The physical mechanism of switching between the sensor's native ISO and dual native ISO circuits

1. Physical dilemma and amplified noise of single native ISO

In a traditional digital sensor, a single pixel photodiode converts photons into electrical charge (a weak analog voltage signal).

• Analog Gain: When you increase the sensitivity from ISO 100 to ISO 1600, the physical sensor does not become more sensitive. Instead, the originally weak electrical signal is linearly amplified 16 times by a single operational amplifier.
• Fatal Byproduct: While amplifying the weak light signal, the originally extremely weak background thermal noise of the circuit system is also amplified 16 times, causing the picture under high ISO to be completely swallowed up by dense variegated noise.

2. The electronic revolution of Dual Native ISO/Double Conversion Gain DCG

Modern high-performance back-illuminated CMOS (such as Sony A7S III, FX3, Panasonic S5II, BMPCC 6K) has designed two completely independent physical analog gain channels (double conversion gain capacitors) in the readout circuit of each tiny pixel unit:

1. Low Conversion Gain Reference Circuit (LCG - Low Conversion Gain):

• Using large-capacity capacitors, it has a huge full well capacity (Full Well Capacity) and is specially designed to cope with strong light environments (such as First native benchmark ISO 640 / S-Log3). The dynamic range is extremely high and the ability to resist highlight bloom is extremely strong.

2. High conversion gain reference circuit (HCG - High Conversion Gain):

• When the sensitivity reaches a critical switching threshold (such as Second native benchmark ISO 12800), the semiconductor microswitch inside the sensor is instantly triggered, switching the signal to a brand new dedicated high-sensitivity amplification circuit with extremely high sensitivity and ultra-low parasitic capacitance!
• Shocking actual measurement miracle: In S-Log3 mode, the noise in the dark areas at ISO 10000 is extremely serious; but when you actively push the sensitivity one step higher to ISO 12800, the internal circuit jumps to the second native reference, and the noise in the background of the picture disappears without a trace in an instant, and the purity is exactly the same as the basic ISO 640!

3. Rules for avoiding pitfalls in the best position for practical photography

• Always avoid shooting at the "one stop before the second native switching point": If your camera's dual native switching point is at ISO 2500, then ISO 2000 is the worst trough of the signal-to-noise ratio of the entire camera! It is better to proactively set the ISO to 2500 than to stick to 2000.
• Dual Gain Mechanism in Still Photography (Sony Dual Gain): In high-pixel bodies such as Sony A7R5 and Nikon Z8, although static RAW is not called Dual Native ISO, the bottom layer actually hides the exact same dual gain logic (for example, it automatically jumps into high gain mode at ISO 320 or 500, and the dynamic range suddenly rebounds). Photographers can refer to the sensor metering white paper for accurate positioning.

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