Astrophotography NPF Calculator
Find the longest exposure that keeps stars as points, not trails, using the accurate NPF rule — and compare it with the older 500 rule.
How it works
NPF max time (s) = (35 × N + 30 × pixel pitch) ÷ focal length · 500 rule = 500 ÷ focal length
As the Earth rotates, stars drift across the frame, so an exposure that is too long records them as short trails instead of points. The old "500 rule" (500 ÷ focal length) ignores aperture and pixel size and is far too generous for today's dense sensors. The NPF rule — named for its aperture (N), pixel pitch (P) and focal length (F) terms — folds in how bright and how small each pixel is, giving a much tighter, more realistic limit. Pixel pitch is your sensor width in millimetres divided by its horizontal pixel count, times 1,000 to reach microns.
Worked example
A 20 mm lens at f/2.8 on a sensor with 4.3 µm pixels: NPF = (35 × 2.8 + 30 × 4.3) ÷ 20 = (98 + 129) ÷ 20 ≈ 11.4 s. The 500 rule would suggest 500 ÷ 20 = 25 s — more than double, and long enough to trail the stars on that sensor.
Frequently asked questions
Why is the NPF rule better than the 500 rule?
The 500 rule only looks at focal length. The NPF rule also accounts for aperture and pixel pitch, so it correctly tightens the limit for fast lenses and high-megapixel sensors, where the 500 rule leaves visible star trails.
How do I find my pixel pitch?
Divide the sensor width in millimetres by the number of horizontal pixels, then multiply by 1,000 for microns. A 36 mm-wide full-frame sensor with 6,000 pixels across has a pitch of 36 ÷ 6000 × 1000 = 6.0 µm.
Does a star tracker change this?
Yes — a motorized equatorial tracker follows the sky's rotation, so you can expose for minutes without trailing. The NPF rule applies to fixed tripods, where the camera stays still while the stars move.
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This calculator is for educational purposes. Double-check important results before acting on them.