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2010 Jan 21 13:25

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Re: [vox-tech] .jpg Mystery
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Re: [vox-tech] .jpg Mystery



Carl Boettiger wrote:
> On Thu, Jan 21, 2010 at 11:41 AM, Rod Roark <rod@sunsetsystems.com> wrote:
> 
>> On 01/21/2010 11:18 AM, Bill Kendrick wrote:
>>> On Thu, Jan 21, 2010 at 10:35:35AM -0800, Bob Scofield wrote:
>>>> In Linux there are a couple of different ways I can get the date and
>> time the
>>>> photo was taken.
>>> I picked a random photo I took with a Canon digital camera a few years
>> ago.
>>> The date/timestamp of the file on my laptop was from 9:05pm on the day
>>> the photo was taken.
>>>
>>> However, using "File->Properties..." in Gwenview, and looking at the
>>> metadata stored in the file itself, it was stamped as 8:05pm of that day.
>>>
>>> I'm guessing the timestamp in the metadata is off by an hour because
>>> I never adjusted the clock inside my camera to account for Daylight
>> Savings
>>> time change.  (In other words, the camera THOUGHT it was 8:05pm, so
>> that's
>>> the metadata it stored.)
>> I would think that the file's timestamp is derived from the time set in
>> the camera, one way or another.  Either the camera creates the file on a
>> standard (probably win32) filesystem when the picture is taken, or the
>> timestamp is assigned from the metadata by some special software that
>> fetches images to your computer from the camera.
>>
>> Sounds like some sort of bug to me, perhaps where some piece of software
>> neglects to take DST into account.
>>
> 
> Sounds plausible to me.  Digital cameras attach metadata in an EXIF file,
> the exiv2 will let you read this file directly:
> http://www.exiv2.org/sample.html  Perhaps the machines.software interpret
> this file differently (i.e. DST stettings))
> Of course there's lots of reasons a timestamp won't match reality:
> http://www.slate.com/id/2140303/
> 
> 
> -Carl
> 

Since I'm a photographer I should probably help clear this up. Digital
cameras when shooting photos do in fact write the timestamp from the
camera clock to the EXIF tags of the image. This value should not change
no matter what computer you view it on. Looking at the EXIF 2.2 spec,
DateTime is stored as ASCII not an encoded unix time or anything like
that. So missing DST time on the computer viewing should not affect it.

This value can easily be wrong if the camera was set wrong in any way
(but the error is on the camera side), or the tag was edited.Yes EXIF
tags can be edited, I've actually written python scripts to do this.
Note an image resize or other operation can change this value, since the
new copy of the photo was changed, this should however change the some
of the other fields in the EXIF to indicate it's no longer a camera
generated photo.

There are several add ons for Windows XP that add more photo tools,
that's why you don't have that extra tab. Keep in mind file properties
will also show date created, and date modified, these are not the EXIF
timestamp.

Digikam is a photo management tool that will let you see the EXIF data
and edit the tags the camera didn't auto insert for tags, comments, gps
location and a bunch of other stuff.

See for more information
http://en.wikipedia.org/wiki/Exchangeable_image_file_format
http://www.exif.org/specifications.html

Alex
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