Astrophotography Exposure Calculator

How long can you expose the night sky before the stars stretch into trails? Enter your lens and camera to get the classic 500-rule limit, the stricter NPF limit, and the number both are really guessing at: the trail length in pixels on your own sensor. Everything is computed in your browser.

Your lens and camera

Focal length (mm)

Aperture (f-number)

Use the real focal length printed on the lens, not the 35 mm equivalent; the sensor choice below supplies the crop factor. The f-number only affects the NPF result.

Camera sensor

Target declination (degrees)

0° is the celestial equator, the worst case; stars farther from the celestial equator trail more slowly. Not sure? Leave it at 0.

Pixel size

Resolution (megapixels)

The calculator derives the pixel pitch from the sensor size and the megapixel count.

CycleCalcs.comEnter your lens and camera to see how long you can expose before stars trail.

How the exposure limits are worked out

What this calculator answers: how long you can expose before stars visibly trail, with your camera on a fixed tripod and any lens. For what your telescope shows and what your camera frames through it, use the Telescope Calculator; its imaging tab already works out field of view and image scale for your sensor, so that ground is not repeated here.

The sky moves; the tripod does not

Earth turns once relative to the stars every sidereal day of 23 hours 56 minutes 4.09 seconds, so the sky drifts through a full 360 degrees in that time. At the celestial equator that comes to about 15.04 arcseconds per second of time. Away from the equator the drift shrinks by the cosine of the declination: a star at declination 60° moves at half the equatorial rate, and one near the celestial pole hardly moves at all. Every exposure limit below is a statement about how much of that drift your sensor can absorb before a star stops looking like a point.

The 500 rule, and the stricter 300 rule

t = 500 / (crop factor × focal length in mm)

A 20 mm lens on a full-frame body gets 500 / 20 = 25 seconds; the same lens on an APS-C body (crop factor 1.53) gets about 16 seconds. The rule is a relic of the film era, tuned for a time when a small blur vanished into grain and modest print sizes. The 300 rule is the same arithmetic with 300 on top, a stricter variant once favored for larger prints. Both share the same blind spot: neither knows anything about the sensor recording the image.

The NPF rule

t = (35 × N + 30 × p) / f

N is the aperture f-number, p the pixel pitch in micrometers, f the focal length in millimeters, and t comes out in seconds. The rule was published by Frederic Michaud of the Societe Astronomique du Havre, and unlike the 500 rule it models the star image itself: the aperture term tracks how large a spot the lens draws, and the pixel term tracks how finely the sensor samples it. For a target away from the celestial equator the calculator divides by cos(declination), since the drift is slower there. On typical modern gear the NPF figure lands near half the 500-rule figure, which tells you most of what you need to know about the 500 rule.

The trail the 500 rule actually leaves

The honest test of an exposure rule is the trail it permits, in pixels, on your sensor. Two pieces of arithmetic settle it. The image scale of your setup is 206.265 × p / f arcseconds per pixel, and a star drifts at 15.04 × cos(declination) arcseconds per second. Multiply the exposure by the drift rate, divide by the image scale, and you have the trail length in pixels.

Run that on the 500 rule and something interesting happens: the focal length cancels out. A longer lens drifts across pixels faster, but the rule shortens its exposure by exactly the same factor, so the trail it permits is about 36.5 × cos(declination) divided by (crop factor × pixel pitch). On a 24 megapixel full-frame camera with 6 µm pixels that is about 6 pixels, whatever lens you fit. On a 61 megapixel body it is nearly 10. The 500 rule does not fail because your lens changed; it fails because pixels got small.

Worked example: 24 MP full frame, 20 mm at f/2.8

Pixel pitch: a 24 MP full-frame sensor is about 6000 pixels across 36 mm, so p = 6.0 µm.
500 rule: 500 / (1.00 × 20) = 25 s.
NPF rule: (35 × 2.8 + 30 × 6.0) / 20 = 278 / 20 = 13.9 s at the celestial equator.
Image scale: 206.265 × 6.0 / 20 = 61.9 arcseconds per pixel. In 25 seconds a star at the equator drifts 25 × 15.04 = 376 arcseconds, which is about 6.1 pixels of trail. The NPF exposure drifts about 3.4 pixels, comparable to the size of a well-focused star image, which is why it still looks sharp.

