Star trails are one of my favorite types of photographs to crate, because they go beyond "clicking the button" by requiring a solid combination of mathematical understanding and artistic creativity.
The first concept one must grasp on the road towards creating star trail photos is to understand that desepite the result, the earth is turning, and the stars don't move. Your camera, rooted firmly on it's tripod, is connected to the earth which is flying through space at around 67,000 mph. Here in the northern hemisphere, the earth rotates in such a manner so as to place one star in our sky (Polaris) in a position so that it lines up directly above the axis of rotation of the earth (above our north pole).
Any long exposure at night will show the rotation of the earth by giving stars the look of streaks (instead of points) from the stars in the sky. The length of the exposure along with the size (wide or narrow angle) of your lens together dictate whether stars will be points or leave streaks. There is a mathematical way to determine the time of exposure rule for this called the "rule of 500."
Here's how it works: Divide 500 by the focal length of your lens, and you get the longest possible exposure you can make before stars turn from points to streaks. I typically turn to my 16-35mm lens to shoot wide-angle nightscapes. 500/16 = 31.25 seconds, after which the stars blur into trails.
How do you make a star trails photo? Let's start by coming to a fork in the road. Star trails are made by one of two means - 1. Single, long exposures in bulb mode, or 2. Stacking shorter exposures in Photoshop. In this breif tutorial post, I am only going to go over method #1. Later this summer, I will share with you how I employ a stacking method to create star trails.
Ingredients for ideal single-exposusre star trails photographs:
- Cable release (with lock)
- Digital SLR, full-frame preferable
- Fast, wide-angle lens (if you want to include landscape elements)
- Light-pollution-free skies
- Half-to quarter moon, preferably at your back and out of frame
- Experiment, adjust, then shoot again
- Make a dark frame (lens cap on) with identicaly exposure/time settings
Let's say you are out in a field, it's dark, there is a quarter moon at your back, you have your camera on a tripod, and you are ready to go. Where do you start in terms of exposure? I always like to guess at the exposure, and then make a 600 second (5 minute) test frame, and then evaluate the image and make changes.
A good place to start is f/5.6, ISO 400, bulb mode of 600 seconds. After you get your shot, see where your histogram is at.
The above histogram is from a 900 second long exposure I made of a bristelcone pine I shot in the White Mountains. See how there aren't any fully-dark (underexposed)areas? Or any fully white (overexposed) areas?
Before I made this finished exposure, I made a few test shots to check my focus. At the wide-open aperture of f/1.8, I had to be quite far back from the tree to get both it and the foreground in focus. See this post for more info about knowing the infinity point of your lens.
That's basically it. Guess, adjust, and shoot again. There is one last important thing to consider - MAKE A DARK FRAME! What is a dark frame? Well, if you shoot your desired exposure, then immediately afterwards, put on your lens cap, and make another identical exposure with only total darkness. Then, you use it to remove the noise from your image in Photoshop. I'll cover what you do with this dark frame to remove the noise in another post!
If you want to learn more about star trails, consider signing up for a private workshop where I will meet you in Tahoe and take you out and show you how to create images like the ones in this post!