longexposure.net

Sven April 20th, 2006

This is a repost of a piece I wrote on my previous blog, but hadn’t moved across – it’s here because someone asked what Exposerator was all about…

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Taking long exposures at night is good fun, but trial and error takes a while when you’re shooting twenty-minute frames, so I tend to use the following technique for getting the exposure more-or-less right before I commit myself to the shot.

Now, this technique, while my own invention, is hardly the most innovative, and there may well be other approaches that work as well.  However, I haven’t seen any documented, so maybe this will provide some useful starting information for someone new to all this. 

I’ll start by composing the shot, and setting the camera to about f/4, ISO1600, and, say, 10 seconds, and take the shot, with noise reduction disabled (though on the Nikon D70, one can just switch the camera off and back on during the noise reduction stage). Depending on the outcome on-screen, I’ll adjust the time, re-take, and so on, until I’m relatively happy. Remember that we’re only measuring light levels here, so this can even be hand-held – the shots themselves will be discarded.

With this sample exposure determined, I’ll then convert to the actual ISO and aperture settings I want for the shot.  So, as an example, say I was happy with f/4, ISO1600, and 10 seconds, and I actually wanted f/8, and ISO200. This is an exercise in counting stops:

ISO1600 –> ISO200 is three stops (we halve the figure three times)
f/4 –> f/8 is two stops (doubling the aperture number is two stops)

So, in total, we have five stops of difference, so we double the exposure time five times:
10 seconds –> 20 –> 40 –> 80 –> 160 –> 320 seconds

So, I’ll set the ISO to 200, set the aperture to f/8, and start a timer for 320 seconds, and that’s all there is to it.

As an addendum to this, Chris convinced me to write a mobile phone application to support this, which I did, and called Exposerator. It’s a J2ME application that takes sample exposure details and desired details, and calculates the exposure time (in minutes and seconds). It also has a timer built-in, but oddly enough that part didn’t work on Chris’s phone, despite the fact that he has the same model as I do…

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