Why can't Preview Export 16-bit HEIC images?

Question

When I open a Nikon RAW .NRW file (16-bit image) with Preview, and select "File > Export...", when the Format setting is set to HEIC, the Depth option for "16 Bits/Channel" is grayed out. Why? I can export as PNG or TIFF with 16-bit depth, so why not HEIC?

And, what software can I use to convert .NRW files into lossless 16-bit .heic files?

(I am using MacOS 11.3.1)

Answer 1

Apple doesn't appear to support 16-bit HEIC.

I can't find anything official on this, but I can find a whole lot of user complaints bemoaning it.

https://discussions.apple.com/thread/251072024

I'd be seriously tempted to try Nikon's own ViewNX-i [freeware] to do your conversions. Though as yet it doesn't support HEIF at all, it is far better at interpreting Nikon RAW files than any other app. Overall you will probably lose less information going NRW > ViewNX-i > PNG than you would via Preview, or even Photoshop.

BTW, NRW isn't 16-bit, it's 12-bit. Nikon's higher spec RAW file NEF is 12 or 14-bit.