Role of sensor in color rendering?

Started 3 months ago | Discussions thread
JimKasson
JimKasson Forum Pro • Posts: 29,184
Re: Sensor Cross-Talk

lélé wrote:

JimKasson wrote:

lélé wrote:

But do you think that, for example, the hue shift towards a 'slightly greenish brown' of caucasian skin tones when viewed from an angle and in the shadow is really caused by cross-talking?

Brown isn't a hue. There's a green hue shift, and something else. What is that something else? Luminance? Desaturation?

Measure the skin tones in Photoshop in CIELab. Make the skin tones look right. Then measure again. Post the two sets of CIELab values.

When you say at an angle, do you mean that the lens axis is not orthogonal to the person's skin? Maybe you should post an example photograph.

Shadows are usually bluer than sunlit areas, because they are lit by the sky.

Jim

Hello again Jim!

Got the Sony a7R IV before lockdown, so limited pictures library for the moment. All pictures with skin tones I got are family portraits, so I won't post them on a public Internet forum.

That said, here is an example from DPReview samples that illustrates what I'm talking about:

https://www.dpreview.com/sample-galleries/0635778599/sony-a7r-iv-sample-gallery/8559074145

Here is what it's looks like converted with a matrix only color profile, then exported to sRGB:

With color patches (from 11*11px average):

link to full resolution : https://ibb.co/JkYpfLy

Do that again and put in the Lab values.

Of course, I do expect lightness, but also hue and saturation variations accross her face. Due to natural variations in her skin color. Due to her makeup (she obviously has some). Due to the lighting (the main light may have a color cast). Due to the angles between light sources, skin surface and camera. Etc. In addition, her skin may be catching some indirect light with strong color cast, scattered/reflected by her surrounding. Finally, we don't know the particular lighting that was used for this shot.

With all these reservations about that particular example (please take it as an illustration, not more! ), I'm surprised how skin hue shifts towards green when viewed from an angle. This is something I noticed in almost all my familly portaits, no matter they were shot under direct sunlight, shadow, in halogen light (yeah, I still have halogen bulbs at home) or flash. And it's, in my opinion, unpleasant.

This is attenuated when using a color profile with LUT (very same than previous one except LUT, generated with Lulariver from a ColorChecker Digital SG shot under midday natural light). But in order to do so, I have to allow 'sharp bends' in the LUT (Lumariver has two settings for that), because light skin tones patches of the CCSG lean towards negative hues shift while dark skin tones patches leans towards positive hue shift. My understanding is that this may result in some posterization...

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