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I'm running GraphicsMagick 1.4-0.020111127 installed on OS X 10.7.2 via Fink. Identify does not appear to pick up lens metadata that ExifTool catches. For example, this image should report a lens ID of 742172430, which corresponds to a AF-S Nikkor 28-300mm f/3.5-5.6G ED VR, but Identify doesn't report anything about the lens ($ identify -verbose AGY_7503.jpg). Am I doing something wrong?
I don't think you are doing anything wrong. GM only collects EXIF info that it knows about. Some EXIF information is proprietary or not deciperable (or assumed to not be decipherable). If you can provide EXIF identifying information and the data type then I can add that to what GM knows about.
Sorry for the long delay in responding—forgot to select "monitor this". I'm seeing that ExifTool is able to determine the lens based on the Lens ID and a look-up table. Each manufacturer uses a different set of lens tags, but the general idea appears here. There's a non-trivial amount of processing going on under the hood.