Comparing CRF Settings — Measure Encoder Quality, Don't Guess

The CRF folklore problem

Every encoding guide repeats the same folklore: “CRF 18 is visually lossless, CRF 23 is fine for the web.” Sometimes true — but CRF is content-dependent. A talking-head clip at CRF 26 can look pristine while a confetti-filled concert shot falls apart at CRF 20. The only honest answer for your footage is a measurement.

The two-encode test

1. Take a short, representative slice of your footage — include the hardest content you have (motion, grain, fine detail). A 20–30 second cut is ideal.
2. Encode it at the two candidate settings, e.g. CRF 20 and CRF 24.
3. Compare each encode against the original with DiffALL: upload original + encode, read the SSIM score and the per-second chart.

If CRF 24 scores 97%+ SSIM against the source on your hardest section, the extra bitrate of CRF 20 is buying you nothing you can see — ship the smaller file. If it dips to 90% during the action scene, you’ve found your reason to spend bits, and the chart shows exactly which seconds suffer.

Why per-second beats a single score

Encoders fail locally. An average SSIM of 96% can hide a two-second block-artifact disaster during the one fast pan in the clip. DiffALL scans every second, flags the worst moments, and charts quality over time — so you judge an encode by its weakest seconds, which is exactly how viewers do.

Reading the numbers

• SSIM 97%+ — differences are essentially invisible at normal viewing distance.
• SSIM 93–97% — fine for most web delivery; inspect the flagged seconds.
• Below 90% — visible degradation; raise the bitrate or lower CRF for this content.

The same method settles any encoder question, not just CRF: x264 vs x265 at equal bitrate, preset slow vs fast, hardware vs software encoding, or whether your streaming platform’s re-encode mangled your upload — compare what you uploaded against what it plays back.

Keep clips short

Comparison on the free plan covers clips up to 30 seconds and 25 MB (1 minute and 150 MB with Pro) — which fits this workflow naturally: you want a short, hard test slice anyway, not a full episode.

Try it

Cut a test slice, encode two candidates, and measure them — SSIM, PSNR, per-second chart, and a synchronized side-by-side player. Free, in the browser.