Dual native ISO is one of the most confusing topics in modern videography. Almost any professional camera Alexa, Varicam have dual native ISO. So what is it and does it matter to underwater shooters?
Sensitivity and ISO
Most of the confusion stems from the fact film no longer exists. When you had film you could choose different ASA or film sensitivities and once loaded in the camera you were stuck with it until it was finished.
With digital cameras having a memory storage you can flexibly change the ISO but there is some confusion about ISO and sensitivity so let’s have a look at some details.
In the schematic above the film represent the sensor. As with film the sensor has a fixed level of sensitivity that does not change.
The two triangles are gain circuits those will amplify the signal coming from the sensor that is still analog has yet to be converted into digital signal. A camera has typically a single gain circuit but some camera have two in this case we will have a dual gain circuit like the Panasonic GH5s or the Blackmagic Pocket Cinema Camera 4/6K.
Base or Native ISO?
When the gain circuit is set to 1 or passthrough an ISO measurement is taken according to ISO 12232 https://www.iso.org/standard/73758.html
It follows that as the amplifier is in pass through the camera can only have a single native ISO. So the whole definition of dual native ISO is incorrect and this should be called as Dual Gain camera as the sensor really has only 1 ISO. The ISO formula defines speed in lux*sec with the following formula this gives the native ISO of the sensor and then gain levels on the amplifier are mapped to an Ev or Stops scale.
It is worth noting that ISO values as seen on a camera are typically off and real values are lower, this is because manufacturers tend to leave headroom before clipping.
And how do I find out the native ISO of my camera? This is typically not defined clearly but generally is the lowest ISO you can set on the camera that is outside the extended range, where extended really means additional digital enhancement range.
For simplicity this is a snapshot of the GH5 manual where you can see that the native ISO is 200. The extended gain is below 200 and above 25600.
A method to check what gain circuitry is installed in the camera is to look at Read noise graphics from PhotonsforPhotos.
When we look at a camera with a dual gain circuit the same graph has a different shape.
In the case of the Panasonic GH5s the sensor has a native ISO of 160, this is the value without any gain applied. You can also see that at ISO 800 when the high gain amplifier is active the read noise is as low as at ISO 320. This is why there is a common misconception that the GH5s native ISO is 800 but as we have seen it is not.
The GH5s manual mentions a dual native ISO setting, as we have seen this is actually an incorrect definition as the sensor has only 1 native ISO and this is 160.
The first low gain analog amplifier works from 160 to 800 ISO and the high gain amplifies works from 800 to 51200, values outside this range are only digital manipulation.
Gain and Dynamic Range
In order to understand dynamic range, defined as the Ev difference between the darkest and brightest part of the image, we can look at a DR chart.
This chart looks at photographic dynamic range (usable range) so it is much lower than the advertised 12 or 13 Ev from Panasonic but neverthless shows that dynamic range is always higher at the lowest ISO. This may or not be the native ISO, in the GH5s case is actually ISO 80 in the extended range. First of all is not possible to increase dynamic range by virtue of amplification so it is not true that the camera DR will be higher at say ISO 800. So why you find plenty of internet posts and video saying that the GH5s native ISO is 800? It is because of confusion between photo styles, gain and gamma curve.
Dual Native ISO VLOG Confusion
VLOG is a logarithmic gamma curve. https://en.wikipedia.org/wiki/Gamma_correction
When the gamma curve is logaritmics the camera will no longer reach saturation at the native ISO of 160 but will require an additional stop of light. This is explained in the manual where we can see that the values 160 and 800 have shifted to 320 and 1600.
We can also see that when in variable frame rate the camera needs additional gain to record VLOG so the ranges are 320-2500 and 2500 25600. Values above 25600 are not implemented for VLOG because actually the camera has already at 51200.
So what has changed in the situation above are the base ISO of the Low and High gain setting depending on the gamma curve.
The compression of the gamma curve allows further dynamic range to be recorded despite higher noise due to a higher gain applied.
Comparison of Standard Styles and VLOG
From what we have seen before VLOG has higher dynamic range due to gamma curve compression compared to a standard photo style this has been measured by EBU. Full report here https://tech.ebu.ch/docs/tech/tech3335_s29.pdf
In terms of EV or stops HLG has more dynamic range than VLOG however is not grading ready and really is more an alternative to Like709. In this evaluation the knee function has not been activated so the real gap between HLG and Like709 is less than 4.3 Ev.
When it comes to VLOG vs CineLike D we can see that VLOG has a higher maximum exposure of cinelike D however in virtue of the additional gain applied also a higher minimum exposure resulting in 0.4 Ev less dynamic range. However what really matters is the maximum brightness as displays typically are not true black and a lot of the lower darks are just clipped.
Due to the difference of gamma curve and impact on ISO and the in variance of native ISO it is totally pointless to compare a linear style like CineLike with a log one (vlog) at same ISO setting. The comparison has to be done with VLOG set 1 stop higher ISO.
So most of the videos you see on YouTube comparing the two settings at same exposure settings are flawed and no conclusion should be drawn from there.
Because VLOG needs higher gain and higher gain means higher noise log footage in dark conditions may as well appear more grainy than linear photo styles. As VLOG really excels on highlights you need to evaluate case by case if it is worth using it or not for your project. In particular when the high gain amplifier is engaged it may make more sense to use CineLike D so that the gamma is not compressed and there are no additional artefacts due to the decompression of the dark tones.
Underwater Video Implications
When filming underwater we are not in situation of extreme brightness except specific cases and this is the reason why generally log profiles are not useful. However dual gain camera can be useful depending on the lens and port used.
In a macro situation generally we control light and therefore dual gain cameras do not offer an advantage.
For wide angle supported by artificial lights the case is marginally better and strongly depends on the optics used. If appropriate wet optics are used and aperture f-numbers are reasonably low the case for low gain cameras is not very high.
For ambient light wide angle on larger sensor cameras with dome ports dual gain cameras are mandatory to improve SNR and footage quality. This is even more true if colour correction filters are used and this is the reason a Varicam or Alexa with dual gain are a great option. However considering depth of field equivalence you need to assess case by case your situation. If you shoot systematically higher than ISO 800-1250 than a camera with dual gain is an absolute must even in MFT format.