If you’re into night sky photography in any way, then it’s only a matter of time before you run across somebody talking about the “500 Rule” – sometimes referred to as the “rule of 500.” It’s a classic photographic rule of thumb that most experienced photographers will commit to memory for when they are out shooting at night and want to capture one of those beautiful images of the Milky Way that tend to blow audiences off their feet and keep the stars nice and sharp.
One of the first challenges beginners faces when getting into astrophotography is taking an in-focus photo with round stars. Because the night sky appears to move from our vantage point on Earth, capturing a long exposure starry sky image on a fixed tripod may reveal star-trailing.
One of the best ways to combat star-trailing when capturing astrophotography images on a stationary (non-tracking) tripod mount is to use the 500 Rule.
If you’ve played around with night photography before, you’re well aware of the many differences to traditional 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.
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Despite thoroughly covering night photography settings in our Beginner’s Guide to Night Photography, there’s one setting that tends to be often overlooked: the shutter speed. Yes, a slow shutter speed is needed but can’t we 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
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 available 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 the Earth’s rotation, stars will appear blurry once the shutter speed exceeds a specific period. These are 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 will 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:
What is the 500 Rule?
The 500 rule is used to measure the maximum exposure time you can shoot before the stars become blurry or before star trails appear. Setting the shutter speed for longer than allowed by this rule will result in images that do not have sharp stars. It is a simple formula to calculate proper exposure time/shutter speed with a particular lens, full-frame, or crop sensor camera. This formula, if done correctly, will produce those pin-point, razor-sharp stars without trailing in your Milky Way photos or images of the night sky.
The 500 rule can be helpful when photographing the night sky on a fixed tripod. The technique works on images of many focal lengths (up to about 200mm) but can be especially effective when photographing the Milky Way with a wide-angle camera lens. Choosing the right wedding photographer in Melbourne to capture every moment on your wedding day.
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 exciting 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 evident that the star trail is accidental instead of deliberate. That’s where the 500 Rule comes in handy.
500 Rule for Night Photography
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.
The 500 Rule for Full Frame Camera
The 500 rule for a full-frame camera requires you to set your camera to ISO 3200 or 6400, Aperture to f/2.8 (or as wide as possible) and your shutter speed to 500 divided by the focal length of your camera.
The calculation is most straightforward when using a full-frame camera, although you should expect to cut down on exposure time a bit. Because you do not need to multiply the focal length by a crop factor, the formula is simply 500 divided by your focal length. For example, if you are shooting with a 50mm lens, your shutter speed would be 10 seconds (500 / 50 = 10).
If you are shooting with a 24mm lens, your shutter speed would be 21 seconds (500 / 24 = 21 approximately).
Note: Depending on your camera sensor’s response to low light, you may want to reduce the iso values sometimes. Some cameras have perfect sensors to get ideal star photographs at around iso values close to 500.
The 500 Rule for a Crop Sensor Canon Camera
The crop factor of a Canon crop sensor camera is 1.6, so you need to account for that in your equation. Then it becomes this:
500 / (focal length x 1.6)
For example, with a 50mm lens it is 500 / (50 x 1.6) = 6 seconds (approximately).
The 500 Rule for a Crop Sensor Nikon Camera
The crop factor for a Nikon crop sensor camera is 1.5, so you account for that like this:
500 / (focal length x 1.5)
For example, with a 50mm lens, it is 500 / (50 x 1.5) = 7 seconds (approximately).
Shutter Speed Settings for the 500 Rule
It is important to remember that this rule is not a perfect solution and that slight adjustments will have to be made according to circumstances. Factors such as light pollution, atmospheric haze, and the stars’ angle are something you should keep in mind.
Here are the settings you would use, according to your camera and lens. Just remember that the 500 rule is not perfect – you probably have to adjust slightly for your particular circumstances, such as light pollution, the stars’ angle, or even atmospheric haze.
How to Calculate Shutter Speeds With the 500 Rule
So what is this magic formula that calculates the maximum shutter speed? It’s quite simple:
500/FOCAL LENGTH = MAXIMUM SHUTTER SPEED
You’re photographing with a 14mm, a standard 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.
It would help if you multiplied 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
Be careful when using the 500 rule for night photography.
While the 500 rule is an excellent guideline for night photography, it’s essential to take care. It’s not a perfect guideline, and by following the above recommendations, you might still see slight blurs in your stars.
We 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, never go beyond 30 seconds when photographing the night sky.
Why Photographers use The 500 Rule
The 500 rule is more of a guideline, but that does not mean it is not applicable. This simple formula can make a big difference in your night sky photography because (in theory) you’ll be able to create photos with sharp stars to the edges of the field.
It serves to discover the maximum exposure time allowed before the stars become blurry or before star trails appear. Setting the shutter speed for longer than allowed by this rule will result in an unclear photograph (the stars will appear as trails rather than dots).
Using this formula requires math knowledge, but you do not have to be a nuclear scientist to understand it and apply it. It is effortless, but often the minor changes make the most significant differences.
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How it Works
The rule is as follows:
SS = 500 / (CF x FL)
If you are perplexed by this formula, there is no need to worry; you just need to understand the abbreviations. SS stands for the shutter speed expressed in seconds, CF is the crop factor of your sensor (the ratio between your sensor and a full-frame one), while FL refers to the focal length in millimetres.
