HLG: The Lazy Person's Way to HDR, but...
Camera manufacturers (and Atomos for its Inferno) are now offering an HLG mode in camera (or recorder) that results in video files that if played on HLG-enabled TV's will be HDR. No grading, nothing to do - instant HDR! And reportedly the HLG files look really good shown that way. Sounds great. Especially if you don't know how to grade or use LUTs or are too lazy to do so. You can now shoot in HDR!
But, what if you play the same video clips on a regular monitor or TV that is not HLG, that is REC709 like the monitor you are using right now? Unfortunately, the video looks way worse than if you shot in regular mode. It's not awful, just odd and not nearly as good as if you did not shoot using HLG.
And: it is very difficult to convert the baked-in HLG video clips to REC709 in an editor so the clips might look good on all regular viewers, especially if the HLG file is shot with 8bit highly-compressed codecs (like the Panasonic GH5 in 4K 60P or the FS5 in 4K, both of which offer this option).
So, if you shoot in HLG your files will be exclusively for HLG TV's, and will not look good otherwise.
That is why the preferred route to HDR is to shoot with regular log files (Slog, V-log). They maximize dynamic range (HLG actually has less dynamic range than log files but more than REC709) and the log-gamma clips can be rendered both to HLG (or PQ) HDR or to REC709 in post. You can produce both HDR video and REC709 video. You just have to be willing to actually use an editor to get either.
Here is a link to a really good explanation of HLG, it's advantages and its pitfalls from a person who is an official Sony payee:
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