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I'm going through some of the settings on a Nikon Z8, and one of the options when shooting raw is to switch the tone mode from SDR to HLG.

In principle, I understand what these two modes do: HLG offers better dynamic range for colours and brightness than SDR in a compressed image format. What I don't understand is whether or not this makes a difference when shooting raw, since as I understand it, 14-bit raw is always going to be 14-bit raw, and those 14 bits of colour information are (in this camera's case) a hardware limitation of the sensor.

In real life, the effect of toggling between these two tone modes is a change in how the live view image is displayed: HLG shows those trademark flat colours, highlights, and shadows often associated with similar video formats like N-Log.

Does HLG offer any benefits when shooting raw stills?

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  • \$\begingroup\$ 14-bit raw values have no "bits" of color information. 14-bit raw values are single monochrome luminance values for each photosite (a/k/a sensels or pixel wells). Since the color filters of a Bayer mask are not the same colors as the R, G, and B subpixels of emissive displays (nor of the CMY colors of printing technology), all color values for every pixel in a displayed image are interpolated from the single luminance values recorded by each photosite. \$\endgroup\$
    – Michael C
    Sep 25, 2023 at 6:06
  • \$\begingroup\$ @MichaelC technically correct, but as far as I am (and likely most prosumer photographers are) concerned, it's effectively 14 bits of colour depth, just not necessarily encoded in the most intuitive way to a layperson. \$\endgroup\$
    – Jules
    Sep 25, 2023 at 22:46

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The use of Hybrid Log Gamma (HLG) when recording .nefs is primarily for the camera to generate a .heif image rather than a .jpeg. Whether singularly, embedded into the raw file, or both.

Unfortunately, it does have some negative (IMO) impacts to the raw file, much as selecting D-lighting does. It affects the metering, resulting in underexposure just like D-lighting does for jpegs (to protect highlights). And it also increases the minimum ISO to 400, which is potentially also additional underexposure (lack of light recorded). This underexposure does affect the raw data; which results in increased noise and a reduced ability to recover shadow details, as one would/should expect. The color space is also changed to rec.2020; but that doesn't directly affect the raw data.

In order to use HLG effectively with raw files (stills or video) you need to pull the exposure up during recording in order to compensate for the raw data underexposure that would otherwise occur; and then recover in post. Because many do not understand this, there is a lot of noise currently about avoiding HLG with still photography.

Here's one example: Steve Perry's HLG-SDR video

While you can compensate for what HLG mode is doing to make effective use of it, and using ISO 400 isn't necessarily/always a negative, I do not personally see any advantage when working with raw still images... I do not do enough video work to have an opinion on that.

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  • \$\begingroup\$ Thanks for the explanation! So in practice, would using SDR be a better approach barring a specific need for HLG (with no generally-applicable penalties to raw shooting)? \$\endgroup\$
    – Jules
    Sep 25, 2023 at 17:13
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    \$\begingroup\$ @Jules, AFAIK there is nothing HLG does that you cannot do manually when set to SDR mode. The negative of using HLG is that it limits your options some (ISO) and maybe doing something the photographer doesn't understand or isn't aware of. The benefit of manipulating the exposure/DR placement in SDR is that everything should be known/intentional. \$\endgroup\$ Sep 25, 2023 at 18:55

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