Aside from gear related questions, probably the most common question I hear related to night photography would have to be, “what are your settings?” It’s a completely valid question, and I remember my puzzled reaction the first time I learned it for myself. Your settings for astrophotography are quite different from most other types of photography, particularly portraits and even general landscapes. But to talk about settings for capturing the Milky Way, we need to talk about your shutter speed and the 500 Rule.
There are basically two different ways to capture the stars in the night sky; single, static points, or blurry, oblong trails. The 500 rule helps us figure out the longest shutter speed before our stars begin to appear as blurry oblong trails. If you are desiring to photograph star trail images, where the stars appear to streak through the sky, this is not the blog post for you - although you can still glean some insight from this topic. During my astrophotography workshops, we go into various ways to capture star trail images, and it’s really a lot of fun.
One of my first night sky photographs, which I was quite proud of at the time, was a long exposure captured over several minutes. The resulting image shows the stars, not as fine points, but as short streaks. At first glance, the stars appear blurry and as a result the overall image suffered - I needed to use a faster shutter speed. There are some instances in which a short trail may be desired, but this post is about capturing static night sky images while minimizing star movement in the image.
The 500 Rule for Full Frame Cameras
To minimize star trails and capture a static image of the night sky, there is a simple formula that can be used to determine your shutter speed. You take the number 500 and divide it by the focal length of the lens you are using, and the resulting number will be the maximum recommend shutter speed in second s.
500/focal length = max shutter speed in seconds
A 16mm lens on a full frame camera works out to just over 31 seconds maximum exposure or 500/16 = 31.25. A 35mm lens would give us a maximum exposure time of approximately 15 seconds, rounding up. Wide angle lenses provide a clear advantage when it comes to maximizing your shutter speed for gathering more light in your scene.
Lenses I use for astrophotography, and their corresponding max shutter speeds:
14mm = 35 seconds (I usually shoot at 30)
16mm = 31 seconds (I usually shoot 30 seconds here also)
20mm = 25 seconds
35mm = 14 seconds (I routinely round up to 15 seconds)
50mm = 10 seconds (pictured left)
For most situations the 500 rule is sufficient for allowing the creation of large prints that will be viewed at a reasonable distance, with minimal visible streaking of the stars across your camera’s pixels.
If you're using a crop sensor camera, you'll need to use a slightly different formula.
300/focal length = max shutter speed in seconds
This is because a lens' effective focal length is longer when used with a crop sensor camera, usually by a factor of 1.6 times (varies by camera model.) So a 16mm lens is effectively 26mm on a crop sensor, rounding up.
Using the same lens examples as above, a 16mm focal length lens on a crop sensor camera works out to just over 18 seconds maximum exposure, 300/16 = 18.75. I would recommend rounding up to 20 seconds or down to 15 for simplicity. Now, a 35mm lens would give us a maximum exposure time of just over 8 seconds. As you can see, a full frame camera gives us an advantage in lengthening our exposure times to gather more light.
Note: rather than using the "300 Rule," you could opt to convert your lens focal length by multiplying it by the crop factor of your camera, then plugging this number into the 500 rule. In this case, 26mm.
Same lens focal length examples as above, but adjusted for a Canon crop sensor, and their corresponding max shutter speeds:
14mm = 22 seconds (rounding down to 20 simplifies things)
16mm = 20 seconds (rounded up)
20mm = 15.6 seconds (round down to 15 for simplicity)
35mm = 8.9 seconds (recommend 8 seconds)
50mm = 6.25 seconds (recommend 6 seconds)
The math clearly indicates that a full frame camera and a wide angle lens are a great combination for photographing the night sky. I typically recommend lenses like the Rokinon 14 f/2.8 lens if you are just getting started. Photographing the night sky with a single lens will provide you with more than enough to experiment with and learn before you might consider adding more specialized lenses to your kit.
Note about the 500 Rule - It’s not perfect
Depending on how much you enjoy math, you may or may not appreciate the information at the following link: http://starcircleacademy.com/2012/06/600-rule/
At that link you can find a much more intricate formula that will allow you to calculate the maximum shutter speed for your precise camera and lens, based on your exact location, etc so that stars do not travel across even one pixel during a single exposure. Did that just get way too nerdy?
It should be stated that the 500/300 rule isn't perfect. If your print is large enough and the viewer is standing close enough to the image, most eyes will see some streaking. Scoot back to a more comfortable viewing distance and you are likely to be satisfied with the results.
To summarize: the 500/300 rule works great for most applications and I recommend experimenting with it for yourself.
What about your other settings?
We spent this blog post talking about maximizing our shutter speed to gather as much light as possible. Next, we want to bump our ISO (camera sensitivity) settings high into the 3200-6400 range, so our camera’s sensor can collect as much light as possible. Finally, set your aperture wide open (hopefully in the f/2.8 or wider range depending on your lens) again, to let in as much light as possible. This is all given that we are far away from bright city lights and we are attempting to capture the splendor of the Milky Way and the surrounding landscape with a wide angle lens.
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