500 Rule Calculator (Star Photography)
Maximum shutter speed before stars trail — classic 500 rule plus the more accurate NPF rule for modern high-resolution sensors.
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How the 500 Rule and NPF Rule Are Calculated
Stars appear fixed to the naked eye, but a camera sensor reveals the truth: Earth's rotation smears point-source stars into visible trails once your shutter stays open too long. The classic 500 rule gives a fast, rule-of-thumb answer to "how long can I expose before that happens?" — max shutter (in seconds) = 500 ÷ (effective focal length, crop factor included). A concrete example: a 20 mm lens on a full-frame body (crop factor 1.0) allows roughly 500 ÷ 20 = 25 seconds before stars visibly trail. Mount that same 20 mm lens on an APS-C body with a 1.5× crop factor, and the effective focal length becomes 30 mm, dropping the allowed exposure to about 500 ÷ 30 ≈ 16.7 seconds.
The 500 rule is explicitly an approximation — it was devised for 35 mm film and modest-resolution digital sensors, and it simplifies away the real variables that determine when a star visibly trails: sensor pixel density and how much you magnify the image when reviewing it. On today's high-resolution sensors (45+ megapixels), individual pixels are small enough to register star movement well before the 500-rule time expires, so images that pass the 500 rule at thumbnail size often show trailing when zoomed to 100%. The NPF rule (named for aperture Number, Pixel pitch, and Focal length) corrects for this: t = (35 × f-number + 30 × pixel pitch in µm) ÷ focal length. Because it accounts for your specific camera's pixel size and the lens aperture, NPF gives a tighter, more realistic maximum exposure — this calculator computes both values side by side so you can treat NPF as the safe number and 500 as the loose upper bound.
What You Should Know
- 500 is a simplification, not a law of physics: it ignores sensor resolution entirely, which is precisely why it was accurate enough in the film era but increasingly optimistic on modern high-megapixel bodies.
- NPF is stricter and camera-specific: because it factors in your sensor's actual pixel pitch, two different cameras with the same lens and focal length can get different NPF-recommended shutter speeds.
- Crop factor always applies to effective focal length: whichever rule you use, multiply your lens's focal length by your sensor's crop factor first — a 24 mm lens on APS-C behaves like a 36 mm lens for star-trail purposes, not like a true 24 mm.
- Both rules assume you are judging trails at typical viewing sizes: pixel-peeping at 100% zoom will reveal trailing sooner than either formula predicts, so treat these as planning guides, not absolute guarantees.
Frequently Asked Questions
Why do my stars still trail at the 500-rule time?
The rule dates from the era of film and low-resolution digital sensors, when it simplified the problem enough to be useful. On a 45 MP body, individual pixels are small enough to record star motion far sooner than the 500 rule assumes — the NPF result (often around half the 500-rule time) is the realistic limit when viewing your image at full resolution.
What settings work for photographing the Milky Way?
A typical starting point: the widest lens you own, an aperture of f/1.8-f/2.8, ISO 3200-6400, and shutter speed set to the NPF value from this calculator. Shoot in RAW for maximum dynamic range, and focus manually on a bright star using live view zoomed to its maximum magnification, since autofocus struggles in near-darkness.
How can I expose for longer than the 500 or NPF limits allow?
A star tracker is a motorized mount that rotates the camera to match Earth's rotation, effectively canceling star trailing and allowing exposures of several minutes at low ISO. Without one, you can stack many short exposures (each within the NPF limit) using software like Sequator or DeepSkyStacker, which averages out sensor noise across frames without introducing trails.
Why does the NPF rule give a shorter time than the 500 rule?
The 500 rule was calibrated for older, lower-resolution sensors where individual pixels were large enough to blur small amounts of star movement into invisibility. The NPF rule accounts for today's much smaller pixel pitches, which resolve star movement sooner, so it typically recommends roughly half the exposure time the 500 rule would suggest for the same lens.
Does crop factor make the 500 rule more or less forgiving?
Less forgiving. A larger crop factor increases the effective focal length used in the 500 ÷ (crop factor × focal length) formula, which shortens the maximum allowed shutter speed — the same physical lens permits a shorter exposure on a cropped-sensor body than on full frame before stars visibly trail.
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500 Rule Calculator (Star Photography)
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