Description of Exif file format 2005-05-18  
Most of the digital camera's use Exif file format to store images. This specification was made by JEIDA. Some information can be found on the internet. I could not find any upto date document. So I started collecting and published my findings on the web. Here is a tiny description about Exif.

Sometimes DCF is mentioned (Design rule for Camera File system). DCF defines whole file-system of digital camera: directory structure, file naming method, character set and file format etc. The file format of DCF is based on Exif2.1 specification and can be found on EXIF.org. This document is more or less based on Exif2.1/DCF specification

This is a FREE / OPEN document, you may use this document for any purpose (commercial / non-commercial) of 'all' / 'part of' this document.

If you have information about
- 'unknown' items
- fautly data
- missing items
Please e-mail me, Ger Vermeulen g e v e 0 0 1@ c a s e m a . n l

http://www.gvsoft.homedns.org/exif/

rev. 1.1 2005-06-03
rev. 1.0 2005-04-01

  Reference materials  
DCF
Exif2.1 Specification
JEIDA
  Contents  
  1 What is Exif file format?  
The Exif file format is based on the JPEG file format. Exif inserts some of image / digital camera information and thumbnail image to JPEG in conformity to JPEG specification. Therefore you can view Exif formatted image files by JPEG compliant Internet browser / Picture viewer / Photo retouch software etc. as normal JPEG image files.
  2. JPEG format and Marker  
Every JPEG file starts from binary value '0xFFD8', ends with binary value '0xFFD9'. There are several binary 0xFFXX data in JPEG data, they are known as 'Marker', and it starts a new part of JPEG information. 0xFFD8 means SOI (Start of image), 0xFFD9 means EOI (End of image). These two special Markers have no data following, the other markers have data with it.

Basic format of Marker is below.
0xFF+Marker Number (1 byte) + Data [ Size (2 bytes) + Info (n bytes) ]

The 'Data' contains Data Size descriptor and Information, Size (2 Bytes) has 'Motorola' byte align. The data size is the size of the total Data: Size + Info ( value = 2 + n).

Example: FF xx - 00 0C - 01 02 03 04 05 06 07 08 09 0A

It means this Marker (0xFFxx) has 0x000C (12) bytes of data. The data '12 bytes' starts with 'Data size' 2 bytes, followed by 10 bytes: '01 02 03 04 05 06 07 08 09 0A'.

In JPEG format, some of Markers describe data, then SOS (Start Of Stream) marker placed. After the SOS marker, JPEG image stream starts and terminated by EOI  (End Of Image) marker. 

marker 'D8'
SOI
marker xx
size=ssss
marker yy
size=tttt
marker 'DA' = SOS
size=uuuu
Image
stream
marker 'D9'
EOI
FFD8 FFxxssssDDDD... FFyyttttDDDD... FFDAuuuuDDDD.... iiiiiiiiiiii.... FFD9
  3. EXIF marker  
The marker 0xFFE0~0xFFEF is named 'Application Marker', not necessary for decoding JPEG image. They are used by user application. For example, older olympus / canon / casio / agfa digital camera use JFIF (JPEG File Interchange Format) for storing images. JFIF uses APP0 (0xFFE0) marker for inserting digital camera configuration data and thumbnail image.

For inserting data Exif uses the Application Marker APP1 (0xFFE1) marker to avoid a conflict with the JFIF format. Every Exif file format starts from this marker. 

SOI marker APP1 marker APP1 Data Other marker
FFD8 FFE1 ssss 457869660000 TTTT...... FFXX SSSS DDDD......

It starts with SOI (0xFFD8) marker, so it's a JPEG file. Then an APP1 marker follows immediately. All the data of Exif is stored in the APP1 data area. The APP1 Data part starts with size 'ssss' (Exif data area). The size 'ssss' includes the size field also.

After the 'SSSS', APP1 data starts. This part is explained in the next section.

After the APP1 marker area other JPEG markers follow. 

