ACR Histogram showing information different to lightroom for same photo

[Steve Moore-Vale]

Good afternoon from England.

For a little while i have been processing my photos using Camera Raw in photoshop (opening canon raw files from photomechanic).

I recently got Lightroom back up and running and have noticed that the histogram in Camera Raw looks different to the one in Lightroom. Not just the appearance which doesn't matter to me, but the actual histogram information from the photo i have opened is shown differently in the two programs. In my test photo ACR shows some shadow clipping whereas lightroom doesn't. This could make me process a photo differently, in this case brightening the photo when i don't need to.

I have attached some reference images. The photo has been processed and the edits are saved to an XMP Sidecar file so everything should be the same in ACR and Lightroom. Any ideas why this is happening?

ACR in photoshop:

Lightroom:

Thank you.

[Guest]

Moving to Adobe Camera Raw from Photoshop

[D Fosse]

ACR shows the histogram for whatever color space you have chosen in your workflow options.

Lightroom displays the histogram for a hybrid color space with ProPhoto primaries, but sRGB tone response curve. This is what is known as "Melissa RGB", presumably after the engineer who came up with it.

This is not to be confused with Lightroom's real internal color space, which is ProPhoto with a linear tone response curve. But you can't show a histogram for linear TRC, that wouldn't make much sense. Hence this little tweak.

If you set Lightroom to soft proof to the same profile as your ACR choice, the histograms will match.

[Akash Sharma · Accepted solution]

Hi SteveMoore-Vale,

Sorry that Histogram in Lightroom and Camera Raw are showing different Clipping info.

I believe that the histogram in Lightroom is in a special colorspace that is not available in ACR. It is in a space with prophotoRGB primaries but with an sRGB tone curve. So you cannot compare them, not even the clipping points. The histogram should look the same if you turn on soft proofing in Lightroom and select the same color space as you selected in ACR.

And the histogram in ACR will look the closest to LR if you select ProPhotoRGB as the output color profile in ACR, using the blue link at the bottom of the main ACR interface.

Thanks,

Akash

[Steve Moore-Vale]

Thank you Akash and D Fosse.

I have tried the second of the suggestions, which was to change the colour space of the document within the ACR window using the blue link at the bottom. As soon as i swap that to ProPhotoRGB (it has always been set to sRGB), the histogram no longer clips the shadows and looks like the one in Lightroom.

Thank you again for taking the time to answer.

Steve

[elie_dinur]

Another factor, not mentioned above, is that in ACR the numerical color value readouts are in the more familiar 8 bit, 0 - 255 scale, while in LR it is in a percentage scale where Black clipping is 0 and White clipping is 100, From LR's inception a dozen plus years ago it was decided to use this scale to prevent a too facile translation from one histogram to another (or to a traditional histogram in PS).

LR and ACR's numerical displays can be brought into closer agreement by setting both of them to use the L*ab scale.