500 rule & NPF rule calculator
The longest exposure you can use before the Earth’s rotation turns stars into short lines. The NPF rule accounts for aperture and pixel size — we work out the pixel pitch from your megapixels.
With a 20 mm lens at f/1.8 on full frame and 24 MP, keep exposures under 12 s (NPF rule) for pinpoint stars — set the shutter to 10 s. The older 500 rule allows 25 s, which shows slight trails at 100% on modern sensors.
- 400 rule
- 20 s
- 300 rule
- 15 s
- Full-frame equivalent
- 20 mm
- Resolution
- 24 MP on 36 × 24 mm
Other focal lengths (f/1.8, 24 MP)
| Focal length | 500 rule | NPF rule |
|---|---|---|
| 14 mm | 36 s | 17 s |
| 16 mm | 31 s | 15 s |
| 20 mm | 25 s | 12 s |
| 24 mm | 21 s | 10 s |
| 28 mm | 18 s | 8.7 s |
| 35 mm | 14 s | 6.9 s |
| 50 mm | 10 s | 4.9 s |
| 85 mm | 5.9 s | 2.9 s |
How the calculator works
The 500 rule is t = 500 ÷ (f × crop); the 400 and 300 variants just use a smaller number for stricter results. For the NPF rule we first estimate the pixel pitch from the sensor size and megapixels: p = √(width × height ÷ pixels). A 24 MP full-frame sensor has 6.0 µm pixels; a 61 MP one has 3.8 µm. Then t = (35N + 30p) ÷ f, using the real focal length.
Example: a Sony A7 IV (33 MP, 5.1 µm pixels) with a 24 mm f/1.4 lens gives (35 × 1.4 + 30 × 5.1) ÷ 24 ≈ 8 s, while the 500 rule says 21 s. The difference is why so many "500 rule" shots look slightly streaky on a big screen.
Read the full comparison in 500 rule vs NPF rule, and check the hyperfocal distance of your astro lens with the hyperfocal calculator so the foreground is sharp too.
More photography calculators
- Depth of field calculatorNear and far limits of sharpness, with a diagram
- Hyperfocal distance calculatorWhere to focus so everything to infinity is sharp
- Equivalent exposure calculatorTrade aperture, shutter and ISO at the same brightness
- ND filter calculatorLong exposure times for ND8 to ND64000, with a timer
- Crop factor calculatorEquivalent focal length, aperture and field of view
Frequently asked questions
What is the 500 rule?
Divide 500 by the full-frame equivalent focal length to get the longest exposure in seconds before stars visibly trail. A 24 mm lens on full frame: 500 ÷ 24 ≈ 21 s. On a 1.5× APS-C camera: 500 ÷ 36 ≈ 14 s. It dates from the film era and is too generous for modern high-resolution sensors.
What is the NPF rule?
A more accurate rule developed by Frédéric Michaud of the Société Astronomique de France: t = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel pitch in micrometres and f the real focal length in millimetres. It keeps stars round when you view the image at 100%.
Which rule should I use?
Use the NPF rule if you print large, crop, or look at your images at full size. The 500 rule (or the 400 or 300 rule) is fine for photos that will mostly be seen on a phone screen. If you want both long exposures and sharp stars, use a star tracker or stack several shorter exposures.
Why do stars trail at all?
The Earth rotates about 15 arcseconds per second, so the sky moves relative to your camera. Stars near the celestial equator move fastest; stars near Polaris (or the south celestial pole) move much less, so you can expose longer when pointing there.
What are good Milky Way settings?
A 14–24 mm lens wide open (f/1.4–f/2.8), ISO 1600–6400, the NPF exposure time, manual focus on a bright star using magnified live view, and RAW. Shoot when the Moon is down and far from city lights.