But in terms of x264 - quality is set by the CRF value. CRF-19 will always produce the same quality (roughly speaking). With slower values (the so-called right values) it will produce the same quality as the “fastest” preset but at a (slightly) lower bitrate. You can’t feed settings into x264 that will magically make CRF-19 look like CRF-16.
^ This is absolutely true, You can’t feed settings into x264 that will magically make CRF-19 look like CRF-16. However, you can choose to do a multi-pass VBR encode INSTEAD of using a CRF value. If we choose the exact same average bit-rate as CRF-16, the file size will of course be the same, and the quality (according to the documentation) will probably not be visibly or measurable better. But, if CRF 17 produces a 21 GB file and CRF 16 produces a 25 GB file, then a 2 Pass encode could be targeted to produce a 23 GB file which would still fit on a single layer disc, but would be slightly higher quality than CRF 17 - call it CRF 16.5. Also, there are many other x264 options that can be adjusted from the defaults which may improve picture quality at the same file size. If anyone knows what they might be, we’re open to suggestions. Jan, I’m testing your suggestion out right now.
I will test as many options as I can (and more than one encoder) on a short section of the film, including what it might look like if it was a full BD-50 (not that we are likely to go that route - unless the 25 GB vs 50 GB comparison is like night and day), and post the results somewhere.