The 500 rule and NPF rule, explained

The 500 rule divides 500 by your equivalent focal length for a quick shutter limit; the NPF rule adds aperture and pixel pitch for a sharper, sensor-aware limit. Both exist to answer the same practical question when you shoot the night sky on a fixed tripod: how long can the shutter stay open before the Earth's rotation turns each star from a point into a streak? This guide explains where the formulas come from, why they disagree, and how to choose between them for your camera and your intended output.

Why stars trail at all

The stars are not really moving; the Earth is. It rotates once every 23 hours and 56 minutes, and that rotation makes the whole sky appear to sweep from east to west. Near the celestial equator the apparent speed is about 15 arc-seconds per second of time — fast enough that a wide-angle frame will record a visible smear within a matter of seconds if the focal length is long enough. The job of an exposure rule is to translate that sky motion into a safe shutter time for your particular lens and sensor, so you can set the camera and start shooting without guessing.

The 500 rule

The 500 rule is the classic shortcut: maximum seconds = 500 ÷ (focal length × crop factor). The crop factor converts your lens's actual focal length into a full-frame equivalent, because a smaller sensor crops into the image and magnifies any motion. On a full-frame camera the crop factor is 1.0, so a 24mm lens gives 500 ÷ 24 ≈ 21 seconds. Put that same 24mm lens on an APS-C body with a 1.5x crop factor and the effective focal length becomes 36mm, so the limit falls to 500 ÷ 36 ≈ 14 seconds. Micro Four Thirds, at 2.0x, halves the full-frame time outright. The rule is easy to do in your head at the tripod, which is exactly why it became popular.

Where the 500 rule falls short

The 500 rule was calibrated in the film era, when grain, not pixels, set the limit of visible detail. Digital sensors changed the game. A modern 45- or 60-megapixel full-frame sensor resolves far finer detail, so a star that the 500 rule considers “still a point” may already smear across two or three pixels. The rule also ignores aperture entirely, even though a faster lens renders tighter star images that make any trailing more obvious. The upshot is that on today's cameras the 500 rule tends to be optimistic: the result looks fine on the rear screen and disappoints at 100% or in a large print. That is the gap the NPF rule was designed to close.

The NPF rule

The NPF rule takes its name from the three quantities it balances: aperture (N), pixel pitch (P) and focal length (F). In its standard form the maximum shutter time in seconds is (35 × aperture + 30 × pixel pitch) ÷ focal length, where the aperture is the f-number, the pixel pitch is measured in microns, and the focal length is the actual value in millimetres. Pixel pitch is the physical distance between neighbouring pixels; you calculate it as the sensor width in millimetres times 1000, divided by the horizontal pixel count. A 36mm-wide full-frame sensor with 6000 horizontal pixels has a 6.0-micron pitch; the same sensor at 8256 pixels has a 4.36-micron pitch. Because a denser sensor has a smaller pitch, the NPF rule automatically shortens the allowable time as resolution rises — precisely the behaviour the 500 rule lacks.

Seeing the two side by side

Take a 20mm f/2.8 lens on a 24-megapixel full-frame body. The 500 rule gives 25 seconds. The NPF rule gives (35 × 2.8 + 30 × 6.0) ÷ 20 = (98 + 180) ÷ 20 = 13.9 seconds. Swap to a 45-megapixel body (4.36-micron pitch) and the NPF time falls to (98 + 130.8) ÷ 20 ≈ 11.4 seconds, while the 500 rule stubbornly still says 25. Neither formula is lying; they simply set different standards for “sharp.” The 500 rule targets what looks acceptable at a normal viewing size, and the NPF rule targets pinpoint stars at the pixel level. Knowing which standard you need for a given photo is the whole skill.

The declination refinement

Both rules, in their basic form, assume the worst case: a star on the celestial equator, moving at full speed. But stars near the celestial pole barely move at all — Polaris traces a tiny circle over the whole night. The apparent speed of a star scales with the cosine of its declination, the angle north or south of the celestial equator. That means the safe exposure time scales with one divided by that cosine. Aim at a target 60 degrees from the equator and you can expose twice as long as the equator figure; aim near Polaris and you can go far longer still. The calculator on the home page lets you switch this adjustment on and enter a declination, which is handy for framing a specific constellation rather than a generic patch of sky.

Which one should you use?

For a fast field estimate, or when you will resize the image well down for the web, the 500 rule is perfectly serviceable and quick to compute. When you care about critical sharpness — pixel-peeping, large prints, or stacking many frames where trailing compounds — use the NPF rule and treat its shorter time as the real limit. A sensible workflow is to let the calculator show both, expose at the NPF time when conditions allow, and only relax toward the 500-rule time when you need the extra light and can accept slightly softer stars. If even the NPF time is too short to gather the light you want, that is the signal to consider a star tracker, which is covered in the comparison of the two rules and tracking. To run any of these numbers for your own gear, use the exposure calculator on the home page.