Histogramme luminosité et RVB

Community Beginner ,
Aug 13, 2022 Aug 13, 2022

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Bonjour, je dois envoyer des photos à un imprimeur pour un livre et je veux être sûr de bien les préparer. Il ne me donne comme consigne que de les mettre au format sRVB.  Dans Photoshop, à quel histogramme dois-je me fier pour être sûr d'avoir une exposition correcte : luminosité ou RVB ? Car ils sont nettement différents. Souvent l'histogramme luminosité est correct: les noirs ne sont pas bouchés et les blancs ne sont pas brûlés, alors que l'histogramme RVB indique une sous-exposition (le plus souvent), d'une ou plusieurs couleurs. Merci.

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Community Expert , Aug 13, 2022 Aug 13, 2022

The Histogram tells you nothing about actual exposure if it's from a raw. No Adobe software provides a raw Histogram. Your last screen capture shows the color, not luminance (that's what one Histogram mode is called even though it's not luminance). Again, the video explains the differences. 

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Community Expert ,
Aug 13, 2022 Aug 13, 2022

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You can't trust either, they don't show the full story.

 

This is the histogram view you should use:

histogram.png

 

And I suspect this is how your channels are actually distributed. Note the low end clipping in the blue channel. This is gamut clipping. It is not, as you might think, caused by the blue sky. It is caused by the deep yellows in the shaded foliage. The blue channel "wants" to go lower to represent this very deep yellow, but can't. So it clips.

 

The RGB histogram just averages out the channels. The luminance histogram weights the channels in a way that approaches the Lab L channel (long story). Either way, they don't give you the full picture.

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Community Beginner ,
Aug 13, 2022 Aug 13, 2022

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Do you mean that the luminance histogram doesn't show if there are details in the shadows and the highlights? That's my concern. Actually the colors histogram looks like this. It's much more exposed to the left than the luminance histogram, for blue, green and red layers as well. Does'it mean that this photo is underexposed, even though the luminance histogram is not? Thank you. 

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Community Expert ,
Aug 13, 2022 Aug 13, 2022

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The Histogram tells you nothing about actual exposure if it's from a raw. No Adobe software provides a raw Histogram. Your last screen capture shows the color, not luminance (that's what one Histogram mode is called even though it's not luminance). Again, the video explains the differences. 

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"

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Community Beginner ,
Aug 14, 2022 Aug 14, 2022

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I watched the video and I understand that the histogram that reflects the exposure is the luminosity histo, not the RGB. Thank you for this very helpfull video. 

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Community Expert ,
Aug 13, 2022 Aug 13, 2022

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The histogram is good for info on the overall image, however, I would suggest that you check or set the tonal range with the info panel. I use a colour sample tool with say a 3x3 or higher average in the lightest and darkest areas of the image that contain detail and then set them to something like 245rgb and 10rgb (presuming neutral values) via levels or curves.

 

https://helpx.adobe.com/photoshop/using/making-quick-tonal-adjustments.html

 

https://jkost.com/blog/2018/03/the-eyedropper-color-samplers-and-info-panel-in-photoshop-cc.html

 

https://www.ledet.com/margulis/Makeready/MA21-Defanging.pdf

 

https://www.ledet.com/margulis/PP5E_Ch03_r1.pdf

 

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Community Expert ,
Aug 13, 2022 Aug 13, 2022

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quote

presuming neutral values


By @Stephen_A_Marsh

 

Generally I agree with Stephen, but in this case I suspect the problem is saturation rather than tonal values. In other words, gamut clipping. The attached example looks like a classic case of low blue clipping.

 

The histogram is IMO underrated as a diagnostic tool. It doesn't say anything about how the image looks - but if something's wrong, it can often tell you precisely what in a single glance.

 

The most important thing to watch is not overall value distribution, but how the channels are distributed relative to each other. That's very often the smoking gun.

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Community Expert ,
Aug 13, 2022 Aug 13, 2022

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This may help with your question(s) about brightness, saturation etc (which has nothing to do with Exposure):

 

Everything you thought you wanted to know about Histograms
Another exhaustive 40-minute video examining:
What are histograms? In Photoshop, ACR, Lightroom.
Histograms: clipping color and tones, color spaces, and color gamut.
Histogram and Photoshop’s Level’s command.
Histograms don’t tell us our images are good (examples).
Misconceptions about histograms. How they lie.
Histograms and Expose To The Right (ETTR).
Are histograms useful and if so, how?

Low rez (YouTube): http://www.youtube.com/watch?v=EjPsP4HhHhE
High rez: http://digitaldog.net/files/Histogram_Video.mov

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"

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