In this lesson we’ll compare the quality of the internal XAVC S codec against Prores HQ, 422 and LT, and DNxHR HQ, SQ and LB to understand when to choose which. We’ll be comparing 4K but the same holds true for XAVC S as well.
Here are the things we’re going to test:
- Resolution
- Noise
- Dynamic range
- Latency
- Color grading stress test
- Chroma key stress test with motion
Here’s the comparison:
Chroma keying stress test (including motion)
I’d shot this with the original a7S but it holds true for the a7R II and a7S II as well. This comparison is quite telling (click to enlarge):
The green screen stress test is the easiest way to judge footage. You prop up a poorly lit green screen, and wave a hairy object in front of it. Then, use a really good keyer (I used Keylight in After Effects, color picked at the exact same pixel) to study the images in various stages of motion.
Here’s what the original scene looked like:
The Prores HQ version is miles better in the keying department. I have included versions for 422 and LT as well, but they are really unnecessary. I have spent countless hours keying images in heavily compressed 4:2:0, and trust me, if you want to shoot good chroma key, shoot Prores HQ or DNxHR HQ/HQX.
Takeaways
There is no difference in resolution or dynamic range between internally recorded video and externally recorded video.
The noise levels are the same, though with external recording you do lose a lot of the posterization artifacts caused by heavy compression.
The externally recorded footage does slightly better under extreme color grading, but I believe that’s a result of the heavy compression of XAVC S more than any other factor.
Even though the external recording is 8-bit 4:2:2, it doesn’t really mean anything practically.
There is a one-frame delay between the internal XAVC S recording and the Prores versions. This latency is obviously from the camera via HDMI – first of all due to the inherent latency in HDMI, and secondly because it has to trigger the Shogun to record as well. In a 24p timeline, one frame is the equivalent of 42 milliseconds. In practical use, it is a non-issue.
For chroma keying, use an external recorder because it makes a difference.
To work with DNxHR in QT, download the Avid Codec LE pack (free) and restart your computer.
For a quick flowchart on when to pick which codec, read Should you shoot Prores/DNxHD/DNxHR over XAVC S?
