EXIF Header Information
A digital photograph contains more than just the red, green, and blue pixel values. Additionally, information about the photograph and camera is stored within the JPEG image. The format of this meta information follows the EXIF standard. On top of the stardardised information, some camera makers make use of proprietary extensions to the EXIF standard, known as maker tags.
For WebHDR to be able to determine the image exposure, it needs to know the ISO speed, exposure times, and aperture settings with which the photographs were taken. Almost all cameras that we have tested can be used with WebHDR, but this is only because we use some smart software named ExifTool (see below) to 'tidy up' the EXIF header before feeding the photographs to hdrgen.
EXIF tags in 'cameras/fujifilm_dx10.jpg' ('Intel' byte order): --------------------+---------------------------------------------------------- Tag |Value --------------------+---------------------------------------------------------- Manufacturer |FUJIFILM Model |DX-10 Orientation |top - left x-Resolution |72/1 y-Resolution |72/1 Resolution Unit |Inch Software |Digital Camera DX-10 Ver1.00 Date and Time |1999:01:01 00:03:00 YCbCr Positioning |co-sited Copyright |ANTHONY (Photographer) - [None] (Editor) Compression |JPEG compression Orientation |top - left x-Resolution |72/1 y-Resolution |72/1 Resolution Unit |Inch YCbCr Positioning |co-sited FNumber |f/4.2 ExposureProgram |Normal program ISO Speed Ratings |150 Exif Version |Exif Version 2.1 Date and Time (origi|1999:01:01 00:03:00 Date and Time (digit|1999:01:01 00:03:00 ComponentsConfigurat|Y Cb Cr - Compressed Bits per |14/10 Shutter speed |68/10 sec. (APEX: 10) Aperture |f/4.1 Brightness |56/10 Exposure Bias |0.0 MaxApertureValue |41/10 Metering Mode |Pattern Flash |Flash did not fire. Focal Length |5.8 mm FlashPixVersion |FlashPix Version 1.0 Color Space |sRGB PixelXDimension |1024 PixelYDimension |768 Focal Plane x-Resolu|2151/1 Focal Plane y-Resolu|2151/1 Focal Plane Resoluti|Centimeter Sensing Method |One-chip color area sensor File Source |DSC Scene Type | InteroperabilityInde|R98 InteroperabilityVers| --------------------+---------------------------------------------------------- EXIF data contains a thumbnail (10238 bytes).
Software for editing EXIF information
There is an abundance of software for viewing the EXIF information within an image file. However, there are not quite as many that allow you to edit it as well. I am particularly fond of the following two. They are both command-line utilities, so you can batch-process any number of photographs.
- exiftags
- Command-line tool.
- Image::ExifTool
- Perl module with an included command-line application for reading and writing meta information in image files. It recognises over 1600 different tags. Image::ExifTool is used on the WebHDR site.