Worked example: the same lens on a 61 MP body

A 61 MP full-frame sensor is about 9566 pixels across 36 mm, so p = 3.76 µm and the image scale tightens to 38.8 arcseconds per pixel.
The 500 rule still says 25 s, but that exposure now drifts 376 / 38.8 = about 9.7 pixels.
The NPF rule adapts: (35 × 2.8 + 30 × 3.76) / 20 = 210.8 / 20 = about 10.5 s. Same lens, same sky, half the exposure; the pixels are the difference.

Worked example: aiming higher, declination 60

At declination 60° the drift falls to half the equatorial rate, about 7.52 arcseconds per second.
The NPF limit for the 24 MP setup doubles from 13.9 s to 27.8 s, and the 500 rule's fixed 25 s exposure now trails only about 3 pixels. Cassiopeia and the region around the celestial pole are far more forgiving than Orion, which straddles the celestial equator.

For the models, accuracy and data behind these figures, see the methodology and sources page.

Rules of thumb, trackers, and stacking

Both rules are rules of thumb, and it pays to treat them that way. They assume good focus, a steady tripod, and ordinary standards for what counts as a point of light. Pixel-peep at 200 percent and even an NPF-limit frame shows motion; print at postcard size and the 500 rule may serve you yet. Judge the result at the size you will actually display it.

Neither rule replaces a star tracker. A tracker is a small motorized mount that turns the camera with the sky, removing the drift at its source; aligned on the pole, it swaps exposures of seconds for exposures of minutes, and both rules become irrelevant. The rules answer a narrower question: how far can a plain fixed tripod go? Used well, surprisingly far.

The other half of the craft is stacking: shoot dozens of frames at or under the limit, then align and average them in software. Signal accumulates while random noise averages away, so many short frames outperform any single long one from a fixed tripod, with no trails in the bargain. Pair that with good timing. The Sunrise & Sunset Calculator shows tonight's moonless dark window, and the Full Moon Calendar tells you which weeks to write off entirely.

Frequently asked questions

What is the 500 rule?

The 500 rule is a quick exposure guide for photographing stars from a fixed tripod: divide 500 by your effective focal length, meaning the real focal length times the crop factor, to get a maximum exposure in seconds. A 20 mm lens on a full-frame camera gets 500 / 20 = 25 seconds. The rule dates from the film era, and on modern high-resolution sensors it allows visible trailing, which is why this calculator also shows the trail it would leave on your own camera.

Why does the 500 rule fail on modern cameras?

Because it knows nothing about your pixels. The rule was tuned for film grain and early digital sensors, where a small blur disappeared into the medium itself. A modern sensor with pixels of 4 to 6 micrometers records that same blur, on a full-frame camera, as a trail 6 to 9 pixels long, plainly visible at full size. The finer the pixels, the worse it gets, and the focal length cancels out of the arithmetic entirely: the trail the rule permits depends only on your pixel pitch and crop factor, not on the lens.

What is the NPF rule?

The NPF rule is a stricter exposure guide that reads the three things the 500 rule ignores: N the aperture f-number, P the pixel pitch, and F the focal length. Multiply the f-number by 35 and the pixel pitch in micrometers by 30, add the two, and divide by the focal length in millimeters to get the exposure in seconds. It was published by Frederic Michaud of the Societe Astronomique du Havre; the version here is his widely used simplified form (the full formula also weighs sensor resolution and the blur you will accept). Because it knows your aperture and your pixels it usually comes out near half the 500-rule figure, with visibly sharper stars to show for it.

Does declination really matter?

Yes. The sky drifts at about 15 arcseconds per second of time at the celestial equator, and the drift falls with the cosine of the declination. A star at declination 60 degrees moves at half the equatorial rate, so it tolerates twice the exposure. Orion straddles the celestial equator and gets the worst case; a frame near Polaris barely moves at all. One caution: the scaling applies to the star at that declination, and a wide frame also holds stars closer to the equator, which trail at their own faster rate.

Do I need a star tracker?

Not for wide-field shots of the Milky Way, meteors, or constellations; the limits here plus stacking will carry you a long way. A tracker earns its keep when you want longer focal lengths or exposures of a minute and up, because it turns the camera with the sky and removes the drift at its source. Nearly every deep-sky image you have admired was tracked. These rules tell you how far a fixed tripod can go; a tracker is the step past them.

What about stacking?

Stacking is the standard way around short exposure limits. Shoot many frames at or under the limit, then align and average them in free software such as Sequator, DeepSkyStacker, or Siril. The signal builds with every frame while the random noise averages away, so sixty 10 second frames gather the light of a 10 minute exposure without a single trailed star. From a fixed tripod, more short frames beat fewer long ones every time.

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