Video Log Curve Noise Control Core: S-Log3 / C-Log2 Base ISO and Guide to Over-Expose +1.7EV to Prevent Dirty Noise in Shadows

1. The mathematical nature of the Log curve: sacrificing visual intuition in exchange for high dynamic tolerance

Why does the picture immediately become gray and white with no contrast once the camera switches to S-Log3 or C-Log?

1. Linear conflict between human eye senses and digital sensors:

• The image sensor receives photons purely linearly (the amount of light is doubled and the charge is doubled);
• The human eye's perception of highlights and shadows is logarithmically non-linear (more sensitive in dark areas and slower in highlights);

2. Log logarithmic compression algorithm:

• The Log curve forces the highlights to darken downwards and the extremely dark parts to rise upwards, cramming an astonishing dynamic range of up to 14 to 15 stops into the limited space of 10-bit encoding;

3. Why does S-Log3 base ISO have to start from 800? ：

• In order to leave a full 6 stops of highlights above without overflowing the dynamic headroom (Highlight Headroom), the camera directly increases the gain on the hardware analog circuit;
• So the native base ISO cannot be set to 100 and must be forced to lock at ISO 640 or 800 (or even 12800 for dual native)!

2. Why is the screen full of noise when taking a Log shot under "normal exposure"?

Countless novices follow the habit of taking ordinary photos and center the exposure meter at 0.0 EV:

1. Amplifier Tragedy of Dark Stretching:

• In the Log curve, the weak dark signal that was originally at the bottom was forcibly lifted to the middle gray area;
• If the scene is photographed according to normal exposure (0.0 EV), the dark parts receive extremely few photons, and the signal-to-noise ratio is extremely low;

2. The fatal blow when restoring a set of LUTs:

• When the official 709 restoration LUT is applied in the later stage, the algorithm will force the elevated dark parts down back to pure black;
• But at this time, due to the original insufficient exposure, the suppressed dark parts were accompanied by stretched horrible color spots and noise. The entire dark parts were dirty and flowery, and completely undeliverable!

3. Log golden exposure rule: Expose to the right +1.7EV to +2.0EV

When shooting Log, you must thoroughly implement "It is better to go over than to owe, and to be full to the right":

1. The body light meter must be pushed to +1.7 EV to +2.0 EV:

• When shooting on location, the picture was intentionally overexposed by 2 stops compared to the naked eye;
• Ensure sufficient photons to feed the dark pixels of the sensor and push the dark signals away from the underlying noise area (Noise Floor);

2. Use zebra crossing to lock facial skin color (IRE 45%~50%):

• Under S-Log3, use a gray card to accurately calibrate the 18% gray card level at 41% IRE, and the skin color of the light-receiving surface of the human face falls at 45%~50% IRE;

3. One-click downshift and noise reduction in the later stage:

• During post-color grading, manually lower the exposure by 1.7 stops in the node to restore normal brightness;
• Physical miracle happens: Dark noise is completely compressed and buried into deep pure black with exposure suppression, achieving a silky, pure and transparent high-end cinema quality!

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