Practical Use of Sensor Dual Conversion Gain (DCG) and ISO Invariance Measurement

1. Physical mechanism and charge conversion efficiency of dual gain circuit (DCG)

Traditional sensors use a single readout circuit. At low ISO, large-value capacitors must be used in order to accommodate high charge. However, this will cause the weak dark light signal conversion voltage to be too low and the readout noise to flood the dark parts. At high ISO, if the circuit noise is not reduced, the picture noise will be severe:

1. Low Conversion Gain - LCG:

• Connect to a large capacitor: When the photoelectrons accumulated in the photodiode are fully charged, they will not overflow quickly and have an extremely high full well capacity (Full Well Capacity).
• Applicable environment: Daytime with plenty of light, providing extremely wide dynamic range (usually up to 14 or more stops).

2. High Conversion Gain Mode (High Conversion Gain - HCG):

• Switching small capacitance: When the camera sensitivity increases to a certain threshold (such as ISO 640 or ISO 800 for common Sony full-frame sensors), the microprocessor disconnects the secondary capacitance through the transistor, and the loop capacitance instantly becomes smaller.
• Voltage amplification effect: According to the formula $V = Q / C$, when the capacitance $C$ decreases sharply, the same number of photoelectrons $Q$ is converted into an analog voltage signal several times higher, causing the useful signal to significantly jump above the noise floor of the readout circuit.
• Dynamic range bounce: Measured dynamic range at ISO 800 tends to paradoxically bounce back even higher than ISO 400 or ISO 500.

2. Scientific definition of ISO Invariance

1. What is ISO immutability:

• If a camera has extremely low and constant readout noise, raising the ISO (analog gain) by turning the wheel inside the camera will produce almost exactly the same picture noise and signal-to-noise ratio as shooting underexposed shots with a low ISO on the camera, and then raising the exposure slider (digital gain) in post-production RAW development software such as Lightroom.

2. Comparison of in-camera brightening vs post-processing brightening:

• Increasing ISO shooting in-camera: Highlights are amplified and clipped directly before ADC analog-to-digital conversion, causing the sky and light highlights to be completely white and lost.
• Maintain low ISO underexposure to protect highlights + boost shadows in post: not only the noise in the dark areas is the same as the in-camera improvement, but also the extreme highlight details are recorded intact in the RAW file, and the overall available dynamic range is greatly expanded.

3. Measure the ISO invariance and double gain critical point of your camera

Follow these scientific steps to test whether your camera has high-level ISO invariance:

1. Fixed test platform:

• Mount the camera on a stable tripod and point it at an indoor, high-light-ratio still life scene with bright light bulbs and dark shadows.
• The lens is set to manual focus (MF), the aperture is fixed at f/5.6, the shutter is fixed at 1/125 seconds, and the white balance is manually fixed at 5500K.

2. Stairway shooting comparison group:

• Shot A: ISO 100, underexposed 4 stops (very dark picture).
• Shot B: ISO 1600, normal exposure (same shutter aperture, just raising the ISO to zero the exposure meter).

3. Later Normalization Alignment:

• Import two RAWs into Lightroom.
• Add +4.0 EV exposure compensation to shot A (ISO 100); keep 0 EV for shot B.
• Zoom in to 200% to observe the noise particles and color reproduction in the dark parts: If the noise structure in the dark parts of picture A is almost the same as that of picture B, then the camera has excellent ISO invariance in the ISO 100 to 1600 range; if picture A shows severe color noise, horizontal stripe noise and color cast, it means that the camera model has high analog amplification pre-readout noise.

4. Practical exposure rules for high dynamic starry sky, large light ratio music festival and night scene

1. Crossing the HCG Switching Ladder:

• If the camera switches to HCG at ISO 640 (like many Sony / Nikon Z cameras), never set to ISO 500 in extremely dark environments! Because ISO 500 is still running in the LCG high noise floor mode; directly set to ISO 640 or ISO 800, the microcapacitor conversion gain is immediately triggered, and the noise is significantly lower than ISO 500.

2. The starry sky and landscape leaflet is saved by a large light ratio:

• There is no light in the landscape, there is a shimmer in the starry sky, and there is a weak tent light in the high light on the ground.
• Set at ISO 800 (HCG starting point), shutter 15 seconds, to prevent overexposure of the tent lights.
• In post-production, the shadows of the landscape were separately brightened by 3 levels. The ground details were clear and pure without color stripes, while the color and texture of the tent lighting were perfectly preserved.

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