When photographing the Milky Way, we want to let as much light hit the sensor as possible. With this in mind, it's only natural to assume that we want to use a slower shutter speed. This is certainly true, but the fact that the Earth rotates can make things a little more complicated.
If we choose a shutter speed that is too slow, we will end up with blurry, trailed stars. Make it too fast and we won't capture as much detail as we could otherwise. The trick is to find the correct balance. There are a couple of ways to go about calculating the slowest shutter speed you can get away with. I'll be covering two of these methods here.
So, let's quit talking about it and get to it!
The 500 Rule (The Quick and Dirty Method)
The first rule I'll talk about here is called the 500 rule. I tend to refer to this as the quick and dirty method for calculating your shutter speed. This is because on most modern cameras this rule breaks down and still results in some star trailing. With that being said, it's still better than just guessing and if all you're doing is posting your images to social media it's probably good enough.
The rule is really quite simple. All you do is divide your focal length into 500. So, the 500 rule, on a full frame camera at least (we'll cover crop sensors in a moment), is as follows:
Minimum Shutter Speed = 500 / Focal Length
As an example, let's say that you're shooting at 10mm on a full frame camera. In this case, the slowest shutter speed to still get spot stars, according to the 500 rule, would be:
500 / 10mm = 50 seconds
Pretty simple, huh? Things do get just a touch more complicated with a crop sensor camera though.
What About Crop Sensor Cameras
As I mentioned above, the previous formula is for full frame cameras. When we are dealing with a crop sensor camera, we need to take into account the crop factor of the camera. So, for a crop sensor camera, the new formula is:
Slowest Shutter Speed = (500 / Focal Length) / Crop Factor
So, let's consider an example. Let's assume that we are still shooting at 10mm, but now we are using a camera with a crop factor of 1.6. Our slowest shutter speed, according to the 500 rule, is:
(500 / 10mm) / 1.6 = 50 / 1.6 = 31.25 seconds
As we can see, according to the 500 rule, the slowest shutter speed we can use and still get spot sharp stars would be 31.25 seconds.
As I've already mentioned, however, this rule isn't so great for our modern digital cameras and still gives us a shutter speed that results in some star trailing. It turns out that there is a better, but more complicated, rule.
The NPF Rule
The NPF rule is a much better rule for determining the slowest shutter speed you can get away with before you end up with star trails. That's the good news. The bad news is that it's a much more complicated computation. There is more good news though: There are plenty of apps that will do the calculation for you! I'll discuss one of my favorite options at the end of this post.
The problem with the 500 rule is that it was designed to work on 35mm film cameras. The resolving power of today's digital cameras are far superior to their 35mm ancestors. It fails to take into account the variables of pixel density and refraction. It also doesn't take into account aperture. That's where the NPF rule comes in.
Let's first take a look at the equation:
Slowest Shutter Speed = 35 * aperture + (30 * pixel pitch) / focal length
Everything here should look pretty straight forward, with the exception of this new variable: pixel pitch. This is, in fact, its own equation:
Pixel Pitch = Sensor Width (in millimeters) / number of pixels in width * 1000
As you can see, this pixel pitch variable is where most of the added complexity of the NPF rule comes into play. You can also see how this would be a pain to try to work out in your head when your out shooting in the field!
I feel it's pretty impractical to work this all out by hand, so this is as much detail as I'll go into for the NPF rule. Let's instead look at how to use the PhotoPills app to calculate this for us.
Calculating Your Shutter Speed Using PhotoPills
PhotoPills is a fantastic app that allows you to do far more than just calculate shutter speeds. I can't recommend it highly enough! And no, I'm in no way sponsored or endorsed by the team behind PhotoPills. I just really like the app :)
One of the "pills" within the app is called Spot Stars. This is the tool we use to calculate our shutter speeds to get our slowest possible shutter speed.
From here you will have the options to set the camera you're using, the focal length you are shooting at and your aperture.
The last two options you can set are the declination angle and accuracy options. The declination angle option allows you to set the minimum declination angle of the stars you will be shooting. If you happen to know this, you can set it to get a more accurate result. Zero is the absolute worse case scenario here, though, so it's safe to just leave this at its default.
The last option is the accuracy. The default here can result in some minor, hardly noticeable star trails. You can also set it to accurate, which will get you pin-sharp stars, even when blown up to the size of large prints.
Once you have all of your settings configured, you'll get the results for both the NPF rule and the 500 rule, no mental, mathematical gymnastics required!