Make a raw file 90% smaller with lossy DNG

Friday 12 July 2024, 16:32 by Nando Harmsen | 2,573 views | 0 comments

This article was translated from Dutch with the help of AI. Read the original.

What if you could reduce a 50-megabyte raw file to just 5 megabytes? You could fit ten times as many photos on your hard drive. There are some disadvantages, though, because compressing to a lossy DNG causes quality loss. What should you look out for?

You can turn a raw file into a DNG file. This is a digital negative that many photographers believe is the only guarantee that you will still be able to edit your photos in the future.

The unedited CR3 file, used to show the effect of lossy DNG on a photo.
There is now also such a thing as a lossy DNG file, which means that your raw file is not only converted to DNG, but also made smaller. This can save you up to around 90% of hard drive space per photo.

Creating a lossy DNG file

There are several ways to convert a file to a lossy DNG. One of them is exporting to a DNG file in Lightroom Classic. You will then see the option 'use lossy compression'. This creates a lossy DNG file.

Creating a lossy DNG file by exporting in Lightroom Classic. You can also do this using a DNG converter or the 'convert to DNG' option in the menu.
Converting a photo to a lossy DNG file does come at a price. Since a (relatively new) JPEG compression method is used, you lose image quality. The mosaic information, the unprocessed sensor data, is no longer in the file, for example, which means it is no longer a raw file.

The differences in size between a normal DNG file, a lossy DNG file, and the original CR3 raw file, compressed in the camera. You can see that the mosaic information disappears from the file with lossy DNG.

The quality of a lossy DNG file

At first glance, there is no difference at all between a normal DNG file and a lossy DNG file. Apart from the file size, of course. What does this mean when editing a photo?

The photo was edited in exactly the same way in both the DNG file and the lossy DNG file. The photo's dynamic range was used to the maximum.
I edited the photo and lifted the shadows as much as possible. This is an edit that demands the maximum from the photo's dynamic range. I exported the edited DNG and lossy DNG files to a full-size JPEG file at a quality setting of 75%.

The differences between a normal DNG file and a lossy DNG file side by side, or underneath each other. There is a clear decline in quality.
If you compare the normal DNG file with the lossy DNG file, you can see a clear decline in quality. The lossy DNG file is more heavily sharpened, shows loss of detail, colour noise and JPEG artefacts.

Applying AI noise reduction

There is something else you need to bear in mind. Since the mosaic information has been removed from the lossy DNG file, modern AI noise reduction can no longer be applied to it. If you still want to use this type of noise reduction, do it before creating a lossy DNG file.

AI noise reduction can no longer be applied to a lossy DNG file. If you still want to do this, carry out this editing step before converting the photo to lossy DNG.
It is striking that when you apply AI noise reduction to a raw file first, the resulting DNG file becomes twice as large. If you then compress the DNG with noise reduction to a lossy DNG file, the file size becomes even smaller. At the same time, there is less colour noise and loss of detail.

The difference between a lossy DNG made directly from the raw file, on the left, and a lossy DNG made only after AI noise reduction. The quality is better.
If you place the lossless DNG with AI noise reduction next to the lossy DNG with AI noise reduction, there is no noticeable difference at all. Even with the relatively extreme edit, both photos look exactly the same.

Future-proof, or not?

Just as you lose the option of using AI noise reduction after compressing to a lossy DNG file, possible future editing options may no longer be available either. This is something to bear in mind if you convert your photos to lossy DNG files.

The point, and lack of point, of saving hard drive space

Is it worthwhile to save 90% of your hard drive space for your photos? That is easy to work out. I am assuming that your raw file is 25 megabytes in size. How many raw files will fit on a hard drive?

On a 1-terabyte hard drive, that would be 40,000 photos. This means that a 4-terabyte hard drive can hold 200,000 photos. A 4-terabyte hard drive costs between 100 and 150 euros.

Suppose you take an average of 20,000 photos per year. You could then get around ten years out of a 4-terabyte hard drive. There is a good chance you will not keep, or need to keep, all those photos. You can delete failed photos, duplicates and photos that are not worth keeping.

Do you never delete photos, and do you also keep the failed and duplicate photos? Then you are simply filling up your hard drive. In that case, those unused photos could be compressed to a lossy DNG file. But if you never do anything with them again, that is still a waste of space.

Conclusion

For general use in a photobook or online, a lossy DNG file is reasonably usable. You cannot see the difference at those small sizes. Certainly not if the edit is minimal.

If the quality of your photo is important and you want to get the most out of it, after all, you do not shoot raw for nothing, a lossy DNG file causes a noticeable loss. This becomes particularly clear with more extreme edits.

Although lossy DNG files can indeed reduce a raw file by 90%, in my opinion it is not worth using them. To save hard drive space, it is not even financially worthwhile.

Do you still want to use lossy DNG files? Then I recommend applying AI noise reduction and editing the photo first. Then convert the result to a lossy DNG.

After that, do not forget to delete the original raw file and improved DNG file. Otherwise, you will still not save any storage space.

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