Quality Control — CRF, Bitrate, and the Tradeoff

Two ways to ask for quality

Software video encoders accept two different quality controls, and you have to know which one to use:

CRF (Constant Rate Factor) — "keep this quality, file size is whatever it ends up." Lower number = better quality. Ranges 0 (lossless) to 51 (terrible). For libx264 the sweet spot is 18–23. For libx265 use 22–28 (HEVC's CRF scale is offset). For libsvtav1 try 25–35.
Bitrate (-b:v) — "give me this many bits per second." Quality is whatever fits. Used when you have a target file size or are streaming under a network cap. Hardware encoders (videotoolbox, nvenc) usually require bitrate mode; they ignore CRF.

Default to CRF

For 95% of one-off jobs (uploading to YouTube, archiving, sending to a friend), use CRF. You ask for a quality and let the encoder figure out the bitrate. For libx264, CRF 18 looks visually transparent; CRF 23 is fine for casual viewing.

Use bitrate when you must (live streaming bandwidth caps, hardware encoders) or for two-pass encoding (Track 3 lesson 6) where you want a precise final file size.

Code

CRF — quality target·bash

# Visually transparent, larger file (libx264)
ffmpeg -i in.mp4 -c:v libx264 -crf 18 -preset slow -c:a copy out_quality.mp4

# Web-friendly, ~half the size (libx264 default)
ffmpeg -i in.mp4 -c:v libx264 -crf 23 -preset slow -c:a copy out_web.mp4

# Same idea for HEVC — note the higher CRF for similar perceptual quality
ffmpeg -i in.mp4 -c:v libx265 -crf 24 -preset slow -tag:v hvc1 -c:a copy out_h265.mp4

# AV1 (slow encoder, smaller files)
ffmpeg -i in.mp4 -c:v libsvtav1 -crf 32 -preset 6 -c:a copy out_av1.mp4

Bitrate — file-size target·bash

# Single-pass, target ~5 Mbps video
ffmpeg -i in.mp4 -c:v libx264 -b:v 5M -maxrate 5M -bufsize 10M -c:a copy out_5mbps.mp4

# Hardware encoders use bitrate by design
ffmpeg -i in.mp4 -c:v h264_videotoolbox -b:v 8M -c:a aac -b:a 192k out_hw.mp4

Quality Control — CRF, Bitrate, and the Tradeoff

Two ways to ask for quality

Default to CRF

Code

External links

Exercise

Progress

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