I just picked up a D70S with 18-70 kit lens.

I once owned a film SLR more than two decades ago, so I have some passing acquaintance with the relationship between shutter speed, aperture, and film speed in terms of exposure.

But how does the Auto ISO function actually work?

Does it assume you want to shoot at the lowest ISO possible and only adjust to higher (and potentially noisier) ratings if your chosen shutter speed and aperture seem insufficient to get decent exposure?

Or does it make some general assumption based on available light and then require you (or the camera) to set shutter speed and aperture around that?

With DSLRs, it seems that ISO becomes a true variable—something that can potentially be changed with each shot as opposed to with each roll of film. I realize in the future I will likely choose specific ISO ratings myself, but I first wanted to see what the camera chooses to get a better idea of what happens at the different ratings. This is the same thinking that has me using some of Digital Vari-Programs in these early days—i.e., a tactic to give me a baseline from which to make my own photographic choices.

Thanks in advance for any help. I hope this query makes sense!