  4. Exif data structure  
The structure of Exif data (APP1) is shown below. This is a case of 'Intel' byte aligns and it contains JPEG format thumbnail. The Exif data starts from ASCII character 'Exif' and 2bytes of 0x00, and then Exif data follows. Exif uses TIFF format to store data. For more details of TIFF format, please refer to 'TIFF6.0 specification'.
 
APP1 Marker   FFE1 Application marker E1
APP1 Data APP1 Data Size ssss  
Exif Header 45786966 0000  
TIFF Header 4949
2A00
08000000
 
IFD0 (main image) xxxx Number of fields in directory 
aaaabbbbccccccccdddddddd Directory Image width
aaaabbbbccccccccdddddddd . . . .
aaaabbbbccccccccdddddddd Exif offset
aaaabbbbccccccccdddddddd GPS offset
xxxxxxxx Next IFD offset (IFD1)
xxxx. . . . Data area of IFD0
 EXIF IFD xxxx Number of fields in directory 
aaaabbbbccccccccdddddddd Directory EXIF version
aaaabbbbccccccccdddddddd Date digitised
aaaabbbbccccccccdddddddd Makernote Offset
aaaabbbbccccccccdddddddd . . . .
00000000 No next IFD
xxxx. . . . Data area of Exif IFD
 GPS IFD xxxx Number of fields in directory 
aaaabbbbccccccccdddddddd Directory GPS version
aaaabbbbccccccccdddddddd . . . .
aaaabbbbccccccccdddddddd . . . .
00000000 No next IFD
xxxx. . . . Data area of Gps IFD
 Makernote IFD FUJIFILM Optional recognise string
xxxx Number of fields in directory
aaaabbbbccccccccdddddddd Directory Familie ID
aaaabbbbccccccccdddddddd ISO speed
aaaabbbbccccccccdddddddd . . . .
00000000 No next IFD
xxxx. . . . Data area of makernote IFD
Interoperability IFD
xxxx Number of fields in directory
aaaabbbbccccccccdddddddd Directory Index
aaaabbbbccccccccdddddddd Version
aaaabbbbccccccccdddddddd . . . .
00000000 No next IFD
xxxx. . . . Data area of Interoperability IFD
IFD1 (thumbnail image)
xxxx Number of fields in directory
aaaabbbbccccccccdddddddd Directory Compression
aaaabbbbccccccccdddddddd XResolution
aaaabbbbccccccccdddddddd . . . .
00000000 No next IFD
xxxx. . . . Data area of Interoperability IFD
Sometimes some unknown stuff
 Thumbnail image FFD8 SOI - Start of Image
FFDB ssss dddd. . . .  DQT - Define quantization table
FFC0 ssss dddd. . . .  SOF0 - Baseline
FFC4 ssss dddd. . . .  DHT - Define Huffman table
FFDA ssss dddd. . . .  SOS - Start of Scan
xxxxxxxx . . . . image
FFD9 EOI - End of Image
  4.1 TIFF header structure  
Byte align TAG markOffset to first IFD
0x4D4D = 'MM' 0x002a 0x00000008
or
0x4949 = 'I I'0x2a00 0x08000000

First 8 bytes of TIFF format is the TIFF header.

First 2 bytes defines byte align of TIFF data. 0x4949='I I' is 'Intel' type byte align, 0x4d4d='MM' is 'Motorola' type byte align. For example, decimal value '305.419.896' is noted as hexadecimal 0x12345678. If Motorola aligned, it is noted as 0x12345678, if it's Intel aligned it is noted as 0x78563412. A lot of digital camera uses Intel align, but not all. Therefore when we need the value of Exif data, we MUST check byte align field every time. Though JPEG data uses Motorola align only, Exif allows both alignments. I can't understand why Exif didn't fix a byte align to Motorola.

Next 2 bytes always have the value of 0x002A.

The last 4 bytes of TIFF header is the offset to the first IFD (Image File Directory, described in next chapter). Includes this offset, all the offset value used in TIFF format counts offset bytes from the first byte of the TIFF header ('I I' or 'MM'). Usually the first IFD starts immediately next to TIFF header, so this offset has often the value '0x00000008'. 

