day/night time lapse exposure correction

[paulgodard]

Hi
This is quite a specific usage of LR. I am taking thousands of photos over a long period of time, usually early morning or late afternoon, in the view of mal=king time lapse movies. Because of the nature of the changing light (night-day-night) and because I have chosen to shot in auto exposure with my D7'', I need to find a way to fix the slightly different exposures in post.
I think that LR might help. The intention is to bring the thousands photos into LR, export the settings via LR/Transporter, analyze the data and correct the sudden small variations of exposure which are not part of the smooth change of light (night-day-night), save these corrections into some kind of metatag or xml file, go back to LR and read the corrections, then finally export the whole set of files 'corrected' to create the time lapse movie.
Does anyone has done something similar?
Where is the exposure correction value recorded for a raw file (dng) or for a jpg file? Can it be recorded in the metatag or in the sister xml file?
I am just at the beginning of the project, so I need to know what is possible or not...
Thanks for sharing if you know something...
Paul

 

[Mark Sirota]

Probably the easy way is to keep the files as NEF (raw), so that Lightroom will write out sidecar .xmp files. Adjsut the exposure in one of those, write out the XMP metadata, and inspect the file to see where it put it. Making adjustments should be straightforward.

Jeffrey Friedl did something similar, but for crops rather than exposure: http://regex.info/blog/2''8-'5-2'/82'

 

[Brad Snyder]

Seconding Mark, that info is readily tagged and accessible in the raw/XMP sidecar file, or in the <crs:> XMP segment of a JPG file. I've just had a twiddle, and it easy enough to adjust the parameters in the XMP data with an editor, and then read the metadata back into Lr for either file type. Easy enough for one or two images, it's going to require some thought for 1'''s tho'.

 

[paulgodard]

Thanks guys. That is what I had in mind.
I suppose that the procedure will include the following steps :

• Import all jpg in LR (3-4''' images)
• Force LR to create sidecars xml files for all of these jpg - how do I do this?
• Analyze how to modify the exposure correction settings in the xml file
• Run a script (php in my case) that will change this setting based on the unwanted variation of exposure in each of the xml file
• Force LR to read the new settings from the xml files - how do I do this?
• Export all jpg files with the correction at the final size for the movie
• Create the QT movie with these new corrected jpg

As I am not familiar with xml sidecars, I just need a bit of help there.
Thanks, Paul

 

[paulgodard]

After reading at Jeffrey's blog, it also seems that the principle will also work within the jpg file, without the xml sidecar.
How can I know how the jpg file is structured, especially to know where is located the exposure correction? Any idea?

 

[Bruce J]

If you use something like EXIFTool to examine the jpg file, you will see the exposure setting and can make changes to it w/o having to understand the internals of the jpeg file structure. Good luck,

 

[Brad Snyder]

You can't force an XML sidecar for JPG, that data gets embedded.

As for the where, as Bruce says, Exiftool is the 'gold standard' tool for this job. That said, while fiddling last nite, I was able to tweak the JPG binary file with a simple programmer's text editor. (A dedicated binary file editor would be better....)

I've attached screen shots of the Exiftool view, and a File Snoop view of the pertinent sections of a Lr edited, XMP written, exported JPG file.