This should give approximately the same file size as before, but better visual quality! Another important advantage of CRF mode is that it will benefit from adaptive quantization, something that QP mode can't do. When switching from QP to CRF mode, you may want to slightly lower the quantizer. It's recommended to prefer CRF mode over QP mode, although CRF is a bit slower. Therefore the CRF mode should give the same subjective quality as QP mode, but it usually achieves a significant higher compression.

For example it will raise the quantizers in “fast” scenes where the loss won't be visible anyway and lower the quantizers in “slow” scenes. The advantage of the CRF mode is that it suits the human perception much better than the QP mode. Internally CRF mode uses the same ratecontrol algorithm as x264's ABR mode, only without a target bitrate. To be more precise, this mode encodes at a constant “rate factor”, which is derived from the specified quantizer. It basically works similar to the QP Mode (see above), but it will encode with an average quantizer instead of a constant one. Single Pass - Constant Rate Factor: This mode is also known as the “CRF Mode” or “Constant Quality” mode.

Avidemux tutorial