  4.2 IFD : Image File Directory  

Next to TIFF header, there is the first IFD : Image File Directory - IFD-0. It contains image information data.

eeee Number of directory entries
tttt - ffff - nnnnnnnn - ddddddddEntry 0
tttt - ffff - nnnnnnnn - ddddddddEntry 1
. . . . . . . . . . . . . . .
tttt - ffff - nnnnnnnn - ddddddddEntry eeee-1
oooooooo Offset to next IFD

The first 2 bytes ('eeee') contains the number of directory entries in this IFD.

Then directory entries (12 bytes per entry) follows.
'tttt' 2 bytesis the Tag number, this shows a kind of data.
'ffff' 2 bytes is data format.
'nnnnnnnn'4 bytes is number of components.
'dddddddd'4 bytes contains a data value or offset to data value.

After last directory entry, there is a 4bytes of data ('oooooooo'), it has the offset to next IFD. If its value is '0x00000000', it means this is the last IFD and there is no linked IFD.

Data format ('ffff') is defined as see table.
Value Format Bytes/component
1 unsigned byte 1
2 ascii strings 1
3 unsigned short 2
4 unsigned long 4
5 unsigned rational 8
6 signed byte 1
7 undefined 1
8 signed short 2
9 signed long 4
10 signed rational 8
11 single float 4
12 double float 8

'rational' means a fractional value, it contains 2-signed/unsigned long integer values, and the first represents the numerator, and the second, the denominator.

You can get the total data byte length by multiplies a 'bytes/components' value by number of components stored 'nnnnnnnnn' area. If total data length is less or equal than 4bytes, 'dddddddd' contains the value of that Tag. If its size is over 4bytes, 'dddddddd' contains the offset to data stored address.
  4.3 IFD data structure example  
At Exif format, the first IFD is IFD0 (IFD of main image), then it links to IFD1 (IFD of thumbnail image) and IFD link is terminated. But IFD0/IFD1 doesn't contain any digital camera's information such as shutter speed, focal length etc. IFD0 always contains special Tag Exif Offset (0x8769), it shows an offset to Exif SubIFD. Exif SubIFD is IFD formatted data also, it contains digital camera's information.

In case of Extended Exif format (Exif2.1/DCF), Exif SubIFD contains special Tag Exif Interoperability Offset (0xa005). It also points to the Interoperability IFD. By the DCF specification, this tag is mandatory and both SubIFD (main image IFD) and IFD1 (thumbnail image IFD) may have Interoperability IFD. In usual, only main image have this tag.

And some of digital camera uses IFD data format for Makernote; Maker-specific magic data area. It's very hard to decide whether makernote is IFD format or not, be careful to coding program. See Appendix for information of Makernote.

