The 500 Rule vs NPF Rule: Which Is More Accurate?
Comparing the two most common rules for maximum astrophotography exposure time. Math, testing, and which to trust.
Every astrophotographer faces the same question: how long can I expose before stars start to trail?
The answer determines your ISO (shorter exposure = higher ISO needed), your noise floor, and ultimately the quality of your image. Get it right and stars are crisp points. Go too long and they stretch into tiny dashes that look soft and amateurish at full resolution.
Two rules dominate the conversation: the 500 Rule (simple, fast, outdated) and the NPF Rule (precise, modern, slightly more complex). This guide explains both, compares them head-to-head with real camera/lens combinations, and tells you when each is appropriate.
The 500 Rule
The Formula
Maximum exposure in seconds = 500 / (focal length × crop factor)
That's it. One variable you know (your focal length) and one you look up once (your camera's crop factor: 1.0 for full frame, 1.5 for most APS-C, 1.6 for Canon APS-C, 2.0 for Micro Four Thirds).
Examples:
14mm on full frame: 500 / (14 × 1.0) = 35.7 seconds 24mm on full frame: 500 / (24 × 1.0) = 20.8 seconds 16mm on APS-C (1.5x): 500 / (16 × 1.5) = 20.8 seconds 10mm on MFT (2.0x): 500 / (10 × 2.0) = 25 seconds
Where It Came From
The 500 Rule (sometimes cited as the 600 Rule with a more permissive constant) originated in the film era. It was designed for 35mm film with typical grain structure — the grain was large enough that short star trails of a few arc-seconds were invisible within the grain texture.
For Kodak Tri-X at ISO 400, pushed in development, exposed through a 28mm lens: the 500 Rule worked fine. The grain masked any trailing below a certain threshold, and the math was simple enough to do in your head at 2am.
Why It Fails with Modern Cameras
Modern digital sensors have pixel pitches of 3-6 μm — significantly smaller than the grain structure of most films. A 45-megapixel full-frame sensor (like the Canon R5 or Nikon Z8) has a pixel pitch of about 4.4 μm. At this resolution, even tiny amounts of star trailing are visible when you zoom to 100%.
The 500 Rule at 14mm on a Canon R5 gives 35.7 seconds. At 100% crop, a star near the celestial equator (worst case for trailing) will have moved approximately 8-9 pixels during that exposure. That's a clearly visible dash, not a point.
The rule wasn't wrong for its era. It's wrong for current hardware.
Variations: The 400 Rule, 300 Rule, 600 Rule
Photographers have proposed adjusted constants to compensate. The 400 Rule tightens the limit. The 300 Rule tightens it further. Some use 600 for a more permissive limit when they don't plan to crop or print large.
All of these are approximations of the same underlying approach — and all share the same fundamental limitation: they don't account for pixel pitch or aperture, which are the variables that actually determine whether a trail is visible at pixel level.
The NPF Rule
The Formula
The NPF Rule was developed by Frédéric Michaud of the Société Astronomique du Havre (SAH). It accounts for three factors the 500 Rule ignores:
N = aperture (f-number) P = pixel pitch (in μm) F = focal length (in mm)
The simplified version:
Maximum exposure in seconds = (35 × N + 30 × P) / F
The full version, which includes the star's declination (angular distance from the celestial equator):
Maximum exposure in seconds = (16.856 × N + 0.0997 × F + 13.713 × P) / (F × cos(δ))
Where δ (delta) is the declination of the portion of sky you're photographing. Stars near the celestial equator (δ = 0°) move fastest; stars near the celestial poles (δ = ±90°) barely move. The cos(δ) factor accounts for this.
What Each Variable Does
Aperture (N): a wider aperture produces larger Airy disks (the diffraction pattern of a point source). Larger Airy disks mask tiny amounts of trailing. At f/1.4, you can expose slightly longer before trailing is visible than at f/2.8, because each star's image is already a slightly larger circle.
Pixel pitch (P): smaller pixels resolve finer detail, including finer amounts of trailing. A camera with 4μm pixels shows trailing sooner than one with 6μm pixels at the same focal length and exposure time.
Focal length (F): longer focal length magnifies the sky, making stellar motion appear faster in the image. This is the variable shared with the 500 Rule.
Examples with Real Cameras
Let's compare 500 Rule vs NPF Rule for popular camera/lens combinations at f/2.8, pointing at the celestial equator (worst case):
Canon R5 (45MP, pixel pitch 4.39μm) at 14mm f/2.8: 500 Rule: 500 / 14 = 35.7s NPF Rule (simplified): (35 × 2.8 + 30 × 4.39) / 14 = (98 + 131.7) / 14 = 16.4s
The NPF Rule gives less than half the time. At 100% crop, the 500 Rule exposure shows clearly visible trailing. The NPF Rule exposure shows round stars.
