About CRF (Constant Rate Factor)

CRF (Constant Rate Factor) is the primary quality/size control setting for h.265 and h.264 encoders. It adjusts the trade-off between file size and visual quality by dynamically allocating bits where needed. It’s on by default, and is a major contributor to the size savings from h.265.

CRF is not the same as bitrate. It adapts per scene, saving space in simpler frames while preserving detail in complex ones.

NanoEncoder defaults to 28 CRF, the same default as FFmpeg. I personally use 23. Of course, that means I’m fine with slightly larger files. If you want to keep as much data as possible, try 17-18 CRF. (Read more about pseudo-lossless here)

Again, too low of CRF may actually INCREASE the size of your file!