How is ISO determined?

[Daniel Browning]

Ahhh somewhat clearer, it sounds like bits do not help the absolute range, but would help in the gradations between the absolutes -

Unfortunately, the bits don't help that, either. The gradations are limited by noise (photon shot noise) to less than 8 bits, even in the very lowest-noise circumstance (ISO 100, ample light). The only thing that helps gradations that is increased full well capacity (FWC) when shooting in ample light, and sensitivity when shooting in low light.

the question is what is a praticable limit.

I'm not sure, but the ideal would be 1 bit -- to record the location of every photon strike individually. Then your effective bit depth would vary by output size, from 1 bit up to 16 bits or more.

Back in the day.... the zone system (preferable is formats larger than 35mm) we would have many more zones and EV values within a single negative than what I have been able to produce w/ the electronic sensor - maybe it is just me and I haven't figured it out yet.

It's not just you. Right now the primary limitation is Canon and typical raw converters. With non-Canon cameras and a good raw converter (e.g. RawTherapee), you can get a full 13 stops dynamic range -- 7 above middle gray and 6 below -- just like the best film (and careful chemical development). But most photographers prefer to stay in the 5-7 stop range because it gives the image "punch" and "pop".

Back to ISO capability. The lower amplificaton levels would improve total dynamic range - noise floor stays down? i.e. what happened to the 50 ISO setting?

It sounds like you're asking if dynamic range would improve by removing the amplification altogether. In general, yes. That's what a CCD is -- the FWC there is limited only by the photodiode itself (so the lowest ISO setting is fixed by the the second place where the energy goes, after knocking an electron loose from the depletion region). In a CMOS sensor, on the other hand, the FWC can be limited by the next link in the chain, one that CCD does not have, the source follower transistor. With a low capacitance, CMOS can suffer very poor FWC and maybe low dynamic range (certainly low max SNR). But designers also have the choice of high capacitance, and the FWC is generally as good as CCD.

As for the ISO 50 setting, the fundamental lower limit of the ISO setting is set by the full well capacity and sensitivity. If you increase sensitivity without increasing FWC, your minimum ISO must increase. But you can always put an ND filter on there and get the exact same result as before. But if you increase the effective FWC somehow (such as with an HDR pixel or even just software), then you can get significantly reduced noise or increased dynamic range with lower ISO settings.

The next question is for a given level of amplification and light, what is the signal - charge level - produced i.e. measure of sensitivity and ultimately S/N = max iso?

I'm not sure what you mean here, but one example might be that on some cameras, 10k electrons gives you the level that corresponds to middle gray in the default raw conversion of some converters. To calculate the S/N, you factor in read noise and photon shot noise, and then the max "usable" ISO could be based on whatever minimum SNR you prefer (I'm happy with 0dB, but a lot of photographers prefer 10dB minimum).

Now the question is how to get the sensor to be more sensitive needing less amplification???

There's not really much left to be done. If you don't count color filters, Modern sensors are already capturing 70-80% of the incident light (at the green wavelength at least). Even theoretically, it's only possible to increase to 100% -- since you can't see light that isn't there, of course. The only big jump would be if we found some other way of sensing color (rather than the filters we currently use).

Where there is a *lot* of room for improvement is in the read noise -- the part added by the sensor. If there were no read noise, we could shoot at ISO 1,000,000 and still be able to see detail (it would be very, very noisy, but still plainly visible).