The answer depends somewhat on whether your source is a standard Blu-ray or a 4K UHD Blu-ray, since I see this is posted under UHD Copy.
If it’s a standard Blu-ray (BD-25 or BD-50): A 50 GB BD-R usually has enough space for the full disc, so you can use Clone/Burn mode in DVDFab Blu-ray Copy for a 1:1 copy with no compression and no quality loss. Audio tracks like DTS-HD, DTS:X, TrueHD, and Atmos stay exactly as-is.
If it’s actually a 4K UHD Blu-ray (which this section is for): That changes things. UHD discs are typically BD-66 (66 GB) or BD-100 (100 GB), so backing one up to a 50 GB BD-R will require compression. In that case:
Yes, compression can affect quality. The more you shrink the disc, the more likely you’ll notice a difference — especially in dark scenes, fine grain, and fast motion.
HDR / Dolby Vision is another factor. Compressing a UHD disc may affect HDR metadata or cause it to fall back to SDR, depending on the settings and player. DVDFab UHD Copy can output to BD-50/25, but a compressed copy is not the same as the original.
Audio is usually less affected if you keep the lossless track (DTS-HD Master Audio, etc.), but this depends on the copy mode you choose.
Bottom line:
Standard Blu-ray → 50 GB BD-R: generally no compression needed, no quality loss.
4K UHD → 50 GB BD-R: compression required, and you will likely see some quality drop compared to the original.
If you want to preserve UHD quality without compromise, your best options are backing up to a BD-100 disc, saving as an ISO on a hard drive, or using a media server instead of shrinking to BD-50.
