Astro 500 Rule & NPF Calculator

NPF is a model with a chosen blur budget, not a measurement — it decides how much trailing counts as acceptable and works backwards. It knows nothing about seeing, tracking mounts, lens coma at the edges or how large you intend to print, all of which can matter more than the arithmetic. Treat it as a starting exposure to test, not a verdict. Nothing is uploaded.

How wrong the 500 rule is does not depend on your lens

Both rules are inversely proportional to the focal length, so dividing one by the other cancels it exactly — the ratio is 500/(crop × (35N + 30p)) and there is no f in it. On a 24 MP full frame at f/2.8 that ratio is 1.798561 at 14 mm, at 20 mm, at 35 mm, at 50 mm and at 85 mm, to six decimal places. Reaching for a wider lens moves both numbers together and changes nothing.

What it does depend on is the sensor, which is exactly what the 500 rule cannot see. Every full-frame body gets the same 25-second answer from it, while NPF ranges from 13.9 s on a 24 MP body down to 10.6 s on a 61 MP one — 1.80× too generous becoming 2.36×. The rule was written for film, where there were no pixels to count.

And stopping down buys you exposure time

Nobody expects this one. A smaller aperture spreads each star over a larger Airy disc, and NPF counts that blur as part of the budget — so the allowed exposure grows as you stop down, from 11.45 s at f/1.4 to 23.00 s at f/8. It is what the model says rather than a licence to stop down: you lose far more light than the extra seconds return.

The two terms cross over in the middle of that table: wide open the pixels dominate, stopped down the aperture does. Declination is the other thing neither rule knows by itself — both answer for the celestial equator, which is the worst case. Stars nearer the pole move slower, and the correction is a plain 1/cos: double the time at 60°, and over elevenfold at 85°.