I took a folder with about 32 minutes of footage (it was about 47,400 frames, at 24p that is about 32 minutes). At the varying resolutions I've been shooting the 5D Mark II at, finder told me that the folder size was 113GB. I fired up Adobe DNG Converter and told it to compress all of those DNG files using lossy compression, no embedded preview, no fast load data, etc. I believe my settings make for the tightest compression possible while preserving pixel count:
I let it run overnight and I compared the file size this morning. WOW, what a difference. The file size dropped from 113GB to less than 30GB. That's an over 70% reduction in size.
I know some guys will say not to use Lossy compression, but I would bet those fellows are probably amateurs or come from a stills photography background, or both. They will for sure say something like, "Hard drive space is cheap, keep the data!" But in reality hard drive space is not that cheap unless you buy the cheapest consumer drives and have room in your computer or a sufficient number of fast external ports for external drives.
In the world of moving pictures, the use of lossy DNG compression will not affect the end result at all. And a 73% reduction in storage requirement is significant.
Agree? Disagree? Let me know in the comments below!