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I want to know the exact UTC time of the moment of the shutter button pushed. A precision of milliseconds order would be ideal, but anything better than 1sec is acceptable.

The photo properties detail doesn't have enough accuracy. The time reported is in minutes of UTC at best.

Also, is there any lag between clicking the shutter button and opening of the shutter? Yes obviously, but how much? and does it depend on the exposure time and anything else?

I'm shooting satellite passes in the night sky, with a final goal of orbit determination. So, precision matters a lot!

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    This depends on the exact model that you have - if you have one that supports GPS (or an app that tags GPS for each shot), then it will be quite simple. So please state which DSLR you have :) – flolilo Apr 28 at 12:25
  • Is a DSLR even a good tool for this? – Hueco Apr 28 at 16:22
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    My Nikon 7200 shows time to 1 second, but this is camera time and not sync'd to anything. I think you're going to need GPS time. Note that GPS time is not the same as UTC time, currently they differ by 18 leap seconds. – user10216038 Apr 28 at 16:27
  • What are you using to view the EXIF Info? There are a significant number of EXIF fields that are there but not displayed by many or most viewers. – Michael C Apr 29 at 7:38
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This question has sort of already been asked.

  • You can determine if the time in the Exif is the time when the shutter opens or when it closes (when it opens makes things a bit simpler because you won't need to factor in exposure time later).

  • before you start shooting at satellites, you can take a picture of an accurate clock, reading the time on the picture and looking at the time recorded in the Exifs you can have a good approximation the camera's clock drift. Your phone (if its clock is controlled by your provider), your PC (if automatically sync'ed with time servers), or specialized web sites. It's all the better if the clock displays tenths of a second, but with just seconds, shooting a burst and checking between which images the seconds change will give you 1/N accuracy (where N is the burst rate), but the EXIF time may be truncated to the second anyway.

A different solution is to trigger the picture from another device (good use for a Raspberry Pi or equivalent), that both triggers the camera and records the accurate time.

  • Ooh, any tether capable application (like Darktable) could do that. Excellent! – user10216038 Apr 29 at 0:48
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Place a precise and accurate clock in the field of view of the camera.

  • That can be problematic for astrophotography when the clock would be either too dark (not illuminated) or too bright (illuminated) relative to the night sky. – Michael C Apr 29 at 7:35
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    and what would its face look like on a 10 second exposure :) – laurencemadill Apr 29 at 11:09
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Your question got me to wondering, so:

There is an official Nikon product GPS Unit GP-1 and 1A that plugs into many (not all) Nikons to provide GPS metadata, GP-1. There are also several 3rd party makers such as di-GPS.

I was unable to find specifications stating the precision that was recorded but this image back display only shows seconds.

@ths's suggestion to put a clock in the field of view made me wonder about your shutter speed. Any clock precison, GPS or physical, is going to be blurred (logically or visually) over your exposure time, so now you would need to know whether your precision time is at the beginning, end, or somewhere else in your shutter exposure.

It's an interesting project and I'd like to hear how it develops.

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With the Canon cameras I have used, the EXIF info includes several fields labeled 'Subsec Time', 'Subsec TimeOriginal', and 'Subsec TimeDigitized'. These fields do not appear to be in the Maker notes section. They are not displayed by most EXIF viewers. There are a significant number of EXIF fields that are there but not displayed by many or most viewers.

When viewing .cr2 files using Irfanview, the three 'Subsec Time' fields are displayed with all three having the same value. The field is populated with a value from between "00" to "99".

enter image description here
The above EXIF info indicates the image was captured at 2019:03:02 12:39:30.17

Bursts taken at several frames per second indicate that the "Subsecond" value recorded is in units of 1/100 seconds. For instance, when the camera is set to shoot at 6 fps, the "Subsec Time" offset between each frame in a burst is usually 16 or 17. One-sixth of a second is 166.667 milliseconds. Combined with the standard "Date Time" tag, this will indicate the time the image was recorded to a precision of 10 milliseconds.

Of course this is all expressed in the camera's clock time.

If the camera has built-in or add-on GPS capability, you should be able to sync the camera's clock to the GPS signal. Be sure to compensate for the current offset between GPS and UTC. It varies over time. That's as practically accurate as you're likely to be able to do unless you use your own properly synced atomic clock (not just a clock that receives radio signals from an atomic clock, but rather an actual atomic clock).

If the camera does not have internal or external GPS capability, you would need to perform a sync with a time server to sync the camera's clock as closely as possible to UTC. I use EOS Utility with the camera connected to an internet connected PC. This gets very close. Immediately before taking your images, you could measure the difference between your camera and an active GPS device By taking a photo of the GPS device. This should get you to within at least 1/10 second if the GPS device displays in increments that small. You would then get the offset when comparing the time stamp (including the "Subsecond" fields) in the EXIF Info with the time indicated by the photo of the GPS device.

¹ When single images are taken, there do seem to be an inordinate number of images with 'Subsec Time' values of "00" with certain camera models (7D, 5D3) while other models (5D2, 7D2) seem to not do this. When more than one image is taken within the same second, or sometimes even within a few seconds of one another, the 'Subsec Time' values are more evenly distributed for all of the camera models. This may be an issue with the way Irfanview is reading the info. The cameras that do not show an inordinate number of '00' entries also show the correct 'Sequence mode' ('Drive mode' in Canon parlance) as 'Continuous'. For the camera models with a high number "00" entries, Irfanview shows 'Single or Timer' as the 'Sequence mode' even when set to 'Continuous' drive mode. YMMV

  • Subseconds are also available in the photos from my smartphone and my 450D., but not in pictures from my ancient Lumix FZ8. – xenoid Apr 29 at 9:35
  • @xenoid Yeah, I looked back at files from a Canon Rebel XTi/400D (a camera introduced in 2006) and it doesn't seem to generate them either. – Michael C Apr 29 at 13:42

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