@ LetTheRightLensIn - Changing the ISO has no direct effect on the amount of light that hits the sensor. At ISO 400, enabling HTP does not result in a change in the amount of light hitting the sensor, no difference in the number of photons. Period. What the camera does is apply one stop less gain to the signal generated from those collected photons, then applies a tone curve to the jpg data to boost everything but the highlights back up that one stop.
this may be a mess since i typed this out quickly with no thought but:
Yes hitting the HTP button doesn't change the total light now hitting the sensor, but HTP mode both secretly changes the gain 1 stop less AND the metering normally used for that gain by to meter 1 stop down from normal.
If you want some HTP thing then you have some sort of scene with extra amounts of highlight stuff say a full stop more than typical so you have some gain that manages to let you maintain a high enough shutter speed to stop motion or to handhold. You then go one gain down and that gain is now getting shot letting in 1 stop less light than you'd normally let in for that gain for a scene that had less highlights.
Or you can go tripod and then you do ISO100 and just raise shutter more and more or you add on more and more ND filters and you are saving highlights by exposing the scene to less and less light.
ISO400 HTP EC0 is no longer ISO400 EC0 it is ISO200 EC0 metered to expose (or suggested to be so in M mode meter readings provided) 1 stop less than the camera normally would do at ISO200 EC0, i.e. it is effectively as if you were to shoot ISO200 EC -1.
You are not gaining a stop of highlights at a given gain and keeping the same light coming in, that can't be done of course.
Yeah it is at the face of it just applying 1 stop less gain but normally when you apply 1 stop less gain you'd also let in 1 stop more light too and in this case you are not so you are basically letting in 1 stop less light than you'd normally do for the gain. ISO400 HTP the camera isn't doing ISO400 at all it is doing ISO200 and it decides to do it at EC -1 metering instead of EC 0 metering and because of the latter part you might look at it that is letting in a stop less light. In an M mode scenario where you end up needing in some case to fix both aperture and shutter exactly it might be weird to think of it in terms of letting less light in since in this scenario you always want to let the same light in, but it is still metering in way that is compatible with thinking about it that way. And your scenario below where it adjusts many stops to match your light sounds more like AutoISO button than HTP button to me.
Anyway, you can only save 1 stop more highlights than the prior shot by either now letting 1 stop less light come in at the current gain (set EC -1) or by keeping same light coming in and lowering the gain 1 stop (by either dialing gain down 1 stop and then setting EC -1 or swapping on HTP because that is HTP). HTP lowers the gain one stop BUT to keep the same light coming in it must be set to meter 1 stop less than normal at the new gain that HTP selected under the hood (or simply fail to report the new actual gain being applied). So HTP is metering to let a given gain get 1 stop less light than it would normally be metered for.
So in the sense that it meters 1 stop darker than it normally would for the gain that it is actually using you might say it is letting in one stop less light than normal. If not it would no different than shooting 1 stop lower with normal metering. When you go to replicate it yourself, that is what you'd do set ISO 1 stop lower than what you had it in with HTP and then set EC -1.
If you set your camera to ISO100 EC -1 and then shoot all day in P,Av,Tv you'll get RAWs that can be made to give same results as ones from ISO200 HTP EC0. In M mode you could get files that can be made to deliver the same results using either ISO100 EC -1 as ISO200 HTP EC0 and meterings suggest to you to use would be the same in either case.
It might make more sense to think of ISO200HTP not as any sort of ISO200 at all but as ISO100 and as an ISO100 that gets exposed to 1 stop less light than typical.
You could think of it as exposed to the same light and then has 1 stop less gain applied which it is but since the basis for that decision was based on the meter thinking it had been getting 1 stop more gain....
So you shoot your fountain at f/4.5, 1/15th, ISO200HTP or you shoot it at f/4.5, 1/15th, ISO100 HTP-off and get the same thing, same shutter, same aperture,same SNR,same DR,RAW files are store a touch differently but are basically 1:1.
Suggesting that at ISO 400, HTP reduces the light hitting the sensor is not just misleading, it's plain wrong.
Yes, swapping ISO400 to ISO400HTP doesn't change the light hitting the sensor. Hit the button and the total light hitting the sensor stays the same.
But saving a stop of highlights for a given amount of sensor gain does mean 1 stop less total light hitting the sensor.
Under the hood, HTP it is doing 1 stop less gain and then setting the metering to expose 1 stop less than normal for that gain. If you have an ISO400 HTP shot and then want to do that on a camera without HTP you would set ISO200 and EC -1. If you shot at ISO400 and EC 0 and wanted to save a stop more of highlights while shooting at ISO 400 what would you do but EC -1 and let 1 stop less total light hit the sensor.
Although it is probably simplest to not talk about getting less photons.
What if you wanted motion blur of a fountain, but to preserve the highlights in the scene - would you sacrifice the motion blur you wanted, or stop down and change your DoF or lose sharpness to diffraction? I would do what the camera does with HTP - underexpose by lowering ISO as many stops as needed, and if that took me to ISO 100, it would be time for an ND filter.
HTP doesn't lower as many stops as needed. AutoISO does.
HTP just shoots one stop lower and then meters that ISO one stop under.
In this particular scenario of M mode, we are not talking about an at any given gain scenario any more. You just set the gain to balance how much shadows detail you want vs highlights saved. Once you get it ballpark you can then +/- 1 it to save more or less highlights. (although you might want finer tuning than whole stops and might want to tweak aperture or shutter 1/3 of a stop unless you MUST have it left as is exactly).
In the ap AND shutter MUST be exactly locked sub-case of M mode scenario deciding to lower gain a stop might be a bit odd to think of in terms of shooting a given ISO with 1 stop less light coming in since you are in a we decided to keep that fixed scenario, granted so yeah a bit weird. It is still true though that if ISO200 HTP is what ended up working for the particular scenario then so would ISO100 instead and if you cared what the meter told you, then setting that EC -1 same as always though.
I disagree with your statement, "...if I want to do HTP myself what do I need to do? To get the exact same result I shoot at my selected ISO, keep my selected aperture and then I raise the shutter speed 1 stop faster."
It is probably best to just say that shooting HTP at a given gain an EC 0 is 1:1 in RAW to shooting without HTP but at 1 stop less gain and metering set to EC -1. That is normally what I say. Of course in the latter case the histogram and image review may be a bit tougher to judge at first.
At high ISO where they disallow HTP is would actually make sense to always have it on since the digital gain just lops off the top stop each ISO you go up.