How dual native ISO works and why the two base settings have different noise/latitude tradeoffs
Asked 3/26/2024
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I’m trying to understand dual native ISO (or dual gain) on digital cinema and stills cameras.
I understand that shot noise rises with signal, but signal-to-noise ratio generally improves as more light is captured. I also understand that a sensor has a true base sensitivity, and that changing ISO/EI often just changes how the captured signal is mapped, not the amount of light the sensor receives.
What confuses me about dual native ISO is that the camera appears to have two different base settings with different latitude distributions. For example, if a camera has base ISOs of 800 and 3200, how should I think about the noise difference between something like EI 1250 on the 800 base versus EI 640 on the 3200 base?
Are those effectively the same noise level with different tone-curve mapping, or does the high-base mode actually use a different sensor gain/readout path? If so, why wouldn’t you always just use the higher native ISO if it needs less light?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
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First, photosite exposure is simply a ratiometric value of the voltage the photodiode collects (acting as a capacitor in electro conductive mode) versus the maximum voltage it can collect (the full well capacity). So if a photodiode/pixel is measured (read out) at .5mv when it could hold 1mv it is recorded as 50% exposed.
Dual ISO/dual gain places a second capacitor in parallel with the pixel well... in the case of the Z8/Z9 it approximately doubles the capacity. But that also means it has to receive 2x as much light, and there are two capacitors to switch (read noise). Requiring 2x as much light means it has a lower conversion gain... it is less sensitive to light. It is also noisier in dark regions for the same reason, and it may have greater read noise due to the additional readout/switching. The tradeoff is that it has a much greater capacity to record brighter scenes or scenes of a greater dynamic range... which is the reason you would (normally) use a lower ISO.
Note that there are other variants called dual ISO... but they are not "native" sensor sensitivities. And video ISO's tend to be higher than native sensor ISO due to the gamma curve applied to the output.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
2y ago
0
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Dual native ISO is not just a different log curve or EI label. It usually means the sensor/readout uses a different analog gain path (often called dual gain), so the camera is behaving differently before digitization.
The key idea: the sensor itself is not becoming magically more photon-efficient at the higher native ISO. Quantum efficiency doesn’t increase just because you switch bases. What changes is the conversion gain/readout behavior.
In a lower-gain/base mode, the system can tolerate more signal before clipping, which helps highlight latitude, but it generally needs more light and can look noisier in darker regions. In the higher-gain/base mode, the signal is amplified earlier, which improves shadow performance/read noise in low light, but usually trades away some highlight headroom.
So EI 1250 on the low base and EI 640 on the high base are not necessarily “the same noise with different mapping.” They can differ because the camera may be using different analog circuitry and different highlight/shadow allocation.
That’s why you wouldn’t always use the higher native ISO: it is better for low light/shadows, while the lower native ISO is usually better when you have enough light and want maximum highlight latitude.
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