Is it safe to work with DNG in Lightroom?

Started Feb 8, 2012 | Discussions thread
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eos 5d user
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Is it safe to work with DNG in Lightroom?
Feb 8, 2012

I have scanned slides in Vuescan and got a Vuescan “DNG TIFF-file” of approx 35 MB (2000x3000 pixels/48 bit). I have adjusted the images in Lightroom 3.6 and then used “Update DNG preview & metadata”. The file size decreased to 23 MB, i.e. by 12 MB or by 1/3 of the original file size!

(Normally I have not used “Update DNG preview & metadata”, but I saw an advantage of seeing the Lightroom adjustments in my MS Expression Media 2.)

Now, in Lightroom the file is still said to be 2000x3000 pixels, but in MS Explorer it says approx 700x1024. I assume that this latter might be the size of a new revised Jpg-preview… Then I used “Reset” (and “Update DNG preview & metadata”) in Lightroom and the file size in MS Explorer increased to 2000x3000 pixels but the file size in MB is unchanged!

I thought a dng-file (even if they are made through Vuescan, which is somewhat different compared to dng-files made from “proprietary raw”-files) was impossible to change, so my questions are:

1. Have I lost all original image information from my scan?

2. Is it true that the image information always is safe in my dng-files and that I only can change the “instructions” (Parametric Image Edits, PIE) and perhaps the Jpeg preview?

3. Can I now reset completely and get back to my original file (of size 35 MB)? Or, perhaps it is of no value to reset completely?

4. Can anybody explain what I may have done? Perhaps there is a compression of the dng file hidden in the command "Update DNG preview & metadata"?

5. Has anybody seen anything officially written about the above matter?

I appreciate any information on what I might have done and suggestions on what to do next "to be safe"!

Thanks!
Hans

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