Astrophotography exposure calculator
To keep stars as points rather than streaks, divide 500 by your full-frame equivalent focal length for a quick limit — or use the NPF rule below for a sharper, sensor-aware answer. The Earth's rotation drags the stars across your frame, so beyond a certain shutter time each point smears into a short trail. This tool gives you that time for your exact setup: enter a focal length, pick your camera (or type in your sensor), add the aperture, and it returns both the familiar 500-rule number and the stricter NPF-rule number that holds up when you zoom to 100% or print. Everything runs in your browser; nothing you enter is uploaded.
Advanced: crop factor & sensor details
These feed the NPF rule's pixel pitch. Choosing a preset above fills them in; switch to “Custom sensor” to type your own.
How it works
Stars appear to move because the Earth turns underneath them, completing a full circle in about 24 hours. At the celestial equator that is roughly 15 arc-seconds every second of time. Whether that motion shows up as a trail in your photo depends on how much of the frame each star crosses during the exposure, which is why focal length, sensor size and pixel density all matter. The 500 rule captures the biggest factor — focal length — in one division: 500 divided by the full-frame-equivalent focal length gives a shutter time in seconds. The NPF rule goes further, adding your aperture and the physical size of a pixel so the answer reflects trailing at the pixel level, not just what is visible on a small screen.
Worked example
Say you shoot the Milky Way with a 20mm f/2.8 lens on a 24-megapixel full-frame body. The effective focal length is 20mm, so the 500 rule gives 500 ÷ 20 = 25 seconds. The NPF rule, using an aperture of f/2.8 and a 6.0-micron pixel pitch, works out to (35 × 2.8 + 30 × 6.0) ÷ 20 = 13.9 seconds — barely more than half. Both are “right”; they answer slightly different questions. Twenty-five seconds looks clean on the back of the camera, while fourteen seconds is what keeps the stars as crisp points when you view the file at full resolution. Put a denser 45-megapixel sensor behind the same lens and the NPF time drops again, because each pixel now covers a smaller slice of sky.
500, 300 or 200?
You will see the “rule” quoted with different numbers. 500 is the traditional, forgiving value that dates from the film era; 300 and 200 are stricter variants people adopted once high-resolution digital sensors made trailing easier to see. Think of them as a tolerance dial: 500 for a fast look, 300 for careful web-sized images, 200 for large prints. The calculator shows all three alongside the NPF figure so you can pick the trade-off that matches how the shot will be viewed.
| Lens (full-frame equiv.) | 500 rule | 300 rule | 200 rule |
|---|---|---|---|
| 14 mm | 35.7 s | 21.4 s | 14.3 s |
| 20 mm | 25.0 s | 15.0 s | 10.0 s |
| 24 mm | 20.8 s | 12.5 s | 8.3 s |
| 35 mm | 14.3 s | 8.6 s | 5.7 s |
| 50 mm | 10.0 s | 6.0 s | 4.0 s |
Frequently asked questions
What is the 500 rule?
Divide 500 by your full-frame-equivalent focal length to estimate the longest exposure before stars trail. A 20mm lens on full frame gives 500 ÷ 20 = 25 seconds; on an APS-C body (1.5x) it is 500 ÷ 30 ≈ 17 seconds.
Why is the NPF rule shorter than the 500 rule?
Because it accounts for aperture and pixel pitch. Modern sensors pack more, smaller pixels, so a star crosses several pixels in less time than the film-era 500 rule assumes. The NPF rule reflects that and gives a tighter limit for critically sharp stars.
Do I use the actual or the equivalent focal length?
Both, in different places. The 500 rule uses the full-frame equivalent (actual focal length times the crop factor). The NPF rule uses the actual focal length together with the real pixel pitch, which already encodes the sensor size. This calculator handles the bookkeeping for you.
Can I expose longer near Polaris?
Yes. Stars near the celestial pole barely move, so you can use a longer shutter there than near the equator. Turn on the declination option to scale the result by one over the cosine of the declination angle.
Does this store my inputs?
No. All maths runs in your browser. Nothing you type is uploaded, logged or kept after you leave the page.
Want the reasoning behind the numbers? Read the guide to the 500 rule and NPF rule, or see 500 rule vs NPF rule vs a star tracker for when each approach wins.