If you’ve played around with night photography before you’re well aware of the many differences to standard landscape photography. Forget about the rules and guidelines you know about general settings for photography; most of these are likely to do more harm than good when the sun goes down and stars take over the sky.
Instead, you have to go against everything you know and increase the ISO, open the aperture and use a long exposure time.
Despite thoroughly covering the settings for night photography in our Beginner’s Guide to Night Photography, there’s one setting that tends to often be overlooked: the shutter speed. Yes, a slow shutter speed is needed but can’t we just keep a low ISO, an aperture of f/11 and then extend the exposure time to several minutes?
Yes, we can do that but that also introduces an effect that you might not want in your images. The solution is the 500 Rule.
What happens when we extend the shutter speed too much
Long exposure photography is a topic we’ve covered extensively here on CaptureLandscapes but let’s do a quick summary of what happens when using a slow shutter speed:
The camera picks up any motion that appears while the shutter is open. In other words, anything that moves within the frame while the shutter is open will be picked up by the camera; leaves blowing in the wind during a five-second exposure will appear blurry.
What does this mean for night photography? Due to earth’s rotation, stars will appear blurry once the shutter speed exceeds a certain period of time. This is known as star trails.
In other words, if you decide to keep your ISO at 100 and aperture at f/11, you might need a shutter speed of over 20 minutes. These settings are going to result in a much cleaner image file (less noise and better front-to-back sharpness) but the long shutter speed will result in star trails.
Now, this can be a fun technique to explore but what do you do if you want razor-sharp stars? Which shutter speed is acceptable? Let me introduce you to the 500 rule:
Razor-sharp stars with the 500 rule
The trick with night photography is that you’ve got to make some sacrifices with image quality. There’s no getting around the fact that you need a high ISO and open aperture to get sharp stars.
While star trails can be an interesting technique from time to time, there’s one thing you want to avoid at any cost: star blur. This is when the shutter speed is only slightly too slow and the stars are beginning to become blurry. At this point, it’s obvious that the star trail is accidental instead of deliberate.
That’s where the 500 Rule comes handy.
The 500 rule is a simple guideline you can use to calculate the maximum exposure time you can use before stars begin to blur. Is it perfect? No. But it gives a good indication of what shutter speed you can use with your current setup.
How to calculate shutter speeds with the 500 rule
So what is this magic formula that calculates the maximum shutter speed? It’s actually quite simple:
500/focal length = maximum shutter speed
Let’s say that you’re photographing with a 14mm, a common focal length for night photography. The slowest shutter speed you can use is then 500/14 = 35.7; anything below 35 seconds should give sharp stars.
That’s it? Finished? Not quite. The formula above isn’t going to work for everyone reading this. If you use a crop sensor camera (APS-C or Micro Four Thirds) the calculated shutter speed above will result in blurry stars as it’s intended for full-frame sensors.
You need to multiply the focal length with the crop factor to calculate the maximum shutter speed for an APS-C or Micro 4/3 sensor. Most APS-C sensors have a crop factor of 1.5 and Micro 4/3 have a crop factor of 2. Canon’s APS-C sensors are slightly different and have a crop factor of 1.6.
APS-C: 500/(focal length * 1.5) = maximum shutter speed
APS-C (Canon): 500/(focal length * 1.6) = maximum shutter speed
Micro Four Thirds: 500/(focal length * 2) = maximum shutter speed
Calculate the maximum shutter speed for your equipment
Now that you know the formula for your camera it’s quite straight forward to calculate the shutter speed you shouldn’t exceed. However, to make it even easier for you, I’ve put together a list of maximum shutter speeds based on your crop factor and focal length:
|Focal Length||Full-Frame Sensor||APS-C 1.5||APS-C 1.6 (Canon)||Micro 4/3|
|14mm||36 sec||24 sec||22 sec||18 sec|
|16mm||31 sec||21 sec||20 sec||16 sec|
|20mm||25 sec||17 sec||16 sec||13 sec|
|24mm||21 sec||14 sec||13 sec||10 sec|
|35mm||14 sec||10 sec||9 sec||7 sec|
|50mm||10 sec||7 sec||6 sec||5 sec|
Not quite sure what crop factor you’re camera has? Do a quick Google search for “[your camera] crop factor” and you’ll know in no time.
Be careful when using the 500 rule for night photography
While the 500 rule is a great guideline for night photography, it’s important to take care. It’s not a perfect guideline and by following the above recommendations you might still see slight blurs in your stars.
I strongly recommend that you take a test shot with the calculated shutter speed and zoom in on the preview to make sure that the stars are sharp. If they’re not, reduce the exposure time slightly.
As a rule of thumb, I never go beyond 30 seconds when photographing the night sky. That’s despite the fact that I’ve got a full-frame sensor and a 14mm focal length.
More about night photography
Make sure to read through the following articles if you want to learn more about how you can capture stunning images at night:
- Beginners Guide to Night Photography
- 7 Tips for Better Night Photography
- The Best Settings for Night Photography
- How to Plan and Photograph the Milky Way
Want to skyrocket your night photography? Then combine the above articles with Mikko Lagerstedt’s in-depth Star Photography Masterclass; a detailed eBook that will help you take great images in the dark, starting today.