One of the most common questions that come up when discussing night photography is: “How do I get my stars to not look like lines?” Nighttime landscape photography is different from daytime landscape photography in that high ISO is not only perfectly acceptable, but sometimes necessary. You see, you can’t just use a low ISO and keep the shutter open to achieve the proper exposure. You have a time limit at night, unless of course you are shooting star trail images. Because the Earth rotates, celestial objects appear to move across the night sky even though they are completely still. Because of the Earth’s movement, there is a maximum amount of time your shutter can be open before it starts to capture this motion. There is no hard and fast way to determine what is and what isn’t an acceptable amount of captured movement. That is entirely up to you and your interpretation. But there is a starting point which is called the “500 Rule”
The “500 Rule” is where you take the number 500 and divide it by the 35mm equivalent focal length of your lens to get the maximum shutter speed. This gives you the longest exposure before any noticeable star movement appears in your image. What I mean by 35mm equivalent focal length is if you are using a crop sensor camera, because they are smaller than full frame (35mm) sensors, there is a magnification crop factor. Canon’s crop factor is 1.6, while Nikon is 1.5. This means a 12mm lens used on a Nikon crop sensor DSLR is equivalent to 18mm on a full frame DSLR. (12mm x 1.5 Nikon crop factor = 18mm equivalent focal length) Including this in our equation helps us include other sensor types in our formula instead of just full frame cameras.
Your equation for the “500 Rule” is:
500/(Focal Length x Crop Factor) = Shutter speed
Now some examples:
For a full frame camera with a 14mm focal length lens your “500 Rule” formula should look like this:
500/14 = 35.7 seconds (or effectively 36 seconds)
Now, if you have a crop sensor, like say a Canon 60D with a 12mm lens your formula would look like this.
500/(12 x 1.6) = 26.04 seconds (or effectively 26 seconds)
Here is a quick table/cheat sheet to show some typical “500 Rule” calculations:
|Focal Length||Full Frame||Nikon 1.5 Crop||Canon 1.6 Crop|
The “500 Rule” evolved from the “600 Rule”. The “600 Rule” is the same as the “500 Rule” except you use 600 instead of 500. The “500 Rule” came about because some thought the “600 Rule” was not effective enough at calculating a shutter speed without star trails. To each their own, I say. Personally, I’m typically somewhere in between.
Using the “500 Rule” formula will give you your starting point to determining how long of a shutter speed YOU think you need to use for your shots. Rules are always meant to be broken, but this should help guide you in the right direction.