Long Exposure Theory

Nautical Dawn [Cropped Version]

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.


8 thoughts on “Long Exposure Theory

  1. Aperture works the same. Keep the ISO the same and for each full aperture stop the shutter speed halves or doubles. And yes Freds computer chart explains the same thing but in more detail..

  2. Example in theory: We want to shoot dark night.

    If we need f/22 with 3200 ISO and for that speed of film and aperture we need 16 min. exposure.

    Now if we use your table and change to ISO 100, the exposure time will be 80 min.

  3. Don’t shoot night at f22!! … and 3200 ISO will be so noisy it will be a waste of time!

    3200 ISO @ f22 = 16 minutes
    is the same as

    200 ISO @ f4 = 8 minutes

    That will give you the same exposure but less noise and half the time!

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