Sharp Stars Calculator
Tell it your sensor, megapixels, lens and aperture, and it works out the longest exposure you can make before Earth's rotation turns pinpoint stars into short trails — using both the classic 500 rule and the stricter, modern NPF rule. Instant, in your browser, no sign-up.
Keep your exposure at or under 11.6 s for pinpoint stars
We recommend the NPF figure — it accounts for your sensor's pixel pitch, so it's the stricter, safer limit on modern high-resolution cameras.
| 500-rule shutter speed | 20.8 s |
| NPF-rule shutter speed | 11.6 s |
| Pixel pitch used | 6.00 µm |
Practical note: round DOWN to a shutter speed your camera actually offers. If the NPF rule says 11.4 s, shoot 10 s — not 13 s or 15 s. Rounding up is exactly how a pinpoint star becomes a short trail.
How to read this
Two rules, two eras. The 500 rule dates from film: divide 500 by your (crop-adjusted) focal length and that was long enough for a print to look sharp. Film grain and modest enlargements hid a lot. A modern sensor resolves far finer detail, and we view the result at 100% on a large screen — so stars that would have passed on film show a tiny trail much sooner. That is why the NPF rule exists, and why it is usually stricter: it brings your pixel pitch into the maths, so a high-resolution camera gets a shorter, more honest limit than a lower-resolution one with the same lens.
Declination is where in the sky you are pointing, measured from the celestial equator. Stars near the equator sweep across the sky fastest, so they trail first — declination 0 is the worst case. Stars near the celestial pole barely move at all, circling a point close to Polaris, so the higher the declination the more time the cos term in the NPF formula buys you. If you do not know the declination, leave it at 0: the answer will be safe anywhere in the sky.
Why stars trail in the first place
The stars are not moving — you are. Earth rotates 15° per hour, which means every star is a moving target crossing your frame while the shutter is open. A star only stays a pinpoint if, during the exposure, it moves less than your sensor can resolve as a line. Think of “sharp” as a budget: how far may a star cross your sensor before the dot becomes a dash? Focal length spends that budget faster because it magnifies the motion, and a fine-pitched, high-resolution sensor spends it faster because it can see smaller movements. This calculator turns that budget into seconds.
The 500 rule
t = 500 ÷ (focal length × crop factor)
At 24 mm on full frame that is 500 ÷ 24 — about 20.8 seconds. On APS-C the same lens crops tighter, so you use 500 ÷ (24 × 1.5), about 13.9 seconds. It is a film-era rule of thumb: quick, easy to do in your head in the dark, and generous on modern sensors. Use it as an upper bound and a sanity check, not as a promise — especially if you plan to view the shot large or crop into it.
The NPF rule
t = (35 × aperture + 30 × pixel pitch) ÷ (focal length × cos(declination))
The NPF rule, from Frédéric Michaud, is built for digital sensors. Aperture is your f-number — a wider aperture (smaller number) tightens the limit slightly. Pixel pitch is the size of one pixel in micrometres (µm), worked out here from your sensor dimensions and megapixel count: a 24-megapixel full-frame sensor lands around 6 µm, while the same megapixels squeezed onto a smaller sensor give a finer pitch and a stricter answer. Focal length is in millimetres, as marked on your lens. Declination is the star's distance from the celestial equator, in degrees, and the cos term is why position in the sky matters — at declination 0, on the equator, cos is 1 and stars move fastest, so that is the worst case. Leaving declination at 0 is always safe, whatever you are pointing at.
Because it accounts for pixel pitch, the NPF rule is stricter than the 500 rule on most modern high-resolution cameras, and it is the figure we recommend shooting to. The gap between the two rules grows as megapixels rise — which is exactly why older advice, written for film or early digital, so often disappoints now.
Practical shooting notes
This calculator only budgets the shutter. You still need the rest of the night kit: a sturdy tripod — the basics in using a tripod for long-exposure shots apply doubly after dark — plus the small stability tricks in using tripod hooks for stability, and if you are travelling light, a pick from our favourite tripods for travel photography. Use a 2-second timer or a remote release so pressing the shutter does not shake the first moments of the exposure. Focus manually, magnified, on a bright star — tap-to- focus and infinity markings will both betray you — and use the widest aperture your lens handles well, which for many lenses means stopping down just a touch from wide open to tame the corners. The same discipline carries over to city work, as in exploring urban photography at night.
Shooting a long exposure in daylight instead? The maths flips — there you are trying to go slower, not faster — and our ND calculator is the daytime long-exposure counterpart to this one.
Frequently asked questions
Why do my stars trail even at the 500-rule time?
Almost always because the 500 rule was not written for your camera. It assumes film-era resolution and modest enlargement. A high-resolution sensor resolves the star's movement in finer steps, and viewing at 100% magnifies it further, so a trail that was invisible in a print is plain to see on screen. Shoot to the NPF time instead, and judge sharpness at the size you will actually share the photo.
Does ISO matter?
Not to trailing. ISO changes brightness — how much the signal is amplified — not motion. A star crosses the same number of pixels per second at ISO 100 and at ISO 6400. Raise ISO freely to get a usable exposure inside the time limit this calculator gives you; it will not make the trails longer or shorter.
What are good starting settings for the Milky Way core?
Start at the NPF time for your lens and sensor, at the widest aperture your lens handles well, and ISO 3200–6400. Take a test shot, zoom in on a star at the edge of the frame, and adjust to taste: shorten the exposure if you see any trailing, lower the ISO if the sky washes out. Those three settings, in that order, cover most dark- sky locations.
What if I am pointing near the pole?
You get more time. Stars near the celestial pole move in tight circles around a near-stationary point, so at high declinations the cos term in the NPF formula shrinks the denominator and the allowed exposure grows — sometimes dramatically. Enter the declination for the part of the sky in your frame and take the longer limit. Just remember a wide frame spans a range of declinations: the lowest one in the frame is the one that trails first.