Can you explain by which export you get a dng of about 28 MB with LR?
If I start with a raw .arw file from a Sony A7RIII, which is actually about 42 MB:
In LR, I export this file in dng, it is about 41 Mo. So roughly the same size.
In PureRaw (or PhotoLab in linear export), the exported dng is 152 MB. Which makes sense as explained in platypus’ post.
But beware: a raw .arw file of 42 MB is a file compressed with the Sony algorithm (with very slight losses). The normal size of an uncompressed raw A7RIII (42 Mpix sensor) is 82 MB.
It makes sense that LR exports a dng of equivalent size (about 42 MB) to that of the raw .arw since both are compressed AND it is initially the same raw data not demosaiced. On the other hand, I don’t see how there could be an “even more compressed” dng of 28 MB.
Take a shot of a uniformly painted grey wall in good light and get a smaller file than if you shoot a busy scene in low light. That is what I get with my Canon files. Other cameras might give other results.
It depends if we are talking about the size of the raw file, or the dgn file.
I don’t have the equipment to test other brands or cameras, but in the case of the Sony A7RIII (the example of PeterK56), I can say that:
An uncompressed raw file is supposed to always have the same size, whatever the content of the image (very detailed or not). In the case of the A7RIII, around 81 Mpix. This is true for this camera, but normally for all others, of all brands (at equivalent sensor size).
But as the raw file embeds other data, and in particular the jpeg thumbnail, the final total size may vary slightly depending on the jpeg compression of this thumbnail. The final size of the raw is then around 82 MB.
A compressed raw file (given its size, it is the case of the example quoted above) can vary according to the type of compression (with loss, without loss…). But always in the example above ( Sony lossy compression), its size is around 41 MB. To which must be added the variable due to the jpeg thumbnail, making a total of around 42 MB.
I did a test with the A7RIII, I get the following results:
I can reproduce the significantly smaller DNGs @gerarto . You need to use “Library > Convert Photo to DNG”.
In my test:
Original ARW Sony raw file: 50MB
Converted DNG: 25MB
DNG from PureRaw (DeepPrime): 80MB
I am very tempted to convert all Sony ARW file to DNG in Lightroom, as it saves a lot of space.
I would also like to find a way to compress the DEEPprime DNG, but so far reconverting to DNG using Lightroom upon importing only produces marginal gains (~10% smaller).
As I explained above, if the raw .arw is an uncompressed file, a dng export in LR will result in a smaller file (around half most often).
If the raw .arw file is compressed (Sony compression), then the size is about the same in dng export (and even sometimes a little larger).
The dng file created by PureRaw will be about three times larger than the compressed .arw. This is an unavoidable constraint of this type of linear dng files.
If in your example you started from an uncompressed .arw, we are indeed in this magnitude ratio. If it’s a compressed .arw, then it’s not normal, unless you checked “lossy compression”, which is not really recommended!