To quote one of DJI's guys from another forum...
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To explain the native ISO of DLOG, let's start with exposure.
1. Exposure
In the imaging engineering point of view, a good exposure is to make the imaging of a 18% gray in the YCbCr domain to be 0.43~0.5 of the white level. So it's very important that the good exposure is measured after gamma, not linear.
2. Gamma
The cameras for photography traditionally use a standard gamma (e.g., Rec709) to compress the pixel to 8bit for digitalizing. So the good exposure means the linear image after a Rec709 gamma, 18% gray will be exposed to 110~128 if the white level is 255. However, as the sensor technology is evolving fast, the dynamic range of a morden sensor is far ahead of standadizing Rec709.
DR=20log10(FWC/Noise), FWC means the maximum capacity of the electrons, and noise means the noise level of dark.
A typical SONY APS-C sensor will have 84dB+ (14 fstops+) dynamic range. The goal of a gamma and exposure is to preserve as large information of the scene as possible, regarding both highlight and darkside. Whether to preserve the highlight or darkside is up to the photographer's tone. Things over-expsoured is forever gone and darkside with noise has chance to recover by strong image processing algorithms. Therefore, the camera vendors make a good balance between the highlight and darkside, by choosing a proper gamma curve, according to their capability of chip and characteristics of sensor.
3. LOG gamma
The cinematography is different from the photography. Cinema camera shoots RAW with 12/14/16 bits, and the footage is generated mostly by post processing software, which is far more powerful than the camera. So given the dynamic range, the highlight preserving is the first priority. Simply speaking, the camera will shoot a under-exposed image and store all the effective bits of the darkside thanks to the RAW, and recover the dark by strong post processing.
Here is a post to introduce the comparison between LOG gamma and standard gamma.
Log is the New Lin
If the LOG gamma is regarded as a kind of digital gain, the cinema camera tends to use the digital gain first. It breaks the rule of analog gain first. Actually the darkside will have dozens of dB amplified when using LOG gamma. Thanks to the strong signal processing algorithms in the post processing software, the darkside is still useable.
4. Native ISO
Usually, when applying the analog gain 1x and LOG gamma, we say the equivlent ISO is called native ISO.
After applying the DLOG, the 18% gray is 2.3 fstops above the normal gamma. Do you remember the exposure is mostly responsible for the 18% gray? Therefore, the exposure should be reduced 2.3ev in order to match the aperture/shutter of other cameras. In our case,
native ISO = Baseline ISO + 2.3ev = ISO500.
That means use the same aperture/shutter/iso, we will make the brightness of 18% gray be aligned with any cameras, no matter for photo or for cinema.
5. More-over
The question is if we can enable 2x/4x/... analog gain? I have to say it's depending on the FPN (fixed pattern noise) of the sensor. Some vendors lock the ISO of LOG while others provide limited but selectable ISOs.
DJI R&D team is testing 2x/4x analog gains with the representative customers. The footage should be projected to a large sceen, under a very strict inspection, to ensure the affordable artifects. So please be kindly patient and we might enable more ISOs in the future.
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The full discussion is at
How to expose correctly if D-Log is fixed to ISO500 !!?? (Ed - presume it's ok to link to it, pls remove if not)