Histogram clipping indicators do not match Photoshop's

@TheDigitalDog wrote:

LRs Histogram outside of soft proofing is always using Melissa RGB.

In Photoshop, the Histogram is based upon the color space of the rendered document. So you should expect to see a difference.

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That’s the most important point to understand here. It may not be obvious, but the histogram and clipping are measured based on the color space of the document. The applications aren’t actually “disagreeing”; in fact they would agree more closely if set the same way.

The demo below helps to show this. Lightroom Classic and Photoshop histograms shift consistently to follow the color space being used for previewing the image. If you set them consistently, the clipping points should be in consistent positions. Although I didn’t show Adobe Camera Raw, the difference is very obvious there: As you change the Color Space in Workflow Options among smaller and larger color spaces, in the preview window the on-image preview of clipping points shows more or fewer tones clipping. Which is absolutely and technically correct, because in a larger color space, you have more room before tones clip.

I’ll leave it to someone else to explain why the histograms don’t look exactly the same for the same preview color space. Like I’m not sure why the Photoshop histogram for the same image has that bump to the right in the red channel, that is missing in the Lightroom Classic image. For ProPhoto RGB that might be explained by the slightly different tone curve, but I would expect them to be more similar in Adobe RGB.

But that doesn’t change the main point: If you set the preview color space consistently in Lightroom Classic, Photoshop, and Camera Raw, the clipping points should be at about the same place in all three.

LRs Histogram outside of soft proofing is always using Melissa RGB.

In Photoshop, the Histogram is based upon the color space of the rendered document. So you should expect to see a difference.

This might help:

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

These warnings say you are close to clipping, not that you have reached that. And it is clipping of the capacity of different working contexts and colourspaces that is being warned of. The exact same thing could be said of ACR vs PS, but we are used to the notion that those two editing contexts have different practical purposes and usages. And that the practical meaning of clipping won't be expected identical across the two. When two people who both normally wear size ten shoes, both put on size nine shoes - these smaller shoes will likely pinch or rub those two people's feet in different places. Just the same applies, when considering LrC vs PS.

Second, "nipping the edge" of clipping should in my opinion not be considered a workflow panacea (universal formula) anyway. Not all images even mind a little clipping: sometimes the picture works (unexpectedly) better with that in. Contrariwise, not all images do need to span the full extent of the histogram. Depends on what is in the photo, and what is the chosen pictorial representation.