500 Rule Calculator: Longest Exposure Before Star Trails
This 500 rule calculator finds the longest exposure at which stars still stay point-like: enter focal length, sensor size, aperture and resolution, and the 500, 420 and NPF rules are compared side by side.
Example: Focal length 24 mm · Sensor size: Full frame (36 × 24 mm) · F-number 2.8 · Resolution 24 MP → Maximum exposure time: NPF rule: 12 s · 500 rule: 21 s. Source: Wikipedia – Astrophotography (500 rule: 500 / (focal length × crop factor)). Updated: .
How it is calculated
With 24 mm on full frame, the 500 rule allows about 21 seconds before stars turn into streaks. The rule divides 500 by focal length times crop factor.
How it is calculated
The Earth rotates, so stars drift across the sensor during an exposure. The 500 rule is t = 500 / (f · crop), and the 420 variant uses 420 instead of 500. The more accurate NPF rule also takes the f-number N and the pixel pitch p into account: t = (35 · N + 30 · p) / f. High-resolution sensors reveal star trails sooner, so the NPF time is often well below the 500 time. Near the celestial pole stars move more slowly, so the calculator lengthens the time by 1 / cos(declination).
What it is useful for
- Photographing the Milky Way and the night sky without a tracker.
- Estimating the usable time when choosing between a wide-angle and a longer lens.
- Setting aperture and ISO once the time is known.
Frequently asked questions
How does the 500 rule work?
Maximum exposure time in seconds = 500 divided by focal length times crop factor. At 35 mm on APS-C that is about 9.5 seconds.
Why is the NPF rule more accurate?
It includes the aperture and the pixel pitch of the camera. On sensors with many megapixels, star trails show up sooner, so the NPF time is shorter than the 500 time.
Which focal length do I enter?
The actual focal length of the lens. The crop factor of the chosen sensor size handles the conversion to full frame.
What does declination mean?
The celestial latitude of your subject: 0° at the celestial equator, 90° at the celestial pole. The closer to the pole, the slower the stars move and the longer you can expose.
Sources and legal basis
- Wikipedia – Astrophotography (500 rule: 500 / (focal length × crop factor))
- Astro Backyard – The 500 Rule (NPF rule by Frédéric Michaud: (35 × aperture + 30 × pixel pitch) ÷ focal length)
- Wikipedia – Image sensor format (sensor dimensions)
As of: · Editorial policy
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