0000: 49 49 2A 00 08 00 00 00-02 00 1A 01 05 00 01 00
0010: 00 00 26 00 00 00 69 87-04 00 01 00 00 00 11 02
0020: 00 00 40 00 00 00 48 00-00 00 01 00 00 00
If the first part of TIFF data is above, it can read as:
Address Hex data Explanation
0x0000~0x00010x4949 the first 2bytes are 'I I', byte align is 'Intel'
0x0002~0x00030x2A00
0x0004~0x00070x08000000IFD0 starts from address '0x00000008'
0x0008~0x00090x0200 number of directory entry of IFD0 is '2'
0x000a~0x000b0x1A01 number for Tag: X-Resolution (0x011A), it
contains a horizontal resolution of image
0x000c~0x000d0x0500 format is 0x0005, unsigned rational
0x000e~0x00110x01000000number of components is '1'
. . Unsigned rational data size is 8 bytes/components, so
total data length is 1 x 8 = 8 bytes
Total data length is larger than 4 bytes, so next
4 bytes contains an offset to the data
0x0012~0x00150x26000000X-Resolution data is stored to address 0x00000026
0x0016~0x00170x6987 next Tag is ExifOffset (0x8769), its value is an
offset to Exif SubIFD
0x0018~0x00190x0400 format is 0x0004, unsigned long integer.
0x001a~0x001d0x01000000one component. Unsigned long integer's data size is
4 bytes/components, so total data size is 4 bytes
. . Total data size is equal to 4 bytes, so next 4 bytes
contains the value for Exif SubIFD offset
0x001e~0x00210x11020000Exif SubIFD starts from address '0x00000211'
. . This was the last directory entry, next 4 bytes is an
offset to next IFD
0x0022~0x00250x40000000next IFD starts from address '0x00000040'
0x0026~0x00290x48000000numerator is 72
0x002a~0x002d0x01000000denominator is '1'
. . So the value of X-Resoultion is 72/1
  4.4 Thumbnail image  
Exif format contains a thumbnail of the image (except Ricoh RDC-300Z). Usually it is located next to the IFD1. There are 3 formats for thumbnails; JPEG format (JPEG uses YCbCr), RGB TIFF format, YCbCr TIFF format. It seems that JPEG format and 160x120 pixels of size are recommended thumbnail format for Exif2.1 or later. By the DCF specification, thumbnail image MUST use JPEG format and image size is fixed to 160x120 pixels.
  4.4.1 JPEG format thumbnail  
If the value of Compression (0x0103) Tag in IFD1 is '6', thumbnail image format is JPEG. Most of Exif image uses JPEG format for thumbnail. In that case, you can get offset of thumbnail from JpegIFOffset (0x0201) Tag in IFD1, size of thumbnail from JpegIFByteCount (0x0202) Tag. Data format is ordinary JPEG format, starts from 0xFFD8 and ends by 0xFFD9.
  4.4.2 TIFF format thumbnail  
If the value of Compression (0x0103) Tag in IFD1 is '1', thumbnail image format is no compression (called TIFF image). Start point of thumbnail data is StripOffset (0x0111) Tag, size of thumbnail is the sum of StripByteCounts (0x0117) Tag.

If thumbnail uses no compression and PhotometricInterpretation (0x0106) Tag in IFD1 has a value '2', thumbnail uses RGB format. In that case, you can see thumbnail image by simply copy data to computer's RGB format (such as BMP format, or copy to VRAM directory). Kodak DC-210/220/260 uses this format. Be mention that at TIFF stores pixel data as 'RGB' order, but BMP stores 'BGR' order.
If that tag has a value '6', thumbnail uses YCbCr format. If you want to see thumbnail, you must convert it to RGB. Ricoh RDC4200/4300, Fuji DS-7/300 and DX-5/7/9 use this format (newer RDC5000/MX-X00 series use JPEG). Next section is brief description to conversion of Fuji DS's thumbnail. For more details, refer to TIFF6.0 specification.

At DX-5/7/9, YCbCrSubsampling(0x0212) has values of '2,1', PlanarConfiguration(0x011c) has a value '1'. So the data align of this image is below.

Y(0,0),Y(1,0),Cb(0,0),Cr(0,0), Y(2,0),Y(3,0),Cb(2,0),Cr(3.0), Y(4,0),Y(5,0),Cb(4,0),Cr(4,0). . . .

The numeric in parenthesis is pixel coordinates. DX series' YCbCrCoefficients(0x0211) has values '0.299/0.587/0.114', ReferenceBlackWhite(0x0214) has values '0,255,128,255,128,255'. Therefore to convert from Y/Cb/Cr to RGB is;

B(0,0)=(Cb-128)*(2-0.114*2)+Y(0,0)
R(0,0)=(Cr-128)*(2-0.299*2)+Y(0,0)
G(0,0)=(Y(0,0)-0.114*B(0,0)-0.299*R(0,0))/0.587

Horizontal subsampling is a value '2', so you can calculate B(1,0)/R(1,0)/G(1,0) by using the Y(1,0) and Cr(0,0)/Cb(0,0). Repeat this conversion by value of ImageWidth(0x0100) and ImageLength(0x0101).