How is ISO determined?

[Busted Knuckles]

So let me wander the topic - if we went to a 16 bit D/A we would have greater dynamic range?

Could the origninal ISO question be answered with the minimum light needed to achieve a certain charge value?

[Daniel Browning]

BTW, great post, ChadS.

So let me wander the topic - if we went to a 16 bit D/A we would have greater dynamic range?

No. Actual instances of a camera being limited by the ADC bit depth are extremely rare. Increasing the bit depth of the ADC is trivially easy. Mass market DSLR have already shipped with 21-bit ADC -- and they could build one with a 1000-bit ADC if they wanted to. Increasing the bit depth alone is easy -- it's like adding a new speedometer to your vehicle. But putting a 500 MPH speedometer on my Pinto isn't going to make it any faster. The hard part, of course, is to build a camera that can actually utilize the higher bit depth. Just like it's hard to make my Pinto actually go 500 MPH.

Canon's noise problems are so severe, they could actually go to 11-bit files without any loss of usable dynamic range. They jumped to 14 bits before their cameras were even making full use of 12 bits. Ah, the joy of marketing.

Could the origninal ISO question be answered with the minimum light needed to achieve a certain charge value?

If you mean ADU (i.e. the numbers in the raw file), then yes, if it's scaled for bit depth or given as a percentage of the maximum value. But if you meant pre-ADC charge units, like mV, then I don't think it would as useful, because in most situations we'll be more concerned with what actually makes it to the file.