When out doing night photography like the shot above, you don’t want to waste time guessing what the exposure time should be (like I did here one bank holiday morning…)
The technique I use is to do a shot at a very high ISO speed and a smallish exposure time, then use that to adjust for composition and check the histogram. Changing the ISO from 1600 to 800 (for example, doubles the exposure time. So to get back down to ISO 100 requires a few doublings to get the right number.
So I made a table to go in my camera bag for reference. Mainly because I’m lazy and rotten at Maths!
Now I can’t imagine where the hell you would have to go to do a 5min exposure at 3200 ISO but its there anyway!
So in essence the table shows that if you did a 20 second exposure at 1600 ISO, you would have to do 5 minutes and 20 seconds worth to get the same amount of light in at 100 ISO.