Why does Sony S-Log2 place 18% gray around 32% instead of 50%?
Asked 11/19/2020
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I’m trying to understand what it means when people say S-Log2 records middle gray at about 32%. I thought camera metering aimed for an average scene brightness of 18% gray, and I assumed that would correspond to a mid-level digital value like 50% gray. In plain English, what does the 32% figure mean? If I meter an 18% gray card, does S-Log2 intentionally record it lower than a normal Rec.709-style profile? Why isn’t middle gray at 50%?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
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It's the difference between the linear response of the digital sensor and the logarithmic response of human vision, which is taken into account by display devices such as monitors.
An 18% grey card "properly" exposed is only RGB (127,127,127) after full gamma correction has been applied to the raw data. When using "log" recording, gamma correction is reduced, thus it does not apply as steep a curve, and applying the rest is delayed until post production. In the "log" file (whether Sony, Canon, or Panasonic, etc. - they all do it similarly) a lower gamma value has been used and the recorded values aren't raised as much as they would be with an application of "full" gamma correction. It's up to the post processor to decide how to shape the final gamma curve as well as the "s-curve" at each end.
Note: When viewing a true "raw" still image file on a monitor, you're not actually viewing "THE raw file." You're looking at one of among near countless legitimate possible interpretations of the raw data with demosaicing, gamma correction, etc. applied to that data.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
5y ago
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Yes — in S-Log2, an 18% gray card is intentionally recorded lower than it would be in a normal display-oriented profile.
The key is that 18% gray, 32%, and “50% gray” are not all describing the same thing.
- 18% gray is the reflectance of the gray card in the real scene.
- 32% in S-Log2 refers to the encoded video signal level where that gray card is placed.
- About 50% gray on screen is closer to how a normal viewing gamma (like Rec.709 / typical JPEG-style rendering) makes midtones appear after stronger gamma correction.
Rec.709 is designed to look more “finished” straight out of camera, so midtones are lifted more and white is placed near the top of the signal range. S-Log2 uses a flatter log curve, so midtones and highlights are recorded lower to preserve more dynamic range and avoid clipping. The image looks flatter and darker before grading, by design.
So no, middle gray is not always encoded at 50%. In S-Log2, 18% gray landing around 32% is normal because the final contrast/gamma is meant to be added later in post.
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