I've been using Vuescan for scanning old slides, and Lightroom to process the images, without any problems. Works like a dream, actually. All scans are stored as 48-bit DNG, with a filesize around 115000 kB. But all my 5000 scans so far have been created with a very small JPEG preview, not very suitable when using organizing software like Faststone. Today I became aware that it's possible to change the preview to a full size one, using Adobe DNG Converter. So I tried this, and it seems to work like a dream too. But the filesize of the new DNG is reduced to about 72000 kB, which is a lot. I imported both the old and new file to Lightroom, and despite thorough investigation, I cannot find any difference at all. So I wonder what's lost, if anything? It seems a bit scary with that kind of filesize reduction, so I want to know what actually happens before I convert all my files. I suspect the VueScan file has no compression whatsoever, and the DNG converter result file has. If so, is the compression lossless? Anyone knows for sure?
One of the advantages touted by Adobe is that the DNG converter will reduce the size of the files. I am not an engineer, and I don't have any connections with Adobe. But if the image size (pixel dimensions) is still the same, and you can't detect any difference in quality, then it's probably the DNG converter just doing its job.