Histogram behavior for a Black and White image.

[Jeffrey Saldinger]

I did a BW conversion of a DNG in Lightroom, then opened it as a smart object in Photoshop, having reset the DNG in Camera Raw to have only the default Adobe BW conversion and no other adjustments.

I noticed what looked like a teensy bit of color (looking like small spikes) along the top edge of the histogram (it was set to “color”).

Wondering whether some color had somehow snuck in (but I doubted it), I applied a Hue/Saturation layer and maximized the “Master” saturation slider (Histogram 1). Then I added a second Hue/Saturation layer and set the “Yellow” slider to maximum (Histogram 2). The image remained Black and White.

Then I used the color range tool (set to select the yellows), thinking I’d see something actually selected, which would explain the yellow in the histogram. But there was nothing selected.

Then, when I checked the histogram after adding a default BW adjustment layer, the colors that had been in the histogram were gone.

I turned off the BW adjustment layer and saved the file as a copy (it was still BW), keeping only the background smart object, and the two Hue/Saturation layers, and the BW conversion layer. When I imported the PSD into my catalog, a large area of it was yellow!

Any thoughts? Thanks.

Lightroom Classic 9.3, Camera Raw 12.3, Photoshop 21.2, Mojave

[digitaldog]

Channel indication and clipping:
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

[Jeffrey Saldinger]

Thank you, Andrew. It was good to go through your video again (you referred me to it in May, after another question about histograms (https://forum.luminous-landscape.com/index.php?topic=135021.msg1172086#msg1172086).

Do you have any further thoughts on the particulars of what I observed that led to my post? After going through your video (especially in the 5 minutes or so beginning at around 19 minutes), I still don’t understand where the color in the histograms I see is coming from, especially because there’s a default ACR BW conversion in the PSD’s background layer (a smart object).

Furthermore, in looking things over again I observed that in Ps the file looks perfectly monochrome at 50% and smaller, but at 66.67% (one zoom click larger) the yellow I saw in Lr yesterday suddenly appears (and disappears when I go back back to 50%).

Is there something at play here, covered in your video, that I didn’t catch?

Thanks.

[digitaldog]

LR/ACR is an RGB engine so even if you 'convert to B&W' there are still three color channels and the Histogram there still shows you them in color (yellow shows blue channel clipping in your one screen shot).
The Histogram in Photoshop can differ and only preview luminance and if set for color, like LR, it can still differ because of the underlying color space and TRC used in LR differs from what you probably provided for Photoshop.

[Jeffrey Saldinger]

Thanks again, Andrew. Your reply has encouraged me to look more deeply into this matter.

I don’t know what TRC stands for. Can you please spell it out?

Appreciatively...

[digitaldog]

Tone Response Curve.

[Jeffrey Saldinger]

Thank you, Andrew. So many acronyms and initialisms; so little time.