"400 ISO" and "0 db" are just arbitrary marks on a scale that the manufacturer is free to manipulate however they see fit. As Barry pointed out in an earlier post, Panasonic's V-log L uses an asymmetrical log scale (-8/+4), which provides 4 stops of highlight range above a preset "stop-0" reference point. Sony's S-log2 scale (-6/+6) provides 6 stops of highlights above its "stop-0" mark. In both cases, "stop-0" is just an arbitrary point (on the selected exposure curve) where the camera's internal light meter and image encoder are calibrated to register "100%". The point where the sensor physically saturates is typically 2-6 stops higher, depending on how much headroom the manufacturer provides in the user-selected tone curve.
So what, then, is "proper exposure"? In practice, it depends on how you plan to use the footage in post. When you're shooting an isolated, stand-alone event and you want to capture as much dynamic range as possible, ETTR will produce the best results. But what if you're shooting multiple scenes in a wide range of lighting and you want to maintain consistent skin tone exposure across numerous takes? In that case, better results will be produced by setting exposure at a consistent midtone level (e.g. 18% gray) representative of your subjects' complexions. When you do that, highlight range will vary depending on ambient light levels and how dramatically the scene is lit, but skin tones will stay consistent and require less grading in post.
It's when you follow those guidelines that Panasonic's and Sony's claims of extended LOG highlight range (e.g. "400% reflectance") make sense, based on their arbitrary reference points. But when you use ETTR, the crucial reference points are where the sensor, tone curve, and H.264 encoding range saturate, and that's where your own experience with your gear comes into play.
So what, then, is "proper exposure"? In practice, it depends on how you plan to use the footage in post. When you're shooting an isolated, stand-alone event and you want to capture as much dynamic range as possible, ETTR will produce the best results. But what if you're shooting multiple scenes in a wide range of lighting and you want to maintain consistent skin tone exposure across numerous takes? In that case, better results will be produced by setting exposure at a consistent midtone level (e.g. 18% gray) representative of your subjects' complexions. When you do that, highlight range will vary depending on ambient light levels and how dramatically the scene is lit, but skin tones will stay consistent and require less grading in post.
It's when you follow those guidelines that Panasonic's and Sony's claims of extended LOG highlight range (e.g. "400% reflectance") make sense, based on their arbitrary reference points. But when you use ETTR, the crucial reference points are where the sensor, tone curve, and H.264 encoding range saturate, and that's where your own experience with your gear comes into play.
