Long Exposures & noise... ISO vs shutter time
it is my understanding that cameras perform at their best at the lowest ISO ...
Pretty much true, although beware that on some cameras, the very lowest ISO (often called "Lo" or similar) actually has the same noise characteristics as the lowest normal ISO (often ISO 100). This is because the camera actually records the same information from the sensor, but just underexposes in software afterwards.
Disclaimer: this doesn't apply to all cameras.
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