How to use the 500 Rule Calculator
You need two pieces of information to get your recommended exposure time:
- Enter the focal length of the lens you are using. For example, if you are using a 14mm lens, enter "14".
- Select whether your camera's sensor is Full-Frame or a crop sensor such as APS-C.
If you do not know your camera's sensor size, you can look it up by searching for your camera model and "sensor size". Press Calculate to get your recommendation.
What is the 500 Rule in photography?
The 500 Rule estimates the shutter speed you should use when photographing stars and the Milky Way with a static (non-tracking) setup. It exists because of two competing needs:
- To photograph the stars you need a long exposure to capture enough light, but
- if the exposure is too long the stars blur, because the Earth rotates while you are shooting and the stars appear to move.
So you want the longest exposure that still keeps the stars as points of light rather than trails. The formula is:
500 / focal length of lens = exposure time (in seconds)
500 Rule for crop sensor cameras
That formula assumes a full-frame sensor. For a crop sensor camera it becomes:
500 / focal length / crop factor = exposure time (in seconds)
A typical APS-C crop factor is 1.5 or 1.6, and Micro Four Thirds is 2.0. The calculator above applies the crop factor for you once you pick your sensor.