Sony a7III (24MP, pixel pitch 5.93μm) at 14mm f/2.8: 500 Rule: 35.7s NPF Rule: (35 × 2.8 + 30 × 5.93) / 14 = (98 + 177.9) / 14 = 19.7s
The difference narrows because the a7III has larger pixels. But the 500 Rule still overestimates by nearly double.
Fuji X-T5 (40MP APS-C, pixel pitch 3.76μm) at 10mm f/2.8: 500 Rule: 500 / (10 × 1.5) = 33.3s NPF Rule: (35 × 2.8 + 30 × 3.76) / 10 = (98 + 112.8) / 10 = 21.1s
APS-C with high resolution — the 500 Rule is significantly too permissive.
OM System OM-1 II (20MP MFT, pixel pitch 3.33μm) at 7mm f/2.0: 500 Rule: 500 / (7 × 2.0) = 35.7s NPF Rule: (35 × 2.0 + 30 × 3.33) / 7 = (70 + 99.9) / 7 = 24.3s
Even MFT benefits from the NPF Rule correction, though the difference is smaller here because of the wider aperture and shorter focal length.
The Declination Factor
The examples above assume δ = 0° (celestial equator) — the worst case. If you're pointing at the Milky Way core (δ ≈ -29°), cos(-29°) = 0.875, giving you about 14% more time. If pointing near Polaris (δ ≈ 89°), cos(89°) = 0.017, giving you enormously more time (stars near the pole barely move).
In practice, most Milky Way photography targets the galactic center, which is at moderate declination. The equatorial worst case is a conservative safety margin.
Side-by-Side Comparison Table
All calculations at f/2.8, celestial equator (δ = 0°):
| Camera | Sensor | MP | Pixel Pitch | Focal Length | 500 Rule | NPF Rule | Difference | |---|---|---|---|---|---|---|---| | Canon R5 | FF | 45 | 4.39μm | 14mm | 35.7s | 16.4s | -54% | | Sony a7IV | FF | 33 | 5.14μm | 14mm | 35.7s | 18.0s | -50% | | Sony a7III | FF | 24 | 5.93μm | 14mm | 35.7s | 19.7s | -45% | | Nikon Z6III | FF | 24 | 5.93μm | 14mm | 35.7s | 19.7s | -45% | | Canon R6 II | FF | 24 | 5.97μm | 14mm | 35.7s | 19.8s | -45% | | Sony a6700 | APS-C | 26 | 3.94μm | 10mm | 33.3s | 18.6s | -44% | | Fuji X-T5 | APS-C | 40 | 3.76μm | 10mm | 33.3s | 21.1s | -37% | | Nikon Z50 | APS-C | 20 | 4.22μm | 10mm | 33.3s | 22.5s | -33% | | OM-1 II | MFT | 20 | 3.33μm | 7mm | 35.7s | 24.3s | -32% |
The pattern is clear: the 500 Rule consistently overestimates safe exposure time by 30-55%, with the largest errors on high-resolution full-frame sensors.
When the 500 Rule Is "Good Enough"
The 500 Rule isn't useless. There are situations where its looser limit is acceptable:
Social media output only. If your images will be viewed at 1080px or smaller (Instagram, Twitter, Facebook), star trails from a 500 Rule exposure are invisible. The image is downsampled so aggressively that pixel-level detail is irrelevant.
Panoramic stitching. If you're shooting multiple frames to stitch into a panorama and will crop the result to a lower resolution, the effective pixel pitch is larger and the 500 Rule approximation works better.
Quick estimation in the field. When your phone is dead, your hands are cold, and you need a number fast, 500/focal length is quick mental math. It'll produce slightly trailed stars, but it's better than guessing.
Timelapse sequences. For timelapses where each frame is displayed for 1/24 or 1/30 of a second, minor star trailing within each frame is invisible.
When You Need the NPF Rule
For any output where pixel-level detail matters:
Large prints. Anything above 40×60 cm (16×24 inches) reveals trailing from 500 Rule exposures on high-resolution cameras.
Cropping. If your composition involves cropping the frame (tighter Milky Way core, removing foreground), you're effectively magnifying and the trailing becomes more visible.
Stacking. If you're stacking multiple exposures for noise reduction, the stacking process actually makes trailing more visible because noise is reduced but the trailing pattern remains consistent across frames.
Portfolio and competition work. Judges and editors zoom to 100%. Trailing is the first sign of technical carelessness.
