2007.07.03 18:37 "[Tiff] BigTIFF extension?", by Phil Harvey

2007.07.04 18:02 "Re: [Tiff] 16-Bit-Per-Channel Lossless Compression", by Stephen Carlsen

Yes, using something like these schemes my 57.7 MB image can be compressed to 39 MB, using an image that is probably pretty close to a best case for that camera.

You might want to have a look at openEXR and the compression schemes it uses (most of them lossless). They have been designed for 16/32bit float and real world imagery (i.e. film, which is close enough to photographs). None of them achieve 1:2 though. You're lucky if you manage to shave off 30%-40%.