Clipping warning in ACR

[Jerry van Dijk]

I'm confused about the two ways in which ACR indicates clipping at the low and high end of the histogram. The two triangles above the histogram light up when one or more channels are being clipped. However, you can also toggle the two triangles to show clipping in the image itself (clipped areas light up in red or blue).
What I've noticed is that the clipping warning in the image shows up quite a bit earlier than the lighting up of the triangles above the histogram.

Why is there a difference and which one is "right"? I do notice a small "mountain" appearing at the edge of the histogram before the triangles light up, which makes me suspect that the triangles are less reliable.

[Jerry van Dijk]

No one knows?

[Roger Clark]

Oops, sorry. I missed this one.

I believe it goes as follows. Histograms are compute intensive, so photoshop makes an approximate histogram (probably from a sub-sampled image). That is the histogram that you first see. When you click on the histogram or the data for the full image is computed and that is the best answer.

Roger

[Jerry van Dijk]

Thanks a bunch, Roger! So looking at the clipping warning in the image itself would be the best option, I conclude from your answer.

[Roger Clark]

Thanks a bunch, Roger! So looking at the clipping warning in the image itself would be the best option, I conclude from your answer.

Hi Jerry,

I experimented with this some, and it seems the best warning is holding down the alt key (on windows) and moving the sliders, e.g. exposure, is the best way to check for clipping.

Roger

[Jerry van Dijk]

Thanks a lot for looking into this, Roger, it's greatly appreciated!