500 rule vs NPF rule vs star tracker

The 500 rule is the fast estimate, the NPF rule is the accurate one for a fixed tripod, and a star tracker removes the time limit entirely by turning the camera with the sky. All three exist to fight the same enemy — star trailing from the Earth's rotation — but they sit at different points on the trade-off between speed, sharpness and effort. This guide compares them so you can pick the right approach for the shot in front of you.

The three approaches in one paragraph

A rule of thumb like the 500 rule accepts a short exposure so the stars don't move far enough to trail, and gathers the rest of the light through a wide aperture and higher ISO. The NPF rule is the same idea done more precisely, tightening the time so the stars stay sharp right down to the pixel. A star tracker takes a different route: a small motorised mount rotates the camera at exactly the speed of the sky, so the stars stay fixed on the sensor and you can expose for minutes instead of seconds. More light means cleaner, less noisy images — at the cost of extra kit, setup and a polar alignment step.

Accuracy: 500 rule vs NPF rule

On a fixed tripod the choice is really between the two rules, and it comes down to how you will view the result. The 500 rule ignores aperture and pixel density, so on modern high-resolution sensors it allows exposures that look clean at small sizes but show trailing at 100%. The NPF rule folds in aperture and pixel pitch and returns a shorter, more honest number. For a 20mm f/2.8 lens on a 45-megapixel full-frame body, the 500 rule says 25 seconds while the NPF rule says under 12 — a two-fold difference that is the gap between crisp and slightly smeared stars in a big print. If your output is a web-sized JPEG, that gap may not matter; if it is a wall print, it absolutely does.

When a rule is enough

For wide-field nightscapes — a landscape with the Milky Way arching above — an untracked exposure guided by the NPF rule is often all you need, and it keeps the foreground sharp too, which a tracker would blur. Fast wide lenses at f/1.4 to f/2.8 gather a lot of light in the ten to twenty seconds these rules allow, and stacking several frames in software cleans up the noise without any extra hardware. If you are travelling light, shooting a scene with land and sky together, or just starting out, the rule approach is the practical default.

When you need a tracker

The rules hit a wall when the light you want simply isn't there in a few seconds: faint nebulae, galaxies, or tightly framed deep-sky targets through a longer lens. Here even the NPF time is far too short, and pushing ISO to compensate buries the subject in noise. A star tracker lets you expose for one to several minutes per frame, dropping the ISO and pulling faint detail cleanly out of the dark. The price is setup: you polar-align the mount to the celestial pole, and because the sky is now sharp but the ground is not, foregrounds usually need a separate untracked frame blended in later.

A practical decision path

Start with what you are shooting. Landscape plus sky, or a quick grab? Use the NPF rule on a tripod and move on. Want the cleanest possible wide-field Milky Way and willing to blend two frames? Still tripod and NPF, then stack. Chasing a specific nebula or galaxy, or want minutes of exposure at low ISO? That is tracker territory. Notice that the NPF rule is the common thread for everything untracked — which is why it is worth having the number for your exact lens and camera before you leave home.

Run the numbers first

Whichever path you choose, knowing your untracked limit is the starting point, because it tells you whether a tripod can get the shot or whether you need to bring the tracker. The exposure calculator gives you both the 500-rule and NPF-rule times for your gear in seconds, and the guide to the two rules explains the reasoning if you want to understand exactly why they differ.