The Practical Workflow
Here's how to use both rules efficiently:
Before your shoot: calculate the NPF Rule value for your camera, lens, and maximum aperture. Write it on tape stuck to your tripod leg. You now have a number that doesn't change unless you change gear.
In the field: use that pre-calculated NPF value as your shutter speed. If conditions are challenging (very cold, battery concerns, limited time), you can extend toward the 500 Rule value as a compromise — knowing you're trading some star sharpness for more light.
After the shoot: at the computer, zoom to 100% on your stars. If they're round, your exposure was right. If they're dashed, shorten next time. Over a few sessions, you'll find the exact sweet spot for your specific gear.
The calculator approach: use the Astrian Light Spot Stars Calculator to compute your exact maximum exposure for any camera, lens, and aperture combination. It applies the full NPF Rule including declination, removing all guesswork.
Beyond Both Rules: The Real Limiting Factors
Both the 500 Rule and NPF Rule model the rotation of the celestial sphere — how quickly stars move across the sensor. But in practice, other factors can limit your usable exposure before stellar motion does:
Atmospheric turbulence (seeing). On nights with poor seeing, stars are blurred into 2-5 arc-second disks by atmospheric instability. This blurring is larger than the trailing from a properly calculated NPF exposure. On nights with poor seeing, you can actually expose slightly longer than the NPF Rule suggests because the atmospheric blur masks the trailing.
Wind vibration. If your tripod isn't rock-solid and wind is gusting, micro-vibrations will blur your stars more than rotational trailing will. A shorter exposure doesn't help if the tripod is moving.
Focus accuracy. If your focus is slightly off — stars are 3-pixel disks instead of 1-pixel disks — trailing within a 3-pixel disk is invisible. Ironically, slightly soft focus makes the trailing tolerance more forgiving.
None of these factors should change your calculated maximum exposure — always calculate for the sharpest possible conditions. But they explain why some photographers report acceptable results with longer-than-NPF exposures: other factors were already limiting their stellar sharpness.
Frequently Asked Questions
Can I just look at the LCD to see if stars are trailing?
Sort of. Zooming in on your camera's LCD gives a rough indication, but camera LCDs are small and not high-resolution enough to show fine trailing. You can catch obvious trailing (20+ pixels), but subtle trailing (5-10 pixels) that's visible in print or on a monitor won't show on the LCD. Trust the math, confirm at the computer.
Does the NPF Rule apply to star tracker work?
No. The NPF Rule calculates maximum exposure before Earth's rotation causes visible trailing on a fixed (untracked) camera. With a properly aligned star tracker, the camera follows the stars and you can expose for minutes or hours without trailing. The limiting factor with a tracker is field rotation (for alt-azimuth mounts) or tracking accuracy (for polar-aligned equatorial mounts).
What if I shoot at f/1.4 instead of f/2.8?
The NPF Rule gives you more time at wider apertures because the larger Airy disk masks trailing. At f/1.4 on a Canon R5 at 14mm: NPF gives approximately 12.1 seconds (vs 16.4 at f/2.8). Wait — that's actually less time, not more. The wider aperture term helps, but the focal length term dominates. The practical difference is small enough to be negligible.
The real benefit of f/1.4 isn't longer exposure — it's two additional stops of light at the same exposure time. That lets you drop ISO from 6400 to 1600, dramatically reducing noise.
Is there an even more accurate rule than NPF?
The NPF Rule itself is an approximation. The actual visibility of trailing depends on your monitor resolution, viewing distance, print size, and visual acuity. Some astrophotographers use even tighter limits (the "200 Rule" or custom calculations based on their specific output requirements). For most practical purposes, the NPF Rule is accurate enough that other factors (seeing, focus, vibration) dominate.
Why not just use 10-second exposures and be safe?
You can, and for ultra-high-resolution cameras (60MP+), 10 seconds is genuinely near the NPF limit at wide angles. The trade-off is that shorter exposures capture less light per frame, requiring higher ISO (more noise) or more frames for stacking (more shooting time). The optimal exposure is the longest you can use before trailing becomes visible — which is exactly what the NPF Rule calculates.
Does this matter for star trails photography?
Not at all. Star trail photography intentionally captures extended trails — the longer the better. The 500/NPF rules are only relevant when you want stars as sharp points.
Calculate your exact maximum exposure time with the Astrian Light Spot Stars Calculator — input your camera, lens, and aperture for an NPF Rule result.
Recommended Reading
Continue reading
Newsletter
A short reading once a month, in your inbox.
A note on the symbolism of the season, recent editorial pieces, and what to look for in next month's sky. No predictions.
Cancel anytime. We don't share your address.
Support this project
Independent, no venture funding, no ads. A contribution keeps Astrian precise and free.
Support on Ko-fi (opens in new tab)