I'm puzzled however, that changes made using the Histogram or using the Basic panel, are not reflected in the Tone Curve. Anyone know why that is?
Just to make sure I understand the problem…it sounds like this is not about the histogram graph updating behind the Histogram and Tone Curve controls, but about changes to one panel’s control set (not the tone graph) showing up in the other panel’s control set?
If so…it is all working as designed, and there is no reason why dragging the Histogram or changing Basic panel values should change the Tone Curve. It’s because they change the image using technically different methods. They should not reflect each other, because they aren't doing the same thing.
The Tone Curve is the old-style (e.g. Photoshop 1990) simple in/out value change. In other words, when you change a level on the Tone Curve, you’re saying something like “take value 56 from the image, and make it 67.” Find all pixels at a certain level, and change them by exactly the same number of levels. That’s it.
That is not what happens when you drag the Histogram or a Basic panel slider. Those two controls are tied together, and what they do is use newer, more intelligent algorithms that are somewhat aware of pixel context and content. For example, if you increase the Shadows value in the Basic panel or Histogram, it is not simply doing a uniform shift of the values of each pixel in that range. Shadows also does some shadow detail recovery, and importantly, it auto-masks shadow values to help isolate the edit and keep it from unintentionally affecting midtones and highlights as much as a similar edit in the Tone Curve would. Part of that involves analyzing nearby pixels to optimize local contrast, and as a result (and unlike the Tone Curve) some pixels of the same original value may change more than others. Similarly, Highlights can apply detail recovery of a clipped channel by analyzing data from non-clipped channels. The Tone Curve is too simple to reflect those types of non-uniform changes that may involve neighboring pixels and neighboring channels.
The Basic panel options also address a long-standing problem with the old Tone Curve: Increasing contrast in one curve segment must decrease it in another, which is not always what you want, or too much of a compromise. The Basic panel options are designed to isolate changes to the intended tonal range with far fewer compromises than the old Tone Curve, and that is one reason the Basic panel is above the Tone Curve.
But there are times when you need exactly what the Tone Curve does, so it still has a job to do. The Basic panel/Histogram drag sometimes doesn’t give you enough control over contrast within a specific narrow tonal range. So for example after I’m done with Basic panel adjustments, if I just want a little more snap near but not exactly at the midtones, I’ll steepen the Tone Curve just there. In this case, a Tone Curve edit is used to refine the Basic panel edits.
So the Basic panel/Histogram and the Tone Curve are provide two technically different approaches to tonal distribution, and because of that, they should not or cannot reflect each other’s changes. And that is a great feature (not a bug), because when used together, they can do more for you than each can do separately.
By the way, there is also a similar perception out there about all of the color panels. It seems like a fair number of users think that White Balance, HSL, Color Grading, and Calibration are different front ends to the same color math so they stick to the one they like to use, but that is also not true; all of them affect color quite differently.
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