F-Stop: f/3.5
Shutter Speed: 30.0
ISO: 1600
The above photos are a single frame and the finished product of my star trail shoot. To begin I set my camera up on a tripod and looked into the sky with a f-stop of 3.5, a shutter speed of 5 seconds, and a very high ISO. This allowed me to take test shots to get my focus perfect. After I had taken around 10 photos, slowly dialing in the focus, I started working on bringing the ISO down and and increasing the shutter speed while retaining an even exposure. Eventually I got an even exposure with a 30 second shutter speed at a 1600 ISO. I wanted to bring down the ISO as much as possible, so I could have the least amount of grain, but I still needed a relatively short shutter speed. If my shutter speed was longer than a minute the star would have moved a little bit by the time the picture had ended.
I set up my intervalometer to take a picture ever 30 seconds for half an hour. Theoretically I should have gotten 60 photos, but the camera had to process each photo after it took it. Thus I actually took 31 photos, because the processing time was roughly 30 seconds as well.
I then used Photoshop to layer all of the photos together and apply a blending mode of lighten to all but the bottom one. This allowed the star light of each photo to show through, which created the trails because the starts had moved a decent amount for the half an hour my camera was taking pictures.
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