Originally posted by stevebrot The in-camera DNG also have checksums. It is required by the DNG spec and if missing, the file will be rejected by processing software (hash calculated at runtime will not agree with hash from file). If Adobe-generated DNG from PEF are smaller, it is only because the user chose other than the full-size JPEG preview.
Steve
Adobe software
can't use the checksum in a from-camera DNG to detect corruption, but it
can use that from a converted PEF, so there's something different about the checksums it creates. This allows Lightroom (using "Validate DNG Files") to notice bit rot and other kinds of corruption.
In my tests the DNG produced by ACR (or Lightroom, same engine) was 15-20% smaller than the out-of-camera one, which is far too big a difference to be due entirely to the preview. I suspect that this is because a PC has far more processing power than the computer in the camera, and time is less of a constraint, so it can do a more computation-intensive ZIP compression (the DNG spec allows for different compression modes).
At least this is the case for my K-5 II, I'd be curious to hear what results other models get. The basic file size experiment is easy: take a pictures in PEF, then switch to DNG and take the same picture, repeat a few times, then convert all the PEFs to DNG and compare file sizes. To correct for different-sized previews, you could recreate them in Lightroom with the same settings.