Rate Control

Constant Quality (CQ) / Constant Rate Factor (CRF)

 In H.264 and H.265, CRF ranges from 0 to 51 (like the QP). 23 is a good default for x264, and 28 is the default for x265. 18 (or 24 for x265) should be visually transparent; anything lower will probably just waste file size. Values of ±6 will result in about half or twice the original bitrate. For VP9, the CRF can be from 0 to 63. Recommended values are from 15–35.

Understanding Rate Control Modes (x264, x265, vpx) (slhck.info)