ISO multiples of 100 are the base ISOs. On Canon cameras at least, all other ISOs are derived from these base ISOs. This has been proven. You can verify the results yourself with your camera and a RAW analyser.
On a direct comparison, 1/3rd reduced ISOs will have less noise because the shot is taken with a slower shutter speed for a correct meter reading
. The data is then compensated digitally by the camera to produce a correct rendering. However, it's not a free lunch! Using a slower shutter speed (for a correct meter reading) will result in 1/3rd of the highlight detail being overexposed. Probably not the end of the world for most shooters, since I guess most people don't even use that top 1/3rd of a stop. But there's a difference between being aware and being ignorant.
1/3rd pushed ISOs don't bring any benefit to the party other then giving the photographer a false sense of security. In this condition, the shot will have more noise because it is taken with a faster shutter speed for a correct meter reading
. To top it off, if you are ETTR this shot, you then lose 1/3rd of the highlight detail when the camera digitally pushes the exposure 1/3rd for a correct rendering.Digital
ISOs work in the same manner as HTP
. The only difference being that it makes 1/3rd stop adjustments rather then full stop adjustments as with HTP. I also wrote a detailed post on bitbucket and will provide a link when I find it.
edit: DxO reports ISOs readings based on the ISO standard. So when a camera says it's shooting at ISO 100 but DxO reports ISO 80, this simply means that the manufacturer didn't map true ISO that is being used to the ISO value displayed by the camera.
And since we know that these digital
ISOs are simply digital manipulation of analog ISO, I don't understand where one can think this results in an increase in DR.
I gave up on Bill Claffs data when he represented ISO 640 (a known digital manipulation of ISO 800) as having less read noise then ISO 100 on a 5D3