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How To Calculate Exposures Quickly at Night Time

17 February, 2010Photography

Night photography generally requires very long exposures, often several minutes at a time. To get the best image quality, we need to keep the ISO as low as possible to keep our noise levels down to acceptable levels and also to minimize the occurrence of hot pixels and sometime banding artifacts.

Now most people probably will wild guess exposures at night time and just adjust the camera settings to suit. Unfortunately since the exposures are so long, a lot of time can be spent making “dud exposures” that might not turn out just because you guess the wrong exposure time. Here is what I do, a lot of modern DSLRs can be set up to ISO 6400 and higher, we’re going to use this to our advantage. Lets say we want to take our images at ISO 100 @ f/11 but we’re not sure what shutter speed to use. But we don’t really want to be guessing 2mins? 4 mins? 8mins and wasting our time. We set everything up and then switch our ISO to 6400. Going from ISO 100 to 6400 is equivalent to increasing our sensor sensitivity by 64 times, and the exposure time (shutter speed) can be reduced by also 64 times. This is very close to 60x. But guess what, there are 60 seconds in a minute! What this means is:

  • 1 second exposure @ ISO 6400 is equivalent to 1 minute exposure @ ISO 100
  • 2 second exposure @ ISO 6400 is equivalent to 2 minute exposure @ ISO 100
  • 8 second exposure @ ISO 6400 is equivalent to 8 minute exposure @ ISO 100
  • 30 second exposure @ ISO 6400 is equivalent to 30 minute exposure @ ISO 100
  • and so forth…

While at ISO 6400, try a few different shutter speeds until you find what you’re after (basically by trial and error), and you’ll only be doing several second exposures so you won’t be wasting any previous time. Then once you find the shutter speed(s) you are after to give you the result you want, switch back to ISO 100 and exposure the same time but in minutes instead of seconds.

This is just not handy solely for night photos, it works great if you are using dark filters such as ND400 filters. You can use this technique to quickly judge long exposures also, and sometimes use it for composition when everything is so dark even Live View is no help (and you don’t want to unscrew/screw the filter all the time).

Some people may not have cameras that go up to ISO6400, that’s okay, you just need to make some quick calculations, but it won’t be as easy as a simple seconds-to-minute translation. So lets say your camera is ISO1600 max, then you can use ISO 1600 the same way but your translation table would be 4s@ISO1600 = 1m@ISO100, 16s@ISO1600 = 4m@ISO100 etc. Note the division of 4x to compensate for the 4x difference in sensitivity from ISO 6400 to ISO 1600.

© Andy Gock 2009−2023