Scene luminance has nothing to do with exposure.

Of course, many people shooting raw don't care about the raw histogram, either. They may target exposure for the OOC/default_conversion on which a JPEG histogram is based, but only shoot raw for more flexibility in making adjustments in post-processing, rather than for optimizing raw ETTR.

That depends on what you expect it to mean. I never thought of it as an absolute light meter; just a way of determining what a "stop" of exposure change does horizontally on the histogram, at different parts of the histogram, with the WB and picture style and subject matter fixed, when only varying exposure.

Thats an awesome explaination thanks for that John, which is why we cant basically set the perfect exposure using any incamera histograms. because we have a 1/3 stop inacuracy in exposure reading as well as 1/9 margin of error in histogram design so there will always be a little or less headroom to play with.

That might be so, but a tone curve only effects the middle of the histogram not the ends.

Are you familiar with the S-curve? Toe, shoulder?

That is not true at all. The sRGB tone curve is a gamma curve with a straight line segment near the origin. In many implementations, the Adobe RGB tone curve is a simple gamma 2.2 curve.

my review histogram does not match the live histogram. I havnt done any real testing of that yet.

it doesnt matter as the ends are still locked. same when you adjust curves in PS to gain more contrast.

You are terribly wrong.
On a side note, the tone curve applied to the image isn't the one applied to the horizontal scale.

when have you ever applied a tone curve and clipped the ends of an image ?

when have you ever clipped the ends of a histogram with colours ? because if you have you need to relearn how to take/process an image.

this is a good example why sony zebras are the most accurate way to expose colours bar none.

Sometimes several times a day. Could be even several hundred.
"a tone curve only effects the middle of the histogram not the ends" - that's wrong, clipping or not.

You have no idea what you are talking about ~<:-)

I can only think of 1 instance when it is the proper way to print. and thats printing a B & W image. there is no other reason.

so you keep saying, post an image with a purpose blown out head shot 😂

Post any image of yours lets see if you live up to your keyboard theroy perfectionism.

Consider off-white background, and generally use your imagination.