500 / Crop-Factor x Focal Length = Ideal Shutter Speed
According to the type of camera you own, you will have to use different crop-factor values. Here is the list you can refer to:
1 X – Full-frame cameras
1.5 (1.6) X – Nikon (Canon) APS-C cameras
2 X- Micro 4/3 cameras
2.7 X and higher – Compact cameras with a one-inch type sensor (or smaller)
Even though it is widely accepted, the number “500” in this formula does not carry any particular meaning. This number is just something astrophotographers figured out works the best for this type of photos.
This formula works because it automatically calculates your camera’s ideal shutter speed to get the most transparent possible photograph. It does it in the shortest amount of time.
Let’s imagine you wanted to take a beautiful photo of the night sky without any previous knowledge of how to avoid star trails. You set your micro 4/3 camera’s shutter speed to 60-seconds using a remote shutter release cable, for example, expecting a fantastic result. On the contrary, you end up with a blurry photo that doesn’t represent how nice the sky looks in reality.
The 500 Rule can give you a point of reference for the length of time you should expose the image using your camera system. It’s not an exact science, but it does work when capturing images like the one below.
The Science Behind It
The basic idea is to provide an easy formula that will guess how long the exposure time can be before the stars’ movement becomes noticeable.
The sky rotates 0.0042 arc degrees per second, or in simple words, 360 degrees in 24 hours. If you own a full-frame camera and use a 24mm lens, its horizontal view will be 73.7 degrees.
Let’s say this camera has a 24-megapixel sensor (6000 x 4000). The before mentioned, 73.7 degrees are projected onto 6000 pixels, resulting in 81.4 pixels/degree. With this kind of lens, the exposure time will be about 21 seconds, according to the 500 rule (500/24).
The sky will move about 0.09 degrees during these 21 seconds (0.0042*21). 0.1 degrees = 7.3 pixels with this kind of camera (81.4*0.1).
Precisely this number of pixels (7.3) is the maximum acceptable movement blur before the stars as we see them in the sky become star trails in the picture. But can you notice this movement in pictures?
Usually, we look at pictures on a computer screen. If you try to zoom in your full-resolution photograph to 100%, you will notice that stars are not dots – this doesn’t matter because you would never see this with a naked eye.
So, if you’re planning on printing large-format versions of your photos, it’s worth thinking about. If you generally share your photos online (Flickr, Instagram etc.), the fine details become much less critical. Looking for a Yarra Valley wedding photographer? Look no further! Wild Romantic Photography has you covered.
Does the 500 Rule Work?
Do these shutter speeds work in the field? The answer is, sometimes.
There was a mix of responses on whether the 500 rules hold true or not, and the consensus was that it does a great job most of the time.
Experienced photographer Craig Stocks noted that taking an image free of star-trailing using the 500 rule will depend on the camera’s resolution and the quality of the lens. The higher the resolution of the system, the more likely you are to see star trailing.
This effect is not as pronounced in a lower pixel resolution camera, and the 500 rule produces a better result.
It’s also important to mention that capturing images close to the horizon will pronounce the effect, as the stars will appear to be moving slightly faster from your vantage point.
The bottom line is that the 500 rule should be considered a great starting point, but it is not a precise measurement, and you will need to experiment using your specific camera and lens system.
Here are some of the many stacking software options available to you. I tend to use Adobe Photoshop to stack my untracked images or DeepSkyStacker. Adobe Photoshop has an image stacking script option to automate the process, and it is worth checking out.
- Senator – It is excellent to use if you are a Windows user because it is free. It makes the process of image stacking extremely easy, and you will end up with a beautiful photograph in no time. It also supports the RAW format, which is essential when trying to achieve the best quality.
- Starry Landscape Stacker – This software is available only for Mac OS X users, and there is a free trial period. You won’t have to worry that your pictures will turn out anything less than excellent when using this software. The only con about the software is that it doesn’t support RAW files, so you have to convert them to TIFF using Adobe Camera RAW or similar software.
- DeepSkyStacker – Another software that is supported on Windows. It simplifies stacking by doing registering, simple post stacking processes, and saving the final result to a TIFF file. We have an exclusive range of wedding photography Mornington Peninsula services. Check them out here.
Star Trail Images
What about times when you want to show star trails? Star trail photography can result in beautiful portraits of the apparent movement of stars in the night sky. The process involves capturing images with much longer exposure times than you would use for the 500 rule.
Photographers will use the longest exposure time possible (without blowing out the highlights) to capture the most movement in the sky possible. In many cases, this is a 30-second exposure on a fixed tripod. The images are then blended in Adobe Photoshop using the “lighten” blend mode to create ultra-long star trails over time.
The 500 Rule is not the be-all and end-all solution to capturing the night sky’s perfect image, but it is a handy point of reference. Many beginners (myself included) start with a crop-sensor, entry-level DSLR camera and kit lens (usually 18-55mm or similar).
I believe the 500 Rule is an excellent formula for users in this situation to try out the next time you set up your tripod. If you have a high-resolution sensor with 30 MP +, you may want to dial back the 500 Rule suggestion or look into the NPF Rule.
By simply understanding how this formula works, you will have a better understanding of the process of capturing photos of the stars at night, which are essentially moving targets.
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Whether you are using a full-frame camera or a micro 4/3, the 500 rule situations are a helpful benchmark in many cases. When combined with image stacking and different software tools, capturing a beautiful image of your favourite constellation or the Milky Way is within your reach.