500 Rule Calculator
The actual focal length of your lens in millimeters (not the 35mm equivalent)
Select your camera's sensor format to automatically apply the correct crop factor
Your camera's sensor resolution — used to calculate pixel pitch and NPF rule
Your shooting aperture (e.g. 1.4, 1.8, 2.8, 4) — used in the NPF rule and EV calculation
Celestial declination of your target — stars at the equator trail fastest; polar targets allow longer exposures
Your shooting ISO — used to calculate the Exposure Value (EV) for Milky Way optimization
Enter Your Camera Settings
Enter your focal length and sensor type to calculate the maximum shutter speed before star trails appear in your astrophotography.
How to Use the 500 Rule Calculator
Enter Your Focal Length
Type your lens's actual focal length in millimeters — for example, 24mm, 35mm, or 50mm. Use the prime or zoom focal length you plan to shoot at, not the 35mm equivalent. The calculator multiplies by your crop factor automatically.
Select Your Sensor Size
Choose your camera's sensor format from the dropdown. This sets the crop factor automatically: 1.0x for Full Frame, 1.5x for APS-C (Nikon/Sony/Fuji), 1.6x for APS-C (Canon), 2.0x for Micro Four Thirds, or 2.7x for 1-inch sensors. Medium Format cameras with a 0.79x crop factor are also supported.
Add Megapixels and Aperture for NPF Results
Enter your camera's megapixel count and your shooting aperture (f-stop) to unlock the more accurate NPF Rule result. The calculator automatically derives your sensor's pixel pitch from the megapixels and sensor dimensions, eliminating the need for manual lookup.
Adjust Declination and Review the Exposure Triangle
Drag the declination slider to match your target's position in the sky — 0° for equatorial targets (Milky Way core), or higher values for circumpolar targets. Add your ISO to see the resulting Exposure Value and check whether your full exposure triangle is within the optimal range for Milky Way photography.
Frequently Asked Questions
What is the 500 Rule in astrophotography?
The 500 Rule is a simple formula for calculating the maximum shutter speed you can use before star trails become visible in a night sky photograph. The formula is: 500 divided by the effective focal length (focal length × crop factor). For example, with a 24mm lens on a full-frame camera, the 500 Rule gives approximately 20.8 seconds. Exceeding this time means the Earth's rotation will cause stars to streak across the sensor, appearing as short arcs rather than pinpoints of light. The rule was originally derived from observations with 35mm film cameras and remains a quick and widely-used starting point for astrophotography beginners and experienced photographers alike.
What is the difference between the 300, 400, 500, and 600 rules?
All four variants use the same formula structure — divide the constant by the effective focal length — but differ in how conservative the result is. The 300 Rule gives the shortest (most conservative) safe exposure, recommended for modern high-resolution cameras with 24MP or more. The 400 Rule is a moderate compromise. The 500 Rule is the traditional standard that most astrophotographers learn first. The 600 Rule allows the longest exposures and was more appropriate for older lower-resolution film and early digital cameras that could not resolve the fine star trailing that modern sensors capture. For cameras with 40MP or more, the 300 Rule is strongly recommended.
What is the NPF Rule and is it more accurate than the 500 Rule?
The NPF Rule is a more mathematically rigorous formula for calculating maximum star-trail-free exposure. Unlike the 500 Rule, the NPF formula incorporates your lens aperture (N), the sensor's pixel pitch in micrometers (P), and the declination of your celestial target (F for declination adjustment). The full formula is: (16.856 × aperture + 0.0997 × focal length + 13.713 × pixel pitch) ÷ (focal length × cos(declination)). The NPF Rule is consistently more accurate for high-resolution cameras and telephoto lenses, giving shorter and safer results than the 500 Rule predicts. Our calculator auto-derives pixel pitch from your megapixels and sensor dimensions, so you don't need to look it up manually.
How does declination affect the maximum shutter speed?
Stars at the celestial equator (declination 0°) sweep across the sky at the maximum angular speed because they travel the full circumference of the celestial sphere in one sidereal day. Stars near the celestial poles travel in much smaller circles and appear to barely move. The declination correction divides the base NPF result by cos(declination), so a target at 60° declination allows twice as long an exposure as the same target at 0°. For Milky Way core photography, your target is near declination −30° to −30°, so the correction is modest. For circumpolar targets like star-trail photography around Polaris at 89°N declination, you can expose for many minutes without noticeable trailing.
What Exposure Value (EV) should I target for Milky Way photography?
Experienced Milky Way photographers typically target an Exposure Value of approximately −7 to −8 EV for optimal results. This range captures enough light from the faint diffuse glow of the Milky Way and individual stars without overexposing the brighter parts of the sky or introducing excessive light pollution. An EV below −8 often indicates underexposure — you may need to raise ISO or widen your aperture. An EV above −5 suggests possible overexposure or that skyglow is brightening the frame. The standard EV formula is: EV = log₂(aperture² ÷ (shutter speed × ISO ÷ 100)). Our calculator computes this automatically from your input values.
Why do high-megapixel cameras require shorter exposures than the 500 Rule suggests?
High-megapixel cameras have smaller individual pixels packed more densely across the sensor. This smaller pixel pitch means each pixel captures light from a narrower angle of sky, making it more sensitive to the angular movement of stars during an exposure. A 61MP Sony camera has a pixel pitch of roughly 3.76µm, while a 12MP camera of the same sensor size has a pitch of about 8µm — more than double. Even the same tiny angular movement of a star translates into a proportionally larger shift across more pixels, making trailing visible sooner. The NPF Rule accounts for this by incorporating pixel pitch directly into the formula, and the 300 Rule was developed as a simpler heuristic to compensate for the limitations of modern high-resolution sensors.