What the 500 rule is
The 500 rule is a rough guideline for the longest shutter speed you can use at night before stars start to look like short streaks instead of points. You divide 500 by the focal length of your lens, in millimeters, and the result is a maximum exposure time in seconds.
Maximum exposure (seconds) = 500 / focal length (mm)
The rule exists because the Earth rotates, so stars appear to drift across the sky. A wider lens covers a bigger area, so the same drift moves the stars across fewer pixels, and a longer exposure is possible. A longer lens magnifies the drift, so exposures need to be shorter.
Examples for a full-frame camera
| Focal length | 500 rule result |
|---|---|
| 14 mm | About 35 seconds |
| 20 mm | 25 seconds |
| 24 mm | About 21 seconds |
| 35 mm | About 14 seconds |
| 50 mm | 10 seconds |
| 100 mm | 5 seconds |
Treat these numbers as an upper limit. Rounding down to the nearest convenient shutter speed is common.
Crop sensors
On a camera with a smaller sensor, the field of view is narrower, so you use the equivalent focal length. Multiply the lens focal length by the crop factor (about 1.5 for many APS-C cameras and 2 for Micro Four Thirds), then divide 500 by that number.
For example, a 16 mm lens on an APS-C body with a 1.5 crop factor behaves like 24 mm. The rule gives about 21 seconds.
Why the rule is only approximate
The rule dates from film-era cameras and lower-resolution sensors. Modern sensors have many more pixels, and a streak that was invisible on an old camera can be obvious when you zoom in on a high-resolution image. Other factors also matter:
- Where you point the camera. Stars near the celestial poles move less than stars near the celestial equator, so exposures can be longer when you face the pole.
- Print or viewing size. Small images hide trailing that a large print would show.
- Your tolerance. Some people accept slight elongation, others want round points.
Because of this, many photographers use a shorter number such as 400 or 300 divided by focal length, which is also the logic behind searches for a “400 rule”. Another approach, sometimes called the NPF rule, accounts for the aperture, pixel size and focal length. Several online calculators implement it.
How to use the rule in the field
- Work out your effective focal length.
- Divide 500 (or a more conservative number) by it.
- Choose the nearest standard shutter speed at or below the result.
- Take a test shot and zoom in to 100 percent on stars near the corners.
- If stars look elongated, shorten the exposure and raise ISO.
This pairs with the other settings in Milky Way camera settings and the broader night photography settings guide.
Full-stop shutter speeds
PAA results ask about shutter speed steps. Full stops are each double or half of the neighboring value: 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1 second, and then 2, 4, 8, 15 and 30 seconds. Many cameras also offer intermediate third-stop values, which are useful when the 500 rule gives something like 21 seconds. The shutter speed chart shows how to read these values.
When the rule does not apply
The 500 rule is for sharp star points. If you want star trails, you intentionally use a much longer exposure or stack many frames; see night long exposure techniques. If you use a star tracker that follows the sky, the rule no longer limits your exposure time in the same way.
Other things that make stars look stretched
Not every elongated star is caused by Earth’s rotation. Focus errors, wind and a loose tripod can cause blur. Corner stars may also stretch because of lens aberrations such as coma, which stays the same however short the exposure.