About the Astrophotography Shutter Speed Calculator
This astrophotography shutter speed calculator tells you how long you can leave the shutter open on a fixed tripod before the stars stop being points and start to smear into short trails. Because the Earth rotates, stars drift across the frame, and the longer your lens and the smaller your pixels, the sooner that drift becomes visible.
Two methods are shown side by side. The classic 500 rule (or the stricter 400 and 300 variants) divides a constant by the full-frame-equivalent focal length and is quick to do in your head. The NPF rule, popularised by the Société Astronomique de France, also accounts for aperture and pixel pitch, so it gives far shorter — and sharper — exposures on today’s high-resolution sensors.
Use it when planning Milky Way, nightscape or meteor shots. Choose your sensor format and megapixels so the pixel pitch can be worked out, then enter the focal length and aperture you will shoot at. Stars near the celestial poles move more slowly, so you can optionally enter the declination of your target for a longer allowance.
With the default inputs, the max exposure (npf rule) is 13.9 s. Change any value above to recalculate instantly.
How to use the astrophotography shutter speed calculator
- 1Enter the focal length you will shoot at (the real focal length, not the equivalent).
- 2Enter the aperture, usually the widest your lens allows for night skies.
- 3Pick your sensor format and enter the camera’s megapixels.
- 4Choose the 500, 400 or 300 rule for the quick comparison.
- 5Optionally enter the declination of your target, then set your shutter at or below the NPF result.
Formula and method
The simple rule divides a constant K (500, 400 or 300) by the full-frame-equivalent focal length. It dates from film days, when enlargements were small, and assumes the same blur tolerance whatever the sensor resolution, so it is generous on modern 24–60 MP cameras.
The NPF rule (simplified form by Frédéric Michaud, Société Astronomique de France) uses the f-number N, the pixel pitch p in micrometres and the real focal length f in millimetres. Pixel pitch is estimated as sensor width ÷ horizontal pixel count, with the pixel count derived from megapixels and the sensor’s aspect ratio. Both results are divided by the cosine of the target’s declination δ because stars away from the celestial equator trace smaller circles and move more slowly across the sensor (capped at 89° to avoid an infinite result).
- t
- Maximum exposure time in seconds
- N
- Aperture f-number
- p
- Pixel pitch in µm
- f
- Focal length in mm
- K
- Rule constant: 500, 400 or 300
- δ
- Declination of the target (0° by default)
Worked examples
20 mm f/2.8 on a 24 MP full-frame camera
A 24 MP 36 × 24 mm sensor is 6,000 pixels wide, so each pixel is 6.0 µm. NPF gives (35 × 2.8 + 30 × 6) ÷ 20 = 13.9 s, while the 500 rule gives 500 ÷ 20 = 25 s — almost twice as long, which shows slight trailing at 100%.
16 mm f/2 on a 26 MP APS-C (Fujifilm)
A 23.5 × 15.6 mm sensor with 26 MP is about 6,258 pixels wide, a 3.755 µm pitch. NPF gives (70 + 112.65) ÷ 16 ≈ 11.4 s. The crop factor of 1.53 makes the lens act like a 24.5 mm, so the 500 rule allows about 20.4 s.
50 mm f/1.8 aimed at the Milky Way core (δ = −29°), 400 rule
A 45 MP full-frame sensor has 4.38 µm pixels. NPF gives (63 + 131.5) ÷ 50 = 3.89 s, divided by cos 29° (0.875) ≈ 4.45 s. The 400 rule gives 400 ÷ 50 ÷ 0.875 ≈ 9.1 s.
Frequently asked questions
What is the 500 rule in astrophotography?+
Divide 500 by your full-frame-equivalent focal length to get the longest exposure in seconds before stars visibly trail. A 20 mm lens on full frame gives 25 s; the same lens on a 1.5× APS-C body gives about 16.7 s.
Why is the NPF rule shorter than the 500 rule?+
The 500 rule was designed for film and small prints. The NPF rule includes pixel pitch and aperture, so on high-resolution sensors it limits star movement to a few pixels and gives pin-point stars when you view the image at 100%.
Should I use the 500, 400 or 300 rule?+
Use 500 for web-sized images from lower-resolution cameras, 400 or 300 for modern 24 MP+ sensors or large prints. If you want critically sharp stars, follow the NPF result instead.
Does declination really matter?+
Yes. Stars on the celestial equator move fastest; at declination 60° they move half as fast, so you can double the exposure. Pointing toward Polaris or the south celestial pole allows much longer exposures.
What ISO should I use for the Milky Way?+
Most photographers start around ISO 1600–6400 at f/1.4–f/2.8 with the NPF exposure, then adjust based on the histogram. The calculator only sets the time limit; ISO controls brightness and noise.