Calculating Exposure at Night

Recently I’ve been experimenting more with night photography, and have had some (in my humble opinion…!) pretty decent results. However, one aspect which persisted in being a grey area for me was calculating exposure times.

At first, most of my exposed shots were a result of trial and error, but I decided I’d like a more scientific method of doing it if at all possible.

Here’s how to do it – it’s not 100% accurate, but it’s pretty close and will save you standing about for minutes on end shivering your bits off, hoping that your shot is correctly exposed:

Let’s say you meter for f11 at ISO1600, and the exposure time is one second. However, you wish to take the shot at f20 at ISO100. The calculations look something like this:

ISO100 is 4 stops down from ISO1600 (800 – 400 – 200 – 100), and f20 is 5 stops up from f11 (f13, f14, f16, f18, f20). This makes a total of 9 stops. The exposure would be worked out by calculating 2∧9 = 512, multiplied by the original exposure time at ISO1600, which in this case was 1 second. Thus, 512×1 = 512 / 60 = just under 9 minutes.

Maths was never my strong point at school but this is as close as it gets before advancing into advanced calculus. Hope this helps somebody out there.