Astrophotography Calculator

The 500 Rule (500 / (focal length × crop factor)) is a fast mental-math approximation from the film era that ignores sensor resolution entirely. The NPF Rule factors in aperture and pixel pitch, which matters because a densely packed modern sensor reveals star trailing at a shorter exposure than the 500 Rule predicts. The calculator computes both and recommends whichever is shorter, so a high-megapixel body doesn't get an overly generous exposure time.

Pixel pitch is derived from sensor width divided by the square root of megapixels — packing more pixels into the same sensor width makes each pixel physically smaller. Since the NPF Rule's formula adds 30× pixel pitch to the numerator, a smaller pixel pitch produces a smaller allowed exposure time before trailing becomes visible at the pixel level, even though the field of view and focal length haven't changed.

The calculator's SNR improvement scales with the square root of the number of subs (√N), a consequence of how random noise averages down when you stack independent frames. Going from 30 to 120 subs (4×) only multiplies your SNR improvement by √4 = 2, which is why chasing dramatically longer integration time has diminishing returns compared to improving your per-frame exposure or gear.

It's a rough approximation built from your entrance pupil diameter (focal length ÷ aperture) and the recommended exposure time, following a standard telescope-limiting-magnitude formula. It doesn't account for real-world factors like light pollution, sky transparency, seeing conditions, or your specific optics' actual light transmission, so treat it as a ballpark for comparing setups rather than a guarantee of what you'll capture at a given site.