Yes. Done lots of testing. (Canon DSLRs and Fuji Mirrorless).
Results of that test don't matter much in the real world, unless you're ONLY EVER shooting absolute non-moving Subjects, with reasonable lenses in good daylight using a Tripod: because to 'display' the FINAL IMAGE usually (almost always) that Final Image Display will have LESS DR than the Camera, when the Camera is set at 'any' ISO: so you will need to finesse the raw file (i.e. Post Produce / Edit / Digital Darkroom) . . . and that finessing doesn't recon for much of whatever ISO you choose to use, so long as you nail the exposure.
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Not wishing to rain on the parade. But that’s theory: and theory is good to know.
Theory is REALLY good to know. I mean that.
But, just as a balance to answer the non-theoretical question “what is the best ISO” . . .
in the real world, the point is If you’re walking at night and you need to get the shot and you have an F/2 lens and a camera which is only capable of ISO 3200, and you NEED ISO 3200 @ F/2 to make the necessary Shutter Speed . . . then ISO 3200 is the “best” ISO to use:
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If I understand the general meaning of that statement correctly: then the statement is generally incorrect or at the least displays a confused understanding.
For any given APERTURE, then Flash as Fill in daylight, will require a specific amount of FLASH POWER to make the desired Fill.
Assuming that one has reached the MAXIMUM Flash Sync Speed and that is what one desires (a fastest possible Shutter Speed) then the AMBIENT EXPOSURE will require a given APERTURE at that FLASH SYNC SPEED.
If one changes the ISO (to a lower ISO), ergo one MUST change the Aperture (open up Aperture).
Opening up the Aperture then requires MORE Flash power to act as Fill.
One might keep the ISO constant when opening up the Aperture but that would require slowing the Shutter Speed - which by implication in the statement is not what is wanted.
There are some minor nuances within various Camera Models, but generally if one is using Flash as Fill and one desires the fastest Shutter Speed WITHOUT engaging HSS, then (for most shooting scenarios) the BEST protocol is to set the MAX Flash Sync Speed; and then set (from what is possible) the desired APERTURE for DoF or for whatever other reason (for example best lens performance) and then check that Flash as Fill is within the realms of possibility and then set the ISO accordingly.
In this shooting scenario, the ISO is the least important factor and it is the last to be set.
Maybe I misunderstood the meaning of the statement - in which case please ignore this last part of the commentary.
WW