Sony a7Rii & Zeiss Batis 25mm f/2.0
[left] 13 seconds | f/2.2 | ISO 10,000
[right] 4,200 seconds (total time) | f4.0 | ISO 800
Sony a7Rii & Zeiss 16-15mm f/4.0
[left] 25 seconds | f/4.0 | ISO 12,800
[right] 6,420 seconds (total time) | f4.5 | ISO 800
Just for fun here are a couple of comparisons between a single exposure and a combination of exposures making up total exposure times of over an hour.
You could capture star trails with a series of short single exposures (around 15-25 seconds) or you could capture it all in one single exposure. I don't use either of these methods. I find that an in between method of slightly longer exposures (say 8-10 minutes each) works best.
The longer exposures in the examples are not done as one long exposure but instead have been split across several exposures of around 8-10 minutes each. Why?
There are a few benefits to capturing this way:
Longer exposures enable you to capture much more detail in the foreground (also with the help of low level lighting during the exposure). A short exposure will capture little or no foreground detail, stacking shorter exposures will not result in more detail.
One long exposure will create a lot of noise (hot pixels), stacking multiple exposures helps to eliminate that noise.
When capturing several exposures you are safeguarding against any external factors that may affect your exposure. i.e. a passerby with a torch. Instead of ruining your whole image they will just ruin one of the exposures. This is much easier to rectify in post.
Capturing any foreground detail at night is difficult! I pushed the camera to its sensitivity limits, as you can see from the single exposures, and there is still very little detail - this is important to know and will help to understand why I capture star trails with longer exposures.
Note: Stacking 150-300 shorter exposures (that equal the same exposure time) will not result in the same amount of detail captured in the long exposures, you will have hardly any shadow detail to play with when post processing. You'll end up with bright star trails but a very dark foreground.
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