On this, I remember where I came unstuck.

I kind of remember the "Trick" about exposoure calculation you up the ISO speed and get a shutter speed.

Then drop the ISO speed and you increase the shutter speed (or rather time) by the same number of steps.

But, this all falls appart when you run out of time for exposure.


To explain:
Say I set ISO 1600 Shutter speed 30 seconds, F7.

If that is 5 steps up from the desired ISO, what are the next steps after 30 seconds?


(Yeah, I'm gonna cop a flogging for asking.)