500 Rule Calculator
Maximum shutter speed for sharp star photography
Calculate Maximum Exposure
About These Rules
500 Rule: Classic rule. Max seconds = 500 / (focal length × crop factor). Shows slight trails on high-res sensors.
300 Rule: More conservative. Better for modern high-resolution cameras.
NPF Rule: Most accurate. Accounts for pixel size: (35 × aperture + 30 × pixel pitch) / focal length. We estimate using typical values.
Tips for Astrophotography
Use a sturdy tripod and remote shutter. Focus manually on a bright star using live view at 10x magnification. Use wide aperture (f/2.8 or wider) and high ISO (1600-6400).
Frequently Asked Questions
What is the 500 rule in photography and how does it work?
The 500 rule gives the longest shutter speed you can use before stars show visible trailing on a full-frame camera: 500 divided by the focal length in millimeters. At 24mm that is about 20 seconds. On an APS-C sensor with a 1.5x crop factor you multiply the focal length first, so 24mm becomes 36mm equivalent, giving roughly 13 seconds.
How do I apply the 500 rule on a crop sensor camera?
Multiply your actual focal length by the crop factor before dividing into 500. A 24mm lens on a Canon APS-C body (1.6x crop) gives an effective focal length of 38.4mm, so the maximum shutter is 500 divided by 38.4, about 13 seconds. Ignoring the crop factor is the most common mistake, producing trails even within the advertised limit.
Is the 500 rule accurate enough for modern high-resolution cameras?
Not quite. High-megapixel sensors resolve finer detail, so tiny trails become visible at exposures the 500 rule would call safe. The stricter 300 rule or the pixel-pitch-aware NPF rule are better for cameras above 30 megapixels. On a 45MP body shooting at 24mm you may need to stay under 12 seconds rather than the 500 rule’s 20 seconds.
What aperture and ISO should I pair with the 500 rule for star shots?
Use the widest aperture your lens offers, ideally f/1.4 to f/2.8, to maximize light collection during the short exposure. Set ISO between 1600 and 6400 depending on how well your camera handles noise. On a full-frame body, ISO 3200 at f/2 with a 20-second exposure is a solid starting point under a dark sky.
Why does the 500 rule sometimes fail even when I follow it exactly?
Two factors can cause failure: pointing near the celestial equator, where stars move fastest, makes trails appear sooner than the rule predicts; and the rule was designed around older, lower-resolution sensors. Stars near Polaris trail much more slowly, so you can often exceed the limit when composing around the North Star without visible smearing.