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Turning Seconds into Minutes

There are many different types of night photography. One of the types of night photography that I enjoy is capturing star trails. The challenge is trying to get the proper exposure. It is easy to either really over expose or under expose our photos. The reason being that lighting can vary based upon moon phase, light pollution, or intentional light painting of certain subjects.

Where to begin? We need to determine the exposure that is going to work with whatever foreground element that we want to use. In the case of this example photo, I wanted to use Split Rock Lighthouse:

I was looking to have some light on the lighthouse without having the sky too bright. There are several factors that need to be considered when photographing star trails. Exposure, number of images, and noise. The first thing to consider is what would be the proper exposure.

We can actually determine the proper exposure by taking some handheld shots with an ISO of 6400. These are going to be throw away shots, so it is ok to hand hold during this process. In this case, the settings ended up being 20 seconds at f/2.8.

Why 6400 for an ISO? It turns out that the number of seconds for an exposure at ISO 6400 equals the number of minutes at ISO 100. That is a pretty easy thing to remember. Unfortunately, there is still a problem. A 20 minute exposure is going to result in a lot of "hot pixel" color noise. The noise would be so bad that the images would be unusable.

Since we are already shooting wide open, the only thing we can do to reduce the amount of time the shutter is open is to change our ISO. In my case, I opted to open up 2 stops from ISO 100 to ISO 400. This allowed me to reduce the length of the exposure from 20 minutes to 5 minutes.

The final image was created by stacking 6 images spanning a 30 minute window.

Here is another example taken on a night with a full moon and some light painting on the foreground element.

In this case, I wanted to get an abstract big dipper over the rock. Using the island out in the ocean as a guide, I discovered that my shutter speed was 2 seconds at ISO 6400. This meant that I could use a 2 minute exposure at ISO 100. In this case, I opted to take 8 shots resulting in a total of 16 minutes. I ended up only using 6 of the shots since I wanted to keep the star trails relatively short in order to get the effect of the big dipper.

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