How does the 500 rule compare?

The classic 500 rule (t = 500/f) is simpler but less accurate, especially with modern high-resolution sensors where trailing is visible at shorter exposures.

Does a tracking mount eliminate this limit?

Yes. An equatorial tracking mount follows the sky, allowing exposures of many minutes without trailing.

Why does declination matter?

Stars near the celestial poles move in smaller circles, so they trail less per unit time. The cosine factor accounts for this.

Free Exposure Time for Stars Calculator

Calculate the maximum exposure time before stars trail using the NPF rule: t = (35 * N + 30 * p) / (f * cos(dec)), with declination correction.

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Focal Length

Pixel Size

f-Number (N)

Declination

deg

Max Exposure Time

1.26 s

Max Exposure Time vs Focal Length

View formula & how it works →

Maximum Exposure Time for Point Stars

The NPF rule estimates how long you can expose before Earth's rotation causes star trailing.

NPF Formula

t = (35N + 30p) / (f * cos(dec))

*N* = lens f-number

*p* = pixel size in micrometres

*f* = focal length in mm

*dec* = declination of the target

Stars near the celestial equator trail fastest; those near the poles trail slowest.

Example Calculation

200 mm f/4 lens, 3.75 um pixels, shooting the celestial equator.

01t = (35*4 + 30*3.75) / (200 * cos(0))
02t = (140 + 112.5) / 200
03t = 252.5 / 200
04t = 1.26 seconds

Free Exposure Time for Stars Calculator

Maximum Exposure Time for Point Stars

NPF Formula

Example Calculation

Frequently Asked Questions

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