EXIF Data Definition

[CharlesHouston]

As a long time EXIFTool user/fan, thanks. I use EXIFTool on a variety of Macs for astronomy, I take ten second duration exposures of satellites going by overhead. Sadly, I need to know the time as precisely as possible and this is a tough problem. Satellites travel at 17,500 mph and a tenth of a second is a LONG time. FYI I also use an JEXIFToolGUI and don't run much on the command line.

I use a Nikon D300s and a D200.

How much information is available (I have done searches, I have talked to Nikon in New York City) about what the various EXIF values really mean? I think that on the D300s, the "time of exposure" is when the first curtain pulls all the way across the image plane. The curtain probably travels across the plane "very quickly" so I understand what the time of exposure is. But I am certain that a "ten second" exposure is Not ten seconds to within a tenth of a second.

Do you have any insight?

Charles
amateur astronomer
C++ software writer

[Neal Krawetz]

Ah, the Nikon D300. I've seen more passport photo forgeries from this camera than anything else...

Here's a sample of real D300 metadata (exiftool -g):

Code Select Expand

---- EXIF ----
Make                            : NIKON CORPORATION
Camera Model Name               : NIKON D300
Orientation                     : Horizontal (normal)
X Resolution                    : 300
Y Resolution                    : 300
Resolution Unit                 : inches
Software                        : Ver.1.11
Modify Date                     : 2022:12:23 14:22:50
Artist                          :
Y Cb Cr Positioning             : Co-sited
Copyright                       :
Exposure Time                   : 1/40
F Number                        : 22.0
Exposure Program                : Manual
ISO                             : 200
Exif Version                    : 0221
Date/Time Original              : 2022:12:23 14:22:50
Create Date                     : 2022:12:23 14:22:50
Components Configuration        : Y, Cb, Cr, -
Compressed Bits Per Pixel       : 4
Exposure Compensation           : 0
Max Aperture Value              : 4.0
Metering Mode                   : Multi-segment
Light Source                    : Unknown
Flash                           : No Flash
Focal Length                    : 70.0 mm
User Comment                    :
Sub Sec Time                    : 86
Sub Sec Time Original           : 86
Sub Sec Time Digitized          : 86
Flashpix Version                : 0100
Color Space                     : sRGB
Exif Image Width                : 4288
Exif Image Height               : 2848
Interoperability Index          : R98 - DCF basic file (sRGB)
Interoperability Version        : 0100
Sensing Method                  : One-chip color area
File Source                     : Digital Camera
Scene Type                      : Directly photographed
CFA Pattern                     : [Red,Green][Green,Blue]
Custom Rendered                 : Normal
Exposure Mode                   : Manual
Digital Zoom Ratio              : 1
Focal Length In 35mm Format     : 105 mm
Scene Capture Type              : Standard
Gain Control                    : None
Contrast                        : Normal
Saturation                      : Normal
Sharpness                       : Normal
Subject Distance Range          : Unknown
GPS Version ID                  : 2.2.0.0
Thumbnail Offset                : 33380
Thumbnail Length                : 9529
Thumbnail Image                 : (Binary data 9529 bytes, use -b option to extract)
NOTE: This isn't from an astronomy photo, but the EXIF data should be similar.

Most of the EXIF data is pretty straightforward. The more interesting fields:

• Exposure Time: How long the shutter was left open, in seconds. For a space photo, this should be long (easily over a second, if there is proper darkness and stabilization, then sometimes minutes).
• F Number: This identifies how big the iris is and how much light can enter.
• ISO: Film sensitivity. Higher numbers are more sensitive. For long exposure astronomy, this is usually really low. (Digital cameras don't use film, but the sensor can approximate film sensitivity.)
• Focal Length and Focal Length In 35mm Format: What is the focal distance?
• Exposure Program: Were the ISO, F-number, and exposure time auto detected or set manually? (AE is auto exposure)

Are there any other specific EXIF fields you are interested in?

[Phil Harvey]

From Neal's output it looks like this camera stores subsecond times to 2 decimals (1/100 sec), so this is the most accuracy you can hope for.

To calibrate the camera time, I would recommend taking a picture of a GPS-synchronized clock that displays 0.01 seconds or better (ideally milliseconds).  But I'm not sure how to best find a clock with that resolution/accuracy.  clock.zone claims 0.01 sec accuracy, but it is offline for me now.  This site claims 0.1 sec accuracy.  You should be able to find others if you look.

Now the problem of calibrating for long shutter times.  This is tricky.  You could put a chopper wheel between the camera and the clock and pan with the camera to get multiple images of a digital clock across the whole exposure time.  This could be a fun little project, but then you could figure out exactly when the shutter opened and closed (it will probably take a lot of trial and error to set it up so you can see the clock time at the start and end of the shutter opening, and will for sure be necessary to take a separate picture for the start and one for the end.  But before you go to all this trouble, you should verify the accuracy/repeatability of your camera clock using a high-speed shutter (1/1000 or faster).

- Phil