Why are some DNG files smaller than the original camera RAW files?
Asked 10/27/2014
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I converted some camera RAW files to DNG and noticed that the DNGs can be smaller than the originals. For example, some Canon CR2 files became about 14% smaller after conversion, while some Sony A6000 files actually became larger.
How does DNG end up smaller in some cases, and why don’t cameras always write smaller files themselves?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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You don't say which camera(s) you're using for the original RAW files, but in general when creating a DNG file on your computer, you've got the enormous processing power of your computer which can be used to drive the most sophisticated compression algorithms around. Compare this with your camera, which has much less processing power and can therefore use only a simple compression algorithm, if any. For instance, Canon RAW files seem to use some form of simple Huffman code (based on analysis of the DCRaw source) and older Pentax cameras produced uncompressed DNGs.
Or put another way: the difference isn't "RAW" vs DNG (RAW in quotes because it's a concept rather than a format), it's "file produced on your camera" vs "file produced on your computer".
Originally by user11371. Source · Licensed CC BY-SA 4.0
user11371
11y ago
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Generated from our catalog & community — verify before relying on it.
DNG isn’t inherently smaller than every camera RAW format. The main difference is often where the file is created.
When a camera writes its RAW file, it has limited processing power and needs to save images quickly, so it may use a simpler compression method—or none at all. A computer converting that RAW to DNG has far more processing power and time, so it can apply more efficient compression.
So the comparison is usually not really “RAW vs DNG,” but “camera-created file vs computer-created file.” That’s why a converted DNG may be smaller than a Canon CR2 in one case, but larger than a Sony RAW in another: it depends on how efficiently the original camera format was already compressed and how the DNG conversion was done.
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