-depth capabilities.
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.
The standard defines a makernote tag, which allows camera manufacturers to place any custom format metadata in the file. This is increasingly used by camera manufacturers to store the myriad of camera setting types not listed in the Exif standard, such as shooting modes, post-processing settings, serial number, focusing modes. As the tag format is a manufacturer specific proprietory binary format, it can be impossible to retrieve information about these camera settings.
Although there is no specified limit on the size of the embedded thumbnail, it is not possible to have Exif metadata larger than 64kB 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 will be lost, and the picture will no longer be compatable with the camera that took it.

Capitalization

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

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.

Viewing Exif

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 Linux, a subset of Exif data can be seen by right clicking the file and selecting properties. Most Linux 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. Visualizer Photo Resize is free software for all Windows operating systems that allows you to study and print the Exif data. Opanda IExif Viewer is a free plug-in for MSIE & Firefox, you can right-click an image and read out detailed exif data online. The Opera browser already includes this data under Image Properties.

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)

External links

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

MakerNote Information

The 'MakerNote' tag often contains interesting image information. It is normally in a proprietory binary format, but some formats have been decoded:

Other

Information about hidden data in JPEG files (Exif, digital fingerprint)

Digital photography | Graphics file formats | Metadata

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