Photography Calculators

Astrophotography

NPF Rule Calculator

NPF folds in f-number and pixel pitch so a 45 MP body does not use the same 20 s as a 12 MP body. The simplified form is t = accuracy × (35N + 30p) / f. Accuracy 1 is a normal ‘sharp enough’ taste; 0.5 is picky. Confirm with a 100% test shot — this still ignores declination.

Inputs

Units
mm

Barrel focal length.

f/

Taking f-number. Faster glass allows a slightly longer NPF time in this fit.

µm

Pitch ≈ sensor width mm × 1000 / horizontal pixels. ~4.3 µm is a common 24 MP full-frame figure.

1 default. 0.5 for large prints or picky stacking. 2 if you only view small.

Results

Max untracked shutter
8.00s

Recommended gear

Calculation method

This is the simplified NPF fit: multiply an accuracy factor by (35 times f-number plus 30 times pixel pitch in micrometres), then divide by focal length in millimetres. The result is seconds. Smaller pixels, longer glass or a picky accuracy factor all shorten the shutter. It is still untracked; a tracker is a different workflow.

Exact formula

t = accuracy × (35·N + 30·p) / f with N = f-number, p = pixel pitch in µm, f in mm. Limits: Simplified NPF; no declination, no tracking, no optical quality; accuracy is a taste factor.

500 vs NPF

500 rule on 24 mm full frame is ~21 s. NPF with f/1.8 and 4.3 µm is much shorter. If your 500-rule shots show dashes at 100%, believe NPF.

Stacking

Shorter NPF frames stack cleaner than one long trailed frame. Set the interval a second or two over the shutter so the buffer keeps up.

Frequently asked questions

Does a faster lens give more time?

Not in this fit. The 35N term grows with f-number, so f/4 recommends a longer shutter than f/1.8. That is a trail-blur budget, not an exposure bonus. Shoot wide open for light, use this time for trails, then inspect a test shot at 100%.

How do I get pixel pitch?

Sensor width in millimetres times 1000, divided by the number of horizontal pixels. A 36 mm-wide 6000 px sensor is 6.0 µm.

Full NPF with declination?

The original NPF has more terms. This calculator uses the compact (35N+30p)/f form plus accuracy. For polar fields you can often go longer; for the equator, do not.

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