I can't really make it any simpler. ETTR is about exposure - that is, amount of photons hitting the sensor.
Check.
What ISO you shoot at has zero affect on this.
Check.
Only your shutter speed and aperture affect exposure.
Check.
I think the reason you are getting confused is that you are assuming that an image taken at higher ISO will be clipped (i.e. the ADC will be fully saturated).
Well, there lies the rub. Try an ETTR at base ISO. Now boost the ISO, what happens?
We did 'expose to the right', and the histogram will be clipped at the right (because it was then amplified).
But that's clearly not necessarily the case, as we all know.
Not necessarily, but for that to
not happen we must first
underexpose. Calling that exposing to the right confuses (me), because you have to underexpose to get there, and its hurting image quality. What should have been done is reducing the ISO, i.e. the amplification, and retain optimal quality. Wasn't that the goal of ETTR, optimizing quality
by exposure? Or is it just to fill out the histogram by changing the amplification? I'd say the former, you seem to say the latter.
Can we agree on that? Because if that's not what is confusing the real issue of quality, then what is?
If there is leeway to increase exposure at ISO 800, then you can apply the principles of ETTR. You can do it at ISO 4 billion if you want. ISO has no effect on the principles of ETTR. The only effect it has is that larger ISO's have reduced dynamic range, and may also saturate the ADC.
Yes, because ISO has nothing to do with exposure as such, it's just like yanking the volume knob, but the band is not playing any louder nor is their music getting any better (if they don't play loud enough to begin with, people at the back won't be able to hear them, so then amplifying the volume would cure that, but band still isn't playing any louder). If it has nothing to do with exposure, then why call it
Expose to the right?
Again, is that what we're dealing with? Although the exposure doesn't change, you call boosting the amplification by increasing the ISO an Exposure boost?
Cheers,
Bart