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I am working to determine the orbit of a satellite based on a long exposure photograph, and need precise observations.

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    \$\begingroup\$ have you tried an experiment? \$\endgroup\$
    – ths
    Commented Dec 23, 2018 at 11:50
  • \$\begingroup\$ My d90 shows the beginning of the exposure so I support the comment by @ths . I'm not sure if I fully got your question, however out of curiosity, what satellite are you tracking? because almost all nearby satellites I know aren't visible nor have the same intensity in their orbit and these precise data are available online. so again, If again I understand correctly the exiff data just would be fine for a later reference if camera date/time synced to your local place of observation. \$\endgroup\$
    – user174174
    Commented Dec 24, 2018 at 3:12
  • \$\begingroup\$ It was from a photo my mum took - and she was curious which satellite it was. I didn't have access to the camera. I ended up just looking up what was around on Calsky (it was an old Falcon booster) \$\endgroup\$
    – pen
    Commented Feb 11, 2019 at 2:58

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The EXIF standard describes the DateTimeOriginal tag simply as "the date and time when the original image data was generated." It gives no guidance about what event (e.g., shutter released, shutter open, shutter closed, sensor read, post processing) should be used to determine the value.

As mentioned in a comment, the best way to find out how your camera defines it is through experimentation. Synchronize its clock to one you can observe, record the start and end times of a long exposure and see which one ends up in the EXIF. Note that the result you get will only be valid for that one camera with whatever firmware it was running.

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  • \$\begingroup\$ I would assume then that the time recorded is at the point after the exposure has ended and the file is generated. \$\endgroup\$
    – David Wilson
    Commented Dec 23, 2018 at 13:51
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    \$\begingroup\$ Rather than using an external clock, just "take 1/100s photo, take 30s photo, take 1/100s photo" should do. \$\endgroup\$
    – Philip Kendall ♦
    Commented Dec 23, 2018 at 14:10
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    \$\begingroup\$ @DavidWilson The standard doesn't make it clear enough for that to be a safe assumption. I just did the experiment with two Nikon bodies and neither behaved as you described. \$\endgroup\$
    – Blrfl
    Commented Dec 23, 2018 at 14:33
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    \$\begingroup\$ @RichardBarber The camera needs to know most of the settings in the metadata, like shutter speed, ISO, flash, before the image acquisition begins, so the argument would be to fill out the metadata in advance. \$\endgroup\$
    – user71659
    Commented Dec 23, 2018 at 19:17
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    \$\begingroup\$ @RichardBarber Without access to the sources for camera firmware, don't know that. I can think of one reason for generating the EXIF data early: it can be done while the shutter is open and the firmware is otherwise idle. It's time that doesn't have to be spent post-exposure, which means you can get to shooting the next frame sooner. \$\endgroup\$
    – Blrfl
    Commented Dec 23, 2018 at 20:14
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Experimentally (on my EOS 70D), this is the beginning of the exposure, and not the end.

But:

  • this seems truncated to the second
  • it depends how accurate is the time of the camera (before doing this I carefully set the time on my camera, but I doubt I can do better than half a second)
  • ... not speaking of clock drift if it hasn't been set recently

IMHO a better method is to trigger the picture from an external device than can have a really accurate clock (Raspberry with NTP clock, for instance)

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  • \$\begingroup\$ Do you mind expanding, or giving a link, ton how to use an external device to help? \$\endgroup\$
    – BruceWayne
    Commented Dec 24, 2018 at 5:29
  • \$\begingroup\$ A Raspberry Pi is a Linux computer which can connect to the Internet and get accurate time over NTP. An RPi can control the wires and plug into a camera's remote trigger port. Hence, you could make an RPi command a camera to take a photo at exactly 01:23:45.6 UTC if that's what you wanted. \$\endgroup\$
    – Nayuki
    Commented Dec 24, 2018 at 6:54
  • \$\begingroup\$ The Rpi is credit-card size computer that typically runs Linux (though there is also a Windows version for it) and can be used for advanced control applications. \$\endgroup\$
    – xenoid
    Commented Dec 24, 2018 at 8:57
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Just did a bulb exposure with a Nikon D850 of over a minute and a half (to make sure that time, during the exposure, chaged the time not just in seconds but also at least in a minute).

In this camera, at least for bulb, the recorded date time is the initial moment the shutter pressed, when exposure start, not when it’s released and the exposure ends. For other models and makers it may vary.

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