Next, let’s get into camera settings. Believe it or not, the exposure approach to shooting start trails versus freezing the stars into clean points is quite different.
First, let’s handle the more traditional approach of freezing your stars as points.
Aperture: For almost all nighttime photography, you’re going to be shooting wide open. So, in the case of our 16-35mm f2.8 lens, we will shoot it at f2.8.
Shutter Speed: This is where things get interesting. So for every focal length lens (24mm, 16mm, 12mm, etc.), there is a different maximum exposure time to be able to freeze the stars. Shoot too long of an exposure, and the movement of the stars across the sky will smear them into streaks in your image. And so, as you use a longer focal length lens (or zoom in), the stars are magnified, and therefore so are their movements across the sky.
So, for instance, if you are shooting at 16mm, you can shoot an exposure up to 31 seconds long without seeing any star trails. But, if you were shooting at 35mm, you would be confined to shooting under 15 seconds. But, there is an easy way to calculate this. It’s called the 500 Rule.
To calculate the exposure time, divide 500 by the focal length at which your lens is set. The equation looks like this:
500 / 35mm = 14.29 second exposure time
or in the case of a 16mm lens…
500 / 16mm = 31.25 second exposure time
So you can see that as you choose a wider focal length, the longest possible shutter time to freeze the stars increases as you “zoom-out” on the night sky.
ISO: When aiming to freeze the stars as points in your image, you can adjust your ISO so that your light meter is at zero, or right in the middle (not under or overexposing).
This, for a 24mm lens at f/2.8 shooting a 26-second exposure requires an ISO of 1000. But generally, with a fast enough lens, you will be able to keep your ISO’s under 2000.