Monday, July 25, 2016

Long exposure 2/2 day

Over the years I was asked what my tricks for long exposure where under certain circumstances.
It all depends on the circumstances!
First of all, the problem with long exposure is that there are neither cameras nor light meters made for this task.

In this post, I would like to cover the day-time aspect of long exposures.
The content of which I assign a new rule to, the 10-stop rule.

For day-time long exposure, many different filters could be used. Some cameras have got a 3-stop ND-filter builtin, but this post is not about 3 stops...
10 stop ND-filters are known by many names. Just make sure your's actually attenuates by 10 stops.
Here's the maths (equivalent exposures), constant ISO assumed:

  • 0: 1s/1000
  • 1: 1s/500
  • 2: 1s/250
  • 3: 1s/125
  • 4: 1s/ 60
  • 5: 1s/30
  • 6: 1s/15
  • 7: 1s/8
  • 8: 1s/4
  • 9: 1s/2
  • 10: 1s/1
OK, what does that mean when putting a 10-stop filter in front of a camera?
It means that when your light-meter indicates 1s/1000, you should set your camera to 1s, in order to get a regular exposure.

So, your scene would require a and exposure of 1s/1000, without any filter, the addition of a 10-stop filter, will get you to 1s for the same exposure.

So, if your exposure would call for a 1s/500, you obviously would like to expose for 2s.

Consequently, the exposure can be calculated as follows, with S being the shutter time of the light-meter reading:

Exposure time in seconds = 1000 * S

Example:
The meter calls for 1s/15 exposure, you would set the camera to 66s.

To smear out any sort of traffic, a minute should be plenty. Mind you, this kind of photography would usually ask for an aperture between f/8 and f/11.

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