What is the 500 rule in astrophotography?

The 500 rule estimates the longest exposure you can use before stars blur into trails. Divide 500 by your full-frame equivalent focal length: at 20 mm equivalent the limit is about 25 seconds. It is a fast field rule but tends to be too generous for modern high-resolution sensors.

How is the NPF rule different from the 500 rule?

The NPF rule accounts for aperture, pixel pitch, and the star's declination, not just focal length, so it gives a shorter and more accurate exposure for pin-sharp stars. High-megapixel sensors resolve trailing sooner, which the 500 rule ignores entirely, so the NPF rule is preferred for critical work.

Why does crop factor matter for the 500 rule?

Star trailing depends on the field of view, which the crop factor changes. A 24 mm lens on an APS-C body frames like 36 mm full-frame, so you divide 500 by 36, not 24. This tool applies the crop factor automatically to find the equivalent focal length.

What is declination and why does it affect exposure?

Declination is the celestial latitude of your target. Stars near the celestial equator, at 0° declination, sweep across the sky fastest and trail soonest, while stars near the celestial poles barely move. The NPF rule shortens the allowed exposure for low-declination targets and lengthens it near the poles.

Should I use the 300 or 600 rule instead?

The 600 rule is more lenient and tolerates slight trailing, useful for small prints or web display. The 300 rule is stricter and suits very high-resolution sensors or large prints where any trailing shows. Pick the variant that matches how sharp you need the stars to look.

500 Rule & NPF Star Trail Calculator

When the Earth rotates, stars appear to drift across the sky, so any exposure that is too long records them as short streaks instead of points. This calculator finds the longest shutter speed you can use before that trailing becomes visible, using both the classic 500 rule and the more precise NPF rule.

How it works

The 500 rule

Maximum exposure (s) = 500 ÷ (focal length × crop factor)

The crop factor converts your lens to its full-frame equivalent because trailing depends on field of view. A 24 mm lens on a 1.5× APS-C body behaves like 36 mm, so the limit is 500 ÷ 36 ≈ 14 seconds. The 600 and 300 variants simply swap the constant for a looser or stricter tolerance.

The NPF rule

The NPF rule is far more accurate because it also accounts for aperture and the sensor’s pixel pitch:

t = k × (35 × N + 30 × p) ÷ (f × cos(declination))

where N is the f-number, p is the pixel pitch in microns, f is the true focal length in mm, and k is an accuracy factor (1 for pin-sharp stars, 2 if small trails are acceptable). Pixel pitch is derived from your sensor width and horizontal pixel count:

pixel pitch (µm) = (sensor width mm ÷ horizontal pixels) × 1000

The cos(declination) term shortens the exposure for stars near the celestial equator, which move fastest, and lengthens it near the poles.

Worked example

A 24 mm f/2.8 lens on a full-frame 24-megapixel sensor (36 mm wide, 6000 px across), pointed at the celestial equator:

500 rule: 500 ÷ 24 ≈ 20.8 s
Pixel pitch: (36 ÷ 6000) × 1000 = 6.0 µm
NPF rule: (35 × 2.8 + 30 × 6.0) ÷ 24 ≈ 11.6 s

The NPF rule recommends barely half the 500 rule’s time for genuinely round stars.

Tips and notes

Use the NPF result for large prints and high-megapixel bodies; the 500 rule is fine for quick web shots.
Stars near the equator (declination 0°) trail first; circumpolar stars tolerate much longer exposures.
If the NPF time is too short for a clean exposure, raise ISO or open the aperture rather than lengthening the shutter.
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