What's the perfect ISO setting?

Question

Assuming that ISO in a DSLR is a 'fake' figure, matching the speeds of wet film & that as everything is calculated digitally, the 'best' calculation is 'no calculation at all'.

Is it a given that the lowest ISO on a camera is the one with no additional amplification or attenuation, or could that vary by make/model?

When I'm shooting under controlled conditions, where I'm in full control of all aspects including the lighting, then I tend to set my camera up at ISO 100 [the lowest it will go] & work from there. I set Aperture to give me my required DoF, exposure high enough to not need extraordinary measures to avoid shake; then I adjust my lighting until I'm in the zone.

Though I'm in full manual, including sometimes focus, for focus-stacked shots, I guess this method could be considered "ISO Preferred"

My camera is a Nikon D5500 & searching the reference manual would seem to give no absolute data, but does give hints that in auto modes such as Aperture Preferred that the camera will attempt to default to 100 if it can.

I didn't want this question to solely concentrate on a single camera model, though, so if there is any general guideline which would cover other models I imagine it would be more useful to future searchers.

As the D5500 has no separate, switch-enabled, ultra-low or ultra-high ISO setting, I am also guessing that perhaps these modes on other cameras do involve an extra amplification or attenuation stage, which a consumer camera may not have.

Answer 1

Assuming that ISO in a DSLR is a 'fake' figure, matching the speeds of wet film & that as everything is calculated digitally, the 'best' calculation is 'no calculation at all'.

Not at all.

The best ISO is the ISO that gives you the required exposure setting to get the shot you want.

By artificially restricting yourself to one ISO setting you are limiting your ability to control shutter speed and aperture. That's like refusing to turn the wheel of your car, or refusing to use high gears.

Is it a given that the lowest ISO on a camera is the one with no additional amplification or attenuation, or could that vary by make/model?

Not necessarily.

Some models have extended low ISO ranges.. The "natural" ISO for the sensor is sometimes 200, not 100. It varies.

Answer 2

As this didn't really garner the answer I was looking for, then just to provide one I'll extrapolate from the information I have available, including Alaska man's comment...

The best ISO is the LOWEST ISO that gives you the required Aperture and shutter speed settings to get the shot you want.

Also from the linked question - What exactly is "base ISO" and how do I find what is base ISO on my camera? - where it appears Canon at least use different amplification methods for whole- & part-stop. Hardware & software, & that software gain is inferior.

As everything in my camera's manual indicates that in any auto mode it will attempt to use ISO 100 I will have to assume that the manufacturers did that for a reason, & that reason is because that generates the least calculation & potentially the lowest noise-floor.

This also assumes that sufficient light is available to do that without compromising any other setting. Of course, if you don't have sufficient light, then you're going to eventually have to change ISO to compensate, once all other technical &/or artistic options have been used or discarded.

As the D5500 has no special "Low ISO" option, then that additional complication is avoided.

In conclusion -
In the studio where I have full control over the lighting I shall continue to use ISO 100.
Outdoors I shall just have to try to keep it as low as practically possible, without compromising the shot.
SNR at different ISOs is discussed elsewhere & beyond the scope of my question.