There is no concept of native ISO a camera has a base ISO that depends on the gamma curve in use
For linear curves that include all normal modes for still pictures and cinelike mode the base ISO is 200.
For logarithmic curves that include VLOG and HLG the base ISO is 400 you cannot go lower.
If you set a minimum ISO of 400 on a normal gamma curve using profiles like natural or cinelike you are increasing noise and in turn decreasing colour depth and dynamic range
For what concerns the gain indicator dB and stops are on different scales the situation is quite weird and there is definitely something going on. When you set a log profile the two scales match 400 800 1600 ISO corresponds to 0 3 6 dB. When you set a linear profile the ISO goes 200 400 800 and the gain goes -6 -3 0 suggesting that 0dB is ISO 800 so I believe there is a bit of bug there when you use extended ISO -12 dB appears. I would completely ignore this gain display as clearly it has issues except in LOG.
In practical circumstances I found that cinelike D with leeming LUT is very useful shooting 10 bit 400 mbps. In situation of bright scenes you can force the camera to ISO 100 this is basically putting an attenuator on the circuit that will give you 1 stop for wider aperture or slower shutter speed so that you get the effect you like. I did some snorkelling in Marsa Alam and I used ISO 100 at the surface with 24p mode and 1/100 shutter speed and the camera stayed below f/13 which is what I wanted. I still occasionally got some clipped highlights (see example here https://youtu.be/T7E9g0z6F0U?t=201)
but overall I was quite happy, by the way skin diving with a video set up is deadly and you just can't get exposure right...
This is basically my understanding as well. I need to find a good article that covers the technical discussion, but in a nutshell, there's no direct relationship between the signal of the CMOS sensor and the ISO. For video, ISO is influenced by the gamma curve of the picture profile used -- ISO 400 in VLOG and ISO 400 in d-cinelike/standard don't produce the same exposure. Changing the ISO doesn't impact the signal from the CMOS sensor, just how the camera processes that signal into the video file. One (somewhat sloppy) way to think of it is that your ISO is a tradeoff between dynamic range, noise, and the distribution of usable dynamic range above and below middle gray. In VLOG, the GH5 can record about 11.5 stops of dynamic range at ISO 400. Go to ISO 800, and your overall dynamic range will decrease to 11 stops or so, BUT you'll get more of that range allocated to the highlights. So, somewhat counter-intuitively, higher ISOs actually protect the highlights a little better. Conversely, if you could shoot VLOG at ISO 200, you'd also lose a bit of dynamic range, but your shadows would be cleaner, so you'd get more of the remaining dynamic range allocated to the shadow regions.
When shooting in d-cinelike/standard however, the gamma curve of the picture profile itself will never allow you to record more than 9-10 stops of dynamic range. So whether you shoot at ISO 200, 400 or 800, your dynamic range is going to more or less be maxed in those profiles anyway. The only difference is that you'll get (slightly) less noise at the lower ISOs. Personally, I find almost no perceptable difference in the footage in terms of either dynamic range or noise in the ISO 100-800 range. So I just use whatever the exposure dictates.
Not to go off on a tangent...
The native ISO (the one where the SNR has been optimised) for the GH5 and GH5s (in Auto or Low mode) is ISO400 - which is where you will get 11-13stops, depending who you speak too.
If you set the camera to show SS/Gain instead of ISO, you will find that 0 Db is equivalent to ISO400. Changing the gain either + or - you will lose dynamic range, so ideally you want to fix that and either play with lighting or aperture.
As Dreifish mentions - ambient light is still bright! the last dive I did over Christmas period, I was shocked how much ambient light there was, as I was running the camera at C4K@30, 1/60, 0Db - and was having to go to f8/f11 to get correct exposure on my waveforms.
Yeah, the ambient light (especially in the tropics) is a big issue for trying to shoot blended-light exposures. Shooting at 60p (and 1/125 shutter speed) helps to keep your aperture around F5.6, which is optimal on a m4/3 sensor to minimize diffraction. I try to avoid F11 unless I really need the increased depth of field for macro (or with a rectilinear wide angle zoom in a dome port). That's why I'm always a little confused when people focus so much on a camera's low-light performance. Maybe they shoot in very different environments than I do. I almost never have to raise my ISO above 800 .
Thanks Andrei! Hope you understand that i am asking to discuss and learn and not to critique
Panasonic states that the native ISO for the GH5 is 400. Dxo numbers are different anyway (eg ISO 100 on the camera corresponds to their ISO 64 if I remember correctly). Most users experiences also agree that ISO 400 they get the best results in terms of dynamic range, color and noise. I am not a pixel peeper so I cant tell. My thinking is that if you work at ISO 400 as a base you still got room to move down the range to ISO 200 if needed and still got very good results.
I understand the third light concept to cover something very close to your port and not wanting to move the lights in but does it have to be so bright since you will be so close?
I understand the reason for wanting to keep the specific exposure settings when it comes to maintaining the quality of the blue color. What IRE are you placing your blues at?
Agreed Vlog is not really needed underwater but its the only way (with the exception of HLG) to produce HDR content (which i think based on your footage you should )
Thanks again for taking the time and for your always informative posts!
I rarely (never) find myself wishing I had less artificial light. Usually the opposite. But yes, technically, if you only use the middle light for situations when you're filming something very close to the lens, it could be weaker. Seeing as I'm usually starved for (artificial) light, I end up turning all my lights on all the time.
The question about the IRE of the water column is an interesting one. I haven't thought about it before.. I usually expose/grade by eye, going for a strong color contrast between warm subjects and blue water column. Looking at my past footage, it seems I like to expose the water column around 25IRE, sometimes a little lower. Almost never above 50IRE. Very curious to hear what other people do.
HDR footage is a whole other topic. In short, I'd like to produce it, but have too many limitations/compromises with my current gear. The biggest is that I don't have an HDR monitor for grading or an HDR display, so can't verify the results. Other minor issues: (a) no option for shooting in HDR and 60p on the GH5, and I favor 60p usually; (b) the 10bit 400mb/s All-I files add up quickly when filming 3 dives a day (150mb/s is already becoming a hassle to manage); and © HDR would likely require significant color grading time, which I don't really have for my day-to-day work. I try to get the colors as close to perfect in camera as I can since I have to produce a 30-45minute film every week