What it's for
When shooting the night sky with long exposures, Earth's rotation makes stars appear as short trails instead of sharp points if the exposure is too long for the focal length used. This calculator gives you the maximum exposure time before that effect becomes visible, using three popular astrophotography rules.
The rules used
500 Rule (older, more lenient): max_exposure = 500 / (focal_length × crop_factor). 300 Rule (more conservative, recommended for modern high-resolution sensors, which reveal trailing more easily): max_exposure = 300 / (focal_length × crop_factor). NPF Rule (the most accurate, created by Frédéric Michaud and named after Aperture-Pixel-Focal, also accounting for aperture and pixel size): max_exposure = (35 × aperture + 30 × pixel_pitch) / focal_length (actual focal length in mm; the crop factor is not used in this rule because the pixel pitch already represents the sensor), in a simplified version without the star declination term.
How to find your pixel pitch
Pixel pitch is the physical size of each sensor pixel, in microns: pixel_pitch = (sensor_width_in_mm ÷ horizontal_pixel_count) × 1000. As a reference, a 24-megapixel full-frame camera has a pixel pitch of roughly 5.9 microns; higher-resolution sensors have smaller pixels (smaller pixel pitch), and so tolerate shorter exposures before showing trails.
Which rule to pick
The 500 rule is a simple approximation, suited to older or lower-resolution cameras. The 300 rule is more conservative and better suited to modern high-resolution sensors, where trailing becomes visible more easily when zooming in. The NPF rule is the most accurate, since it accounts for the sensor's actual pixel pitch and the aperture used, but requires more specific information about your gear.
Limitations
None of these rules account for the declination of the part of the sky being photographed (stars near the celestial equator appear to "move" faster in the photo than those near the poles) — the full NPF rule includes that adjustment, which we've simplified out here to keep the calculation straightforward.
Frequently asked questions
Why do higher-resolution sensors need shorter exposures?
Because smaller pixels (smaller pixel pitch) reveal star movement in more detail when you zoom in — the same star trail that would be imperceptible on a low-resolution sensor becomes visible on a high-resolution one.
What does NPF stand for?
It's derived from Aperture (N, the f-number), Pixel (pitch) and Focal (length) — the three factors the rule considers, on top of focal length and crop factor.
Does crop factor change the result even at the same focal length?
Yes — smaller sensors (higher crop factor) magnify the effect of star movement in the final image, requiring proportionally shorter exposures at the same focal length.