EXIF Time Vs Composite Time

[CharlesHouston]

I have looked around here a bit and still have a question. I am using a Nikon D200, taking photos of satellites as they go by overhead. I need the time of shutter opening (or closing) to within a tenth of a second. The Nikon records time of shutter closing.

After MUCH thrashing I think I am close.

But the "File" information, Date/Time Original is (year month day) time is 01:47:25 for example. The Composite is (year month day) time is 01:47:25.24

Where does the .24 come from? I use ten second exposures and the camera records the time that the shutter closes - but appears to be off by about a half second. I have been using Composite time.

I think that the time is off because my "observations" of the satellites produce a consistent error of about a half second, the time is off on the time of the shutter closing.

Thanks for any help!

Charles

[StarGeek]

Where does the .24 come from? I use ten second exposures and the camera records the time that the shutter closes - but appears to be off by about a half second. I have been using Composite time.

Composite tags are tags that exiftool creates based upon other tags in the file.  In the case of the Composite Date/Time Original (actual tag name is Composite:SubSecDateTimeOriginal), if you look on the Composite Tags page, you'll see that it is created from three tags, EXIF:DateTimeOriginal, SubSecTimeOriginal, and OffsetTimeOriginal.  The latter is the time zone and most likely won't be there unless you filled it out.  The EXIF:DateTimeOriginal only holds full seconds. So the .24 is coming from the SubSecTimeOriginal tag.

[CharlesHouston]

I had seen that SubSecTimeOriginal tag but not quite figured out the significance.

BTW I am using a Mac, JEXIFToolGUI by Harry van der Wolf. I think I am using EXIFtool 12.04

So the camera records the time in two tags, the .24 is a part of the time that the shutter closed? The Nikon D200 records the time that the shutter closes. It seems that the Nikon should record time very accurately but it appears that the time is off by something close to that SubSecTimeOriginal value. I wonder if I should just discard it? But if the camera thinks that it is real, is that a good idea?

Charles
software writer (C, C++), amateur astronomer, orbital mechanics guy

[StarGeek]

I honestly have no idea at what point it's recording the subsecond.  It's something I've never looked into.  I feel lucky if I can remember to take a picture of the seconds on my phone to correct any drift that has happened to the clock on my camera.

[CharlesHouston]

Thanks, I will go to the Digital Photo Review forums and ask the Nikon people there.

The D200 seems to keep very good time for several days in a row, sadly I have to update the time manually.

Charles
software writer (C, C++), amateur astronomer, orbital mechanics guy

[Phil Harvey]

I had seen that SubSecTimeOriginal tag but not quite figured out the significance.

I use this tag all the time:  I keep my raw files archived on a different filesystem in a directory structure based on date/time.  I keep my recent good JPG images on my laptop, but the files have been renamed.  Whenever I want to reprocess one of my JPG images, I find the original raw on my archive disk based on date/time (I have written a script to do this very quickly for me).  But if it wasn't for the subseconds, many pictures would have exactly the same date/time (my camera shoots at 10 frames per second), and I wouldn't be able to find the exact raw file for each JPG.

- Phil