Long Exposure Theory

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!

3200	1600	800	400	200	100
1s	2s	4s	8s	16s	32s
10s	20s	40s	1min20	1min20	5min20
15s	30s	1min	2min	4min	8min
20s	40s	1min20	2min40	5min20	10min40
30s	1min	2min	4min	8min	16min
1min	2min	4min	8min	16min	32min
2min	4min	8min	16min	32min	64min
5min	10min	20min	40min	80min	160min

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.