no Im saying by exposure after 400 iso you have create a head room by 2 stops compare to 100iso
what is so difficult to understand Neuro! = exposure after 400 iso = halving the hitting lights twice compare to 100iso
Hypothetical example: I shoot jpg. I am taking a picture of a forest scene. I am in Av mode, and I set f/8 to obtain the desired DoF, and I chose ISO 400 to get a 1/100 s shutter speed at metered exposure to avoid camera shake, because I foolishly left my tripod at home. Please note, I couldn't care less about what exposure settings would be at ISO 100, 50, or 3200, that's tangential and irrelevant - I choose f/8 and ISO 400 for the reasons I stated. I take a shot, look at the review image, and see blinking highlight alerts where I want detail of the sun-dappled forest floor. I've read that HTP can preserve my highlights.
If that scenario is confusing, I'll summarize - with HTP off, I set the camera in Av mode, f/8, ISO 400, and the metered exposure gave a 1/100 s shutter speed.
Answer these questions about what happens when I set HTP to Enable:
1) Does my selected aperture of f/8 change?
2) Does the camera-selected shutter speed of 1/100 s change?
3) Does the amount of light hitting the sensor change?
Please, no hand-waving, no 'please read my earlier posts', no repeating what you've posted before, no referring to what may happen at some other ISO setting that I didn't select and don't care about - just answer those three, simple questions with a yes or a no.
Answer : yes
Sorry, Mikael - it was a pass/fail test, and you have failed. The answer to all three of those questions is "no". In Av mode, f/8, ISO 400, enabling HTP does not change aperture or shutter speed, and therefore the amount of light hitting the sensor does not change. If anyone believes there a chance Mikael is right, feel free to set Av, f/8, ISO 400 then enable HTP and see if shutter speed or aperture change as a result.
Back in the other thread, I had come to the conclusion that this was semantics and your inappropriate extrapolation of what happens in the special case of ISO 100 in an autoexposure mode to a general explanation of how HTP works.
But...the fact that you answered "yes" to the above questions clearly demonstrates that you do not understand how HTP really works. Despite repeated attempts by several people, including TheSuede, to provide an explanation which you could understand, you fail to grasp some details of the concept. At this point, the only logical conclusion is that you are simply incapable of understanding the mechanism of HTP. Not due to language, semantics, etc. - just a fundamental inability to comprehend this concept. You are wrong, you don't get it, and apparently you never will, much less admit that you're wrong and/or incapable of understanding the concepts.
To paraphrase an earlier statement of yours, it's impossible to discuss this with someone who does not understand the basics - and that someone is you, Mikael. For my part, this discussion is done. Any further statements or questions from you on this matter will be ignored.