[vox-tech] .jpg Mystery

[Brian Lavender]

On Thu, Jan 21, 2010 at 12:25:47PM -0800, Alex Mandel wrote:
> Carl Boettiger wrote:
> > On Thu, Jan 21, 2010 at 11:41 AM, Rod Roark <rod at 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

You can boot with an Ubuntu live CD, and use gqview to view the photo.
Right click on the photo and view "properties". It will give all the
Exif properties.

I believe other programs in ubuntu do the same.

brian

-- 
Brian Lavender
http://www.brie.com/brian/

"There are two ways of constructing a software design. One way is to make
it so simple that there are obviously no deficiencies. And the other
way is to make it so complicated that there are no obvious deficiencies."

Professor C. A. R. Hoare
The 1980 Turing award lecture