http://terragalleria.com/blog/2011/03/22/best-iso-for-low-noise-on-canon-5d-mk2/
The results are actually well expected, the immediate intermediate ISO right below each "real" ISO has better (signal-to-)noise performance than the immediate intermediate ISO right above each "real" ISO.
Indeed, I had tested myself with my 5D long time ago and confirmed the same, when no Pentax DSLR had any intermediate ISO values at that time..
However, is there any disadvantage to use those lower noise but faster speed intermediate ISOs? Yes, I am afraid. Indeed, there is one point most people have overlooked, even though the noise advantage has been over-emphasized. That is, the highlight headroom will be less with those particular ISOs, as it is just using the technique of expose to the Right. So the lower noise is actually achieved with the trade-off of highlight headroom! So, just beware! And you should know what you are actually doing if you are selecting this option.
Related:-
The Effects of Real and Digital Exposure Compensations on Exposure
Tuesday, April 19, 2011
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Flo · 728 weeks ago
They are just arbitrary exposure values, nothing else. Any exposure value could be set by the camera (ie. ISO 104, 105, or 1837 for instance) as it is just the exposure time. Of course there is a lower bound and an upper bound, but in-between, I don't see why we shouldn't be able to set any ISO value we like down to a granularity of one unit (1).
I believe the reason it is how it is is just film history.
In fact many things in digital don't make much sense anymore, but they remain the same for historical/habit reasons.
A perfect example is the new Fuji X100: why on earth does it have a ISO dial? This is totally stupid IMHO as it's so limiting.
ISO should be a totally up to the camera to choose from with a granularity (or step) of 1.
RiceHigh 110p · 728 weeks ago
The fixed ISO speeds are hardware dependent. It is all about the stepping gain of the programmable amplifier of the sensor output circuitry.
R.E. · 728 weeks ago