Astrophotography Exposure Time Calculator
Enter your lens, aperture and camera to see how long you can leave the shutter open on a fixed tripod before the stars stretch into lines.
Untracked exposure time
Longest shutter speed on a fixed tripod before stars start to trail.
The real focal length printed on the lens.
24 MP full frame about 6 µm, 24 MP APS-C about 3.9 µm.
0° for the celestial equator, -29° for the Milky Way core.
Longest exposure (NPF rule)
9.46 s
Stars stay as sharp points at this length. NPF is stricter than the 500 rule and suits modern high-megapixel sensors.
- NPF rule
- 9.46 s
- 500 rule
- 20.83 s
- 300 rule (high resolution)
- 12.5 s
The 500 rule was made for film and lets stars stretch into short dashes on today's sensors. Treat any rule as a starting point and zoom in on a test frame.
How this is calculated
- 1
NPF rule
The simplified NPF rule gives t = (35 × N + 30 × p) / f, where N is the f-number, p is the pixel pitch in µm and f is the real focal length in mm. A 24 mm f/2.8 lens on a camera with 4.3 µm pixels gives (98 + 129) / 24, about 9.5 s.
- 2
500 and 300 rules
The 500 rule divides 500 by the full-frame equivalent focal length (focal length × crop factor). The same 24 mm lens on full frame gives about 21 s. The 300 rule uses 300 instead and suits high-resolution sensors, giving 12.5 s.
- 3
Allow for declination
Stars move fastest on the celestial equator. At declination δ they move cos(δ) times as fast, so each time is divided by cos(δ). The allowance is held at its 80° value nearer the pole, where the simple rules no longer apply.
Frequently asked questions
What is the NPF rule in astrophotography?
It is a formula for the longest exposure before stars visibly trail, which takes the f-number and pixel size into account as well as focal length. Because it allows for small pixels, it gives shorter and more reliable times than the 500 rule on modern cameras.
Is the 500 rule still accurate?
It was devised for film and gives exposures roughly twice as long as a high-megapixel sensor can handle. Stars come out as short dashes when you zoom in. Many photographers now use the 300 rule or the NPF rule instead.
How long can I expose the Milky Way with a 14 mm lens?
On a full-frame camera the 500 rule allows about 35 s. The NPF rule at f/2.8 with 6 µm pixels gives (98 + 180) / 14, about 20 s, which is the safer figure for sharp stars.
Does the crop factor affect the exposure time?
Yes. A crop sensor records a narrower field with the same lens, so star movement is magnified. In the 500 rule you multiply the focal length by the crop factor. The NPF rule handles this through the pixel pitch instead.