Multishot DNG file half size after conversion

[hirohhhh]

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Out of curiosity, I converted one SL2 multishot photo in Lightroom Classic using Library > Convert Photo to DNG and the file size was reduced by half.

I selected medium preview, but even Full size preview doesn't change the size significantly.

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Original file size was 340MB.
After conversion with medium size preview 150MB
After conversion with full size preview 170MB.

Now, I'm curious, does this somehow affects the file? I'd like to reduce the file size on many multishot images I have in my library, but I don't want to screw something else.

Any ideas to find this out?

Edited October 18, 2022 by hirohhhh

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[jaapv]

As you see, you did not tick "lossy compression", so the conversion will be lossless, which means that you can fully restore the original file.

[hirohhhh]

6 minutes ago, jaapv said:

As you see, you did not tick "lossy compression", so the conversion will be lossless, which means that you can fully restore the original file.

I'm not concerned about the converted file. I'm curious how did it reduce the file size so much if I didn't choose Lossy Compression? Should I convert them all in that case? Why having 340MB files when I can have the same at 150MB? But I want to know what's going on in the background before I do this.

Oh, and btw, I cannot fully restore it to the original file size anymore. After converting, it's always between 150-170MB. No matter what options I tick, it never gets back to 340MB.

Edited October 18, 2022 by hirohhhh

[jaapv]

Not the file size, but the file content. Lossless compression works by removing redundancy. If the compression turns out to be effective, there was a lot of redundancy in the original file which is not surprising in this case.

[hirohhhh]

Got it, but how to know if I'd need this redundancy sometimes in the future? Maybe it's there for a reason.

[hirohhhh]

Converting regular single-shot DNG does not affect the file size at all, except when choosing smaller or larger preview size. Only multi-shot files has a signifficant change in the file size.

[01af]

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A multi-shot capture has 187 megapixels. Each raw pixel is 14 bit. That's 2,618 megabit of image data, or 327.25 megabyte. Add some metadata, some profile data, plus one or two embedded JPEG previews, and you arrive at an image file with approx. 340 MB of uncompressed data.

Converting DNG to DNG using Lightroom, Camera Raw, or DNG Converter will, by default, apply lossless compression to uncompressed data. That's why your files become significantly smaller. By the way, if Lightroom and/or Camera Raw are your preferred image processors then it will make sense to tick the 'Embed Fast-Load Data' option.

A regular capture has 47 MP; at 14 bit per pixel that's approx. 82 MB of raw image data. Add some metadata and embedded JPEG previews, and then you'd get an uncompressed image file of approx. 85 - 90 MB. If your image files are significantly smaller than that then they have been compressed, so Lightroom won't be able to compress them any further (at least not losslessly).

[jaapv]

lol

11 hours ago, hirohhhh said:

Got it, but how to know if I'd need this redundancy sometimes in the future? Maybe it's there for a reason.

Wel, the very essence of redundancy is that it is, errm, redundant.