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<title>Exiv2 - Exif and Iptc metadata manipulation library and tools</title>
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<title>Exiv2 - Exif and IPTC metadata library and tools</title>
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<meta name="Author" content="Andreas Huggel">
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<meta name="Description" content="Open source Exif and Iptc metadata library and tools with Exif MakerNote and read/write support">
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<meta name="Description" content="Open source Exif and IPTC metadata library and tools with Exif MakerNote and read/write support">
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<meta name="Keywords" content="exif, iptc, metadata, makernote, manipulation, manipulate, read and write, write, change, update, jpeg, jpg, ifd, image file directory, thumbnail, thumbnails, tag, tags, canon, fujifilm, sigma/foveon, nikon, open source, opensource, huggel, andreas, andreas huggel">
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The Makernote is tag 0x927c in the Exif IFD. According to the
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<a title="The Exif 2.2 standard (PDF)" href="http://tsc.jeita.or.jp/avs/data/cp3451.pdf">
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<a title="The Exif 2.2 standard (PDF)" href="http://www.exif.org/Exif2-2.PDF">
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Exif 2.2 standard</a>, the makernote is <i>"a tag for manufacturers of
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Exif writers to record any desired information. The contents are up to
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the manufacturer, but this tag should not be used for any other than
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its intended purpose."</i></p>
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<p>Unfortunately, most vendors do not publish their makernote specifications. (Why not?)
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Most of the <a href="#R0">specifications available on the Internet</a>
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were reverse-engineered. Judging from the size of many makernote fields, manufacturers
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apparently store a lot of information there and from the available specifications,
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some of this is certainly quite interesting. For example, makernotes may contain
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information about the lens used, contrast, saturation and sharpness settings, image
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quality settings, etc.</p>
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<p>Many (but not all) vendors write the makernote in TIFF IFD (Image File Directory) format,
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i.e., in the same format as the rest of the Exif information is encoded. Although this may
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appear to make sense at first glance, it actually complicates the situation for programs that
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attempt to write to the Exif data. The problem is that IFDs contain offsets relative to the
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start of the Exif data. That implies that if an Exif writer moves a makernote field encoded in
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IFD format to a different location within the Exif data, it gets corrupted. The reason to
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write to the Exif data could be as simple as to add copyright information, an Exif comment,
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etc. In my opinion, this is a bug in the specification. In fact, some vendors seem to have
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recognized this problem and use offsets relative to somewhere at the beginning of the makernote
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field for the makernote IFD.</p>
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<p>The following table summarizes the structures of the makernote field used by some vendors.
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Please let me know if you find further specifications on the Internet.</p>
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<p>Most vendors do not publish their makernote specifications. Most of the
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<a href="#R0">specifications available on the Internet</a>
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were reverse-engineered. Judging from the size of many makernote
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fields, manufacturers store a lot of information there and from the
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available specifications, some of this is certainly quite
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interesting. For example, makernotes may contain information about the
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lens used, contrast, saturation and sharpness settings, image quality
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settings, etc. For some more advanced camera settings, the Exif
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standard doesn't define a standard tag, so camera vendors have to
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resort to using the makernote. Unfortunately, in some cases, camera
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vendors store important information only in proprietary makernote
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fields, instead of using available Exif standard tags. The most
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infamous example for this is Nikon's ISO settings tag.</p>
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<p>Most vendors write the makernote in TIFF format, i.e., in the same
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format as the rest of the Exif information is encoded. This appears to
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be a sensible thing at first glance. Unfortunately, in general it
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means that <i>any</i> change of an Exif tag, which moves the
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makernote field, will corrupt it. It is an inherent problem of the
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TIFF format that a writer must know the format and all extensions
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used, in order to be able to write changes correctly; unknown tags are
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potentially corrupted when they are moved (rearranged). But since
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makernotes are usually proprietary, Exif writers often don't know
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these details. The reason to write to the Exif data could be as simple
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as to add copyright information, an Exif comment, etc. Some camera
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manufacturers seem to have recognized this problem and now use a
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modified TIFF format with offsets relative to somewhere at the
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beginning of the makernote field for the makernote IFD to address the
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<p>The following table summarizes the structures of the makernote
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field used by some vendors. Please let me know if you have additional
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