Scene luminance has nothing to do with exposure.

Typing on phones and not bothering to check before posting is more common. It makes life more easy for the poster and less easy for the reader.
Don

Your histograms rather fall into a category of something you don't understand. Qualified explanations just don't work on you...

That's calculable by fixing your conversion or picture style parameters and varying only exposure of a fixed scene, but it is going to vary through the histogram with most histograms, which use gamma or logarithms on the right side, and are more linear on the left. You couldn't even have black or zero on the left end if the left side was logarithmic, as there is no such thing as the logarithm of zero; a pure logarithmic histogram would go on forever to the left. I would assume that in cameras, the histogram is almost always directly based on an 8-bit converted image, and the values in that image (or a binning or subsampling of it). With such histograms, the number of histogram pixels per stop should peak in the middle of the range, and always be lower on the left end, and lower on the right end when the converter rolls in and compresses an extended range of highlights.

Here's a crude, quantized guess-timation of how objects exactly in a 1-stop luminance series might render with a highlight-extending picture style; just keep in mind that the very left section would actually be denser with the "|" lines, if you kept halving exposure:

|||| |  |  |    |      |       |       |      |  ||

It's very difficult to calculate this for an arbitrary white balance, especially when the histogram is of luminosity. But works for the green channel most of the time.

In the MILC cameras that I've used, the EVF histogram horizontal axis appears to be the tone curve of the JPEG color space selected.

Yes.

Yes, that what I assume is the norm. They already have that data to make the EVF image, so why generate a second set, when the existing one is already applicable for intended purpose?

I think camera manufacturers are going to be reluctant to consider making a EVF histogram that doesn't correspond to what folk are looking at in the viewfinder. Raises too many questions, from folk who won't get the difference between scene-referred and display-referred...

Perhaps it could be a raw histogram if the camera is writing raw files, and a JPEG histogram if it is writing JPEGs. I don't know what it should be if the camera producing both.

If you're not giving a raw histogram, then it makes sense that the RGB histogram is based on what the OOC JPEG would be expected to have (in "live view") or already has (if it is a review histogram).

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.