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I can't figure out why this is happening. I am currently running LR Classic v 10.3, but was having the same issue in at least the one prior version. The histogram shows absolutely zero highlight clipping, gray triangle, but the image itself has red areas with clipped highlights, and indicates individual channels when I press ALT+Highlights. Do I believe the histogram or the image? This is happening on both raw files as well as images I've edited in some of the Topaz plugins. Most of the time moving the highlights slider all the way to the left is not enough to resolve it. Often I have to dig deep in the tone curve to resolve all of it and by the time I've finished the image looks awful, but sometimes just moving the Point Curve output down to 254 will take care of it. But that still doesn't answer the question about why the histogram says everything is fine but the image disagrees. Help?
My 'basic' explanation- The height of the histogram graph illustrates the 'number' of pixels for each tone level.
An extremely small area of clipped pixels may not have enough pixels to register on the (small) histogram graph. You are looking (in your screen-clip) at a very small area clipped! Other LrC experts might explain the 'accuracy' of the histogram in detail.
I see the same result- (a highlight on a balloon)-
Thank you both. In WobertC's example image, the highlight warning is registering on the histogram, but mine does not. I also feel that this is a recent occurrence; I don't recall it being an issue in the past. However, I was just preparing for a show so was scrutinizing every detail in the photos I selected to print, printing being something I rarely do. But I don't remember it being present in the photos I printed for a show a few months ago. Your observation is correct, though, in that it usually only shows up in small detail, often even smaller than my feather image example. So perhaps it is a threshhold issue that keeps it from registering on the histogram, and the tiny blips won't be evident in print.