The 500 rule is a quick way to estimate the longest shutter speed you can use at night before the sky shows trails instead of points of light. Divide 500 by your lens focal length (adjusted for sensor size) and the answer is a first guess in seconds, which you then refine by checking your own images.
Why the Rule Exists
The sky appears to move because the Earth rotates. During a long exposure, each point of light slowly drags across the sensor. A wider lens makes that movement smaller on the frame, so you can leave the shutter open longer. A longer lens magnifies the movement, so the time must be shorter. The 500 rule turns that idea into one simple division.
How to Apply It
- Note your lens focal length. If you use a zoom, check the number at the zoom position you will shoot at.
- If your camera has a crop sensor, multiply the focal length by the crop factor (roughly 1.5 for many APS-C bodies and 2 for Micro Four Thirds). Full frame needs no adjustment.
- Divide 500 by that number. For example, 500 divided by 20 is 25 seconds on a full frame body with a 20mm lens.
- Set the camera to manual mode and dial in that shutter speed or the nearest value below it.
- Open the aperture as wide as the lens allows, then raise ISO until the exposure looks right on the screen. Values in the range of ISO 1600 to 6400 are common first values.
- Take a test frame, zoom in to 100 percent on the playback screen, and look at the brightest points. If they look stretched, shorten the time.
Where the Rule Falls Short
The rule dates from film days, and modern high resolution sensors reveal trails that older cameras would hide. Many photographers now use a stricter number such as 400 or 300, especially with sensors that have small pixels. Direction also matters: points near the celestial pole move less than points near the celestial equator, so a frame aimed north (in the northern hemisphere) tolerates a longer time than one aimed south.
| Focal length (full frame) | 500 rule | Safer estimate (rule of 300) |
|---|---|---|
| 14mm | about 35 seconds | about 21 seconds |
| 24mm | about 20 seconds | about 12 seconds |
| 35mm | about 14 seconds | about 9 seconds |
These numbers are general guidance, not guarantees. Your own camera and lens decide the real limit.
Tips for Cleaner Results
- Use a sturdy tripod and a two second timer or remote release to avoid shake.
- Focus manually on a bright point using live view magnification, then leave focus alone.
- Shoot RAW so you have room to adjust white balance and brightness later.
- Choose a fast wide lens when you can, since a wider aperture lets you shorten the time or lower ISO.
FAQ
Does the 500 rule work for every lens?
It works best for wide and ultra wide lenses. With longer focal lengths the allowed time becomes very short, and many people use a motorized tracking mount instead.
What if my points of light look like short lines?
That means the time is too long for your sensor and lens. Divide by a larger number such as 400 or 300, or shorten the exposure and raise ISO a little.
The Short Version
Divide 500 by your effective focal length to get a rough maximum shutter speed in seconds. Treat the result as a first guess, check a zoomed-in test frame, and shorten the time if the bright points are not round.
