Raw histogram and why does it matter

DR400 is not two stops less exposure. DR400 keeps the set exposure (shutter speed and aperture) and lowers ISO by two stops (hidden). That is why the lowest possible ISO at DR400 is two stops above the base ISO. When the camera converts the raw to JPEG, it boosts the shadows and midtones (by two stops) while keeping the highlights intact.

Adobe reads embedded DR mode data to lift the brightness of shadows and midtones while keeping the highlights intact.

BTW, the DR modes are mostly useful for JPEGs.

You can get otherwise absent values if the camera does the integer math that causes the gaps first, and then interpolates replacement pixels for mapped-out, defective ones.

OSPDAF pixel interpolation can do that, too.

There are usually more positive outliers than negative ones, from thermal noise.

Based on the motive, and I am a so called slow shooter, I set ss and fstop given the motive. That may limit the exposure I want, i.e. I'd like my in camera histogram to move to the right (...). If the motive is static, and the camera is on a tripod, I can adjust ss to get maximum (large?) exposure. If not, i.e. hand hold, I, as far as I know from manyyearso phogography, I am onl left with adjusting the ISO. What a the constraints fro your vanatge point?

DR400 does reduce exposure by 2 stops relative to shooting at base ISO at DR200, the post above clearly demonstrates that. Your are correct that if it makes sense to be at higher ISO anyway it just plays with the ISO implementation and does not result in a loss of sensor exposure, but that's not what happened with the examples in the post above. I did nothing except change from DR200 to DR400, and, with Auto-ISO active, selecting a DR mode is directly responsible for the automatic change in ISO implementation and the 2 stop loss of exposure relative to shooting at 200 instead.

The camera's jpeg processing will apply a softer curve in DR mode (primarily in the highlights), but Adobe simply adds two stops of compensating linear gain with no tonal processing to the DR mode RAWs.

DR200 vs.DR400 RAFs in Lightroom with their nearly identical histograms.

And yes, DR modes are for jpeg shooters (and can be useful for them, typically with some positive exposure compensation). I know how they work, but I normally shoot RAW and don't use them myself.

I wonder what optimum vs maximum exposure entails, as these two terms seem to be used in a not consistent way. If optimum is good enough, some sensor exposure potential is possibly left on the table - not only non-critical highlight clipings are ok, but there is potentially underexposure beyond that, IMO. So maximum to me is when no channel is clipped, but are exposed as close to clipping as is possible, from a RD analysis point of view. This may mean, though, that non even so called non-critical highlights are not clipped. But is that the exposure that adequately considers the motive? How often does a motive have non-critical highlights? So in the spirit of the discussion in this thread, we should IMO not use the term optimum exposure, or maximum exposure, but motive adequate exposure - maximising exposure where I need /want it in the motive, and ignore any highlight clipping(s) where I don't care loosing details.

From my point of view, an "optimal" exposure is really just getting the exposure you were trying to achieve. From a technical standpoint, that generally means maximizing the signal to noise ratio by recording to brightest highlight detail that you want to retain as far to the right as possible - which will often mean that letting direct light sources and specular reflections "go" is very often going to be necessary for achieving an "optimal" exposure. In my book, anyway.

I shot 3 images in a controlled studio situation with uncompressed , lossless and compressed. processed them in ACR saved as psd files and the file size was exactly the same at 187 meg for all of them. so if compressed is suppose to not regain some of its information then why has it saved as the same file size ?

You are correct. This is because you are hitting the Auto-ISO lower boundary with DR400.
DR400 is like shooting with -2 EC and then lifting in the post. If Auto-ISO lower boundary is not hit, it will lower only ISO (internally) and keep the exposure constant.

I also see what you are observing: Adobe lifts everything, including the highlights. However, the highlights data is still there in DR400 mode, whereas they may be lost with DR100 (same effect as when shooting with negative EC). Unlike JPEGs, where the highlights are automatically "preserved," with Adobe you need to do it manually (highlights slider or masking).

im mainly a portrait shooter and focusing on skin tone exposure is most important. whether the white background is white doesnt matter it can be easily fixed in post. and also lets be very clear the white background has be at aprox 245 not pure white 255 as when you print your printer NEEDS to lay ink on the whole image. the perfect exposure has nothing to do with technical jargon of where the white point or black point lands. the question still remains a mystery in the title of my thread, because everyone is different and has different objectives.

whether the white background is white doesnt matter it can be easily fixed in post. and also lets be very clear the white background has be at aprox 245 not pure white 255 as when you print your printer NEEDS to lay ink on the whole image.
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When you say "printer" are you referring to a physical ink jet printer or a commercial printing house?

Rich

RD only shows the total exposure range , it places the total exposure information of your image in a position on the scale where EV0 is equal to 12% mid grey which is calculated from your image. it doesnt show what correct exposure is. use ACR for that ,there is NO correlation between RD and ACR histograms.

I think we both know how this works. The point is, when there is plenty of light for DR200 and base ISO, it is a pretty bad idea to walk around shooting normally, but at DR400 all day instead (and that goes for raw or jpeg shooters). A DR mode with (often significant) positive exposure compensation can often brighten up the mid tones of a jpeg in very high DR situations without blowing the highlights - which is indeed putting the feature to good use (and will reduce any sensor exposure deficit), but I think more than a few people just think. "great, more dynamic range, I'll just shoot in DR400 24/7, win, win right? No, not so much, IMO.

The highlight detail will "be there" in either case if you expose correctly, but some additional noise might also also "be there" with the DR400 file (unless, of course, you shoot at DR400 with +2 stops of exposure compensation - which will produce a functionally identical RAW file to shooting at base ISO and DR200 and 0 EC).

I've experimented a bit shooting jpegs with the DR modes and have found DR200, usually with some positive compensation, to be quite effective for producing well balanced SOOC jpegs in high DR situations. There might be exceptions, but I think DR400 generally produces overly dull results, even in extreme light. IMO, a high DR scene photo typically ought to look like a high DR scene.

Have you not read anything already posted about the "EV 0" scale? It is typically positioned at -3 EV relative to where the clipping threshold is. In my examples I moved to " EV 0" scale to coincide with the clipping threshold instead (I don't care where the mid tones are, I always expose for the highlights). It is the immovable scale on the bottom of the histogram that is relevant for determining the exposure (typically clipping at around 16000 for 14 bit RAW files). The RAW histogram absolutely shows whether your RAW exposure is correct (if you actually know where you want your exposure to be). ACR does not.

He has not read anything and does not understand that where in your final image the mid tones fall are a function of how you process your image.
And still does not understand that if you are do not have any highlights to clip in your image you can further increase your exposure and adjust mid tones that could now be 1 to 2 stops above 0EV back into those 12% grey tone is located in his final image

Your quote: It is typically positioned at -3 EV relative to where the clipping threshold is.
which is 12.5% grey calculated from YOUR image.