... The phase one with the 80mm goes like 2.8, 3.2, 3.5, 4.0, 4.5, 5.0 and then 5.6... Looking at that I assume you added a factor for every two steps, which means the above reflects a half a stop each. Ok, I get that.
George:
Each full stop is 1/2 or 2x the light of the previous. So full stops from f2.8 progress as, 2.8, 4, 5.6, 8, etc. So going from 2.8 to 5.6 is 2 full stops, 1/2 x 1/2 or 1/4th the light, and thus you need to increase the time of the exposure by 4x...
Guy's math was off for number of stops between ISO 1600 and ISO 100, it's only 4: 800 is one, 400 is two, 200 is three and 100 is four.
However, a good rule of thumb with any long night exposure is do the math as above, then add one stop for the tripod
. Seriously, you almost never over expose them!
And after doing all the math, with a clear sky (clouds change everything) and regardless of the phase of the moon, they almost always work out to about one hour at f4 for ISO 100.
A sidebar note: Because of the difficulty composing through a viewfinder in almost total darkness, and because of the relatively limited apertures of f5.6 or maybe f8, I tend to shoot these with wider lenses --- wider angles of view mean you can "zone compose" by pointing the camera in the generally right direction (shoot loose and crop a bit later), and shorter lenses render greater DoF